Air Gun Home :: View topic - New Old Project

rsterne · Moderator

I've learned a lot in the 3 years since I built my .25 cal Disco, in particular from bouncing ideas off of Lloyd.... With recent developments, I decided to revist that rifle and see what improvments I can make.... When I finished the gun before, I had it set up for 11 shots at 45 FPE starting from a 2100 psi fill, although the gun saw a high of 58 FPE during development, right at 2000 psi with Baracudas.... The specs were as follows:

.25 cal LW barrel & bolt from Mountain Air, Disco tube, stock hammer and breech, poly transfer port (0.166"), the hammer spring was 1.75" long by 0.045" wire, running in a homemade RVA.... The valve was bored out to 0.60" ID, the threads shortened to 0.30", the throat drilled to 0.228" and the stem thinned to 0.115" (0.197" equivalent).... The front of the valve was opened up and tapered, and the gauge port was milled off-center to 5/16" to eliminate the original 1/8" hole which restricted the valve from drawing freely on the air reservoir during the shot....

Compared to some of the guns I have done since, that is actually pretty tame.... My .25 cal Millenium Pumper I just finished had porting that was 0.204" or the equivalent as a minimum right through, which is 50% more area, plus it has a heavier hammer with a longer stroke, although the valve is otherwise quite similar.... A couple of years ago, I made a two-piece valve for a guy, but never followed up on it.... Here is what it looked like....

I did use the concept successfully on my Hayabusa, however.... It seemed appropriate to bring that back into the fold for this round of mods to the .25 Disco.... I also wanted to experiment with the new hammer I came up with while working on the Millenium Pumper.... It looks like this....

The hammer is threaded 3/8"-24 NF inside and carries an adjuster which can move back and forth from flush to the end to recessed about 1/4".... Recessing the striking surface increases the stroke (travel) of the hammer before it hits the valve stem.... which in turn increases its energy and momentum.... There is a spring guide / cocking indicator threaded and glued into the adjuster, and a slot in the end provides the means to turn the adjuster.... The threaded hole in the top is for a "locker" to add resistance so that it doesn't self adjust.... The hammer is the stock 1.300" length, but the cocking pin is moved back 0.060" to allow a longer stroke before it hits anything....

Anyway, that is the starting point, and a hint at what is to come.... This should be an interesting project....

Bob

rsterne · Moderator

Here is the progress so far on the new parts for this project.... I decided to make a jig for drilling the valve body.... One of the hardest parts for me is getting the port and the three screw holes perfectly aligned and exactly at 90* to each other, as I don't have an index head for my lathe/milling machine.... What I did was I took a piece of 1 x 1.5" 6061-T6 aluminum and milled off one side to make a 1" square with a bump remaining for a 3/8" setscrew.... I drilled it 3/4" to accept round stock, or a Disco valve (a whisker under 3/4").... By rotating it 90* in my milling attachment I can now drill all the holes in the proper orientation.... I still have to indicate off the top of the round bar each time, but at least the results are accurate....

The valve is designed around an MRod poppet instead of the Disco one.... The head is smaller, and the stem is only 1/8" instead of 5/32" (no thinning required).... I machined a 40* angle (to the stem) on the sealing surface, and the seat in the valve is 45*, so it seals right at the throat, which at the moment is 0.250".... The exhaust port in the valve is 0.219" diameter and angled 20* for better flow.... The mounting screws are low profile 10-32 SHCSs with a 180,000 psi tensile rating, plently for use at 3000 psi....

A stock Disco valve is on the left.... You can see how restricted the flow is past the poppet, although the one in my .25 cal Disco is quite a bit better, having been bored to 0.60" ID.... My new valve is in the middle, and you can see the huge improvement in flow into the throat and around the head of the poppet.... The ID of the valve is the full OD of the valve insert, with the tube acting as the valve wall.... On the right is a PRod gauge port, showing the MUCH larger bore (11/32").... You have to use a 1/8" NPT male to female extension for the gauge instead of the original gauge mount which sealed to the Disco gauge port with an O-ring....

In the bottom row are a stock valve front end, and next to it, my new spring seat and air bypass, manufactured from it.... I added another O-ring groove aft of the trigger mounting screw (where the wide groove is), drilled it out 11/32" from the front, and then milled in four 3/16" holes at a 30* angle to pass air around the outside of the (shortened) spring seat.... The fitting on the right was the original version, which took me an afternoon to make.... It looks pretty, but there is a significant design flaw.... Can anybody see how I wasted an afternoon's work?....

The final photo is how it all goes together.... The upper item is a stock Disco valve, unscrewed until the spring is at zero preload.... Also shown is a stock Disco gauge mount, about the right distance from the valve (~1/16" gap between).... The new parts are directly below it, positioned appropriately for comparison.... The PRod gauge block is a bit longer, and the valve seat is sitting right up against it.... The back of the two valves are very close to the same position, meaning that the valve spring will have about the same preload.... I can adjust the preload by drilling the spring seat slightly deeper if required.... I will prevent the gauge port from sliding forward from the spring pressure (and interfering with the stock) by a simple 8-32 setscrew in the side of the tube.... The back of the new valve has a shoulder machined 0.250" forward to accept a washer of Lloyd's energy absorbing elastomer, and to allow additional hammer travel.... I left the center of the valve full length to act as a stem guide.... Since the photo was taken, I shortened the valve stem so that it projects 0.25" past the guide portion of the valve body, which is more than I need to allow for the lift....

So there are the major parts for the new valve.... I may yet drill the throat of the valve out a bit, as at the moment the total throat area is barely equal to the 0.219" port size I am planning on using.... It would probably be prudent to wait until after initial testing to do that, to see if there is any benefit....

Bob

rsterne · Moderator

I did a lot of measuring today, working on the hammer stroke.... Here are the pertinent details.... First of all, the hammer stroke for a stock Disco is only 0.58".... The lift of the valve is limited by the valve spring going coil bound, which occurs at 0.25" of lift.... The valve stem sticks out 0.32" from the back of the valve, however, so it can be shortened by 0.070" and still allow the valve full lift.... That increases the stroke to 0.65".... That is how I had my .25 Disco set up previously, and there is really no down side to doing that easy mod....

To understand some of the other limitations in the Disco design, it is useful to look at what the maximum possible travel of the hammer is, ignoring the valve stem.... The cocking pin limits the travel significantly.... It can hit the front of the cocking slot in the tube, and the back of the bolt (when closed), at about 0.8" of total travel.... Note, that occurs when the valve is open 0.22", eg. if you dry fire the gun, or use a mondo hammer spring wound tight at low air pressure.... If those are lengthened, then it will hit the front of the cocking slot in the steel breech.... It is easy to shorten the bolt, and grind the slot in the tube a bit longer, so let's assume those are done.... Now the hammer can move 0.90" at which time the face of the hammer will hit the back of the valve body, drive the valve to coil bind (at 0.25" lift), and the cocking pin will hit the front of the breech slot.... basically all at the same time.... If you are using a stock hammer, that is about all you can get, 0.65" of stroke, 0.25" of lift (which you will never need), and 0.90" total travel.... This makes grinding the slot in the tube about 0.10" longer and shortening the back of the bolt a pretty decent mod if you are using a heavy hammer spring.... One word of caution.... If you shorten the back of the bolt too far, it may not push the cocking pin back far enough to cock the gun.... About 0.10" is roughly the maximum, but it seems to vary a bit, gun to gun.... Another advantage is that this gives increased room to load the pellet, particularly useful if you have extended the bolt probe....

So I'm going for broke here, how can I get more hammer stroke?.... Well, there is enough material to machine off 0.25" from the back of the valve body.... The new valves don't have an O-ring, so this can be done easily.... I would recommend leaving a 1/4" diameter stub in the center to keep the valve guide full length.... OK, so that doesn't do anything, right?.... Well, how about if you could drill a hole 0.25" deep in the front of the hammer?.... Yeah, I know, it's hardened, bummer.... OK, how about making a new hammer, that I can do, I wanted to make it adjustable anyway.... So let's duplicate the original hammer except put a 3/8"-24 NF threaded hole in the front.... Now I can set the striker face anywhere I want relative to the front of the hammer..... I can adjust the lift, or the point at which the striker hits the valve.... Now when the outer rim of the hammer face hits the (new) back of the valve, it will have moved another .25", for a total of 1.15".... Darn, the cocking pin can't move that far without hitting the breech slot.... OK, I wanted to use a bumper anyway, and it's 1/8" thick, but I'm not sure if it will collapse a bit, so let's call it 0.10".... Now the hammer hits the bumper after travelling 1.05"....

OK, next idea.... How about moving the cocking pin back on our new hammer?.... There is room to move it 1/16" without making the hammer any longer.... In addition, when the hammer is sitting in the cocked position (against the sear), there is a pretty fair gap at the back, over 1/16".... It turns out that you can move the cocking pin back 0.10" and make the hammer about 0.040" longer and it still works.... Now the cocking pin can travel 1.00" without hitting anything.... Oops, I forgot, we shortened the bolt, and now the gun won't cock.... No biggy, I can cure that by machining the slot for the bolt handle in the breech further back, lengthening it 0.10" to compensate.... BTW, even more loading room....

So where would we be at this stage?.... The hammer can move 1.05" before it hits the bumper on the back of the valve, and 1.00" before the pin hits the end of the cocking slot in the breech.... However, the valve stem sticks out 0.4" from the back of the bumper, and we need to hit it to open it.... Assuming for a minute the hammer face is sitting against the bumper.... If the striker is recessed 0.40" (the maximum), the valve doesn't open at all.... However, if the striker is recessed only 0.15", by the time the hammer face hits the bumper, the valve will have opened 0.25" and the valve spring is in coil bind.... We know it never needs to go that far, so let's make the minimum distance from the striker to the hammer face a 0.20" inset.... The valve can still open 0.20" before the hammer hits the bumper (likely still more than we need).... With the striker adjusted this way, the operational stroke of the hammer is 0.85", and we've gained 0.20" over the 0.65" we had earlier (with the striker flush).... That's a 47% increase in hammer stroke over stock, and 31% more than I had with just the stem shortened.... When the striker hits the valve stem, the cocking pin (further back, remember) is still 0.15" from impacting the front of the slot in the breech.... That's probably enough lift, but I'm going to make sure by moving the front ends of the breech and tube cocking slots forward another 0.10", shortening the bolt more, and machining the slot for the bolt handle back further as well.... In other words, I'm going to make sure that the only thing that can hit is the outer rim of the hammer face on the bumper on the back of the valve.... Since I can adjust the striker face between 0.20" and 0.40" recessed into that face, I will have the following results....

Face recessed 0.20".... Stroke 0.85".... Maximum lift 0.20"
Face recessed 0.30".... Stroke 0.95".... Maximum lift 0.10"
Face recessed 0.40".... Stroke 1.05".... Maximum lift zero

Now you can see why Lloyd chose 0.90" as being the longest practical stroke he could get in a modded Disco.... he can still get about 0.15" lift, which should be all you really need.... In the next post, I'll show you the parts....

Bob

rsterne · Moderator

To continue.... Here are the photos of the new parts....

Let me mention the valve first.... I haven't even assembled the gun yet, and I'm already on the Mark 3 version of the valve front end.... The parts I built yesterday would have worked fine, but there were three O-rings in between the gauge and the trigger screw that weren't needed.... I changed the design (yet again) and came up with what I think is an elegant and simple solution.... The spring seat on the valve poppet on my new valve ends up in the same place as it did in a Disco valve, meaning that by using the same valve spring, the front seat needs to be in the same place as it used to be.... The new valve front end is drilled through 3/8" for most of it's length, leaving lots of volume, a large air passage, and enough wall thickness to carry the gauge mount.... At the back of that hole, I milled four 3/16" holes on a 30* angle, which amounts to the same area.... The air from the reservoir passes through these holes around the OUTSIDE of the valve spring, into the (full diameter) chamber between the two halves of the valve.... The passages into the valve are nearly 3 times as much area as the port leading out, so the valve certainly won't starve for air.... The valve spring sits on the head of an 8-32 screw that I filed down slightly, and if I need to shim the valve spring tension I can add a washer under the screw head.... The clearance slot for the trigger mounting screw has been reduced to a shallow hole milled with a 5/32" mill.... The screw sitting in that hole will locate the front half from moving forward due to the force of the valve spring, and keep the gauge centered in the hole in the stock....

Now for the new hammer.... It is 0.040" longer than a Disco hammer, and the cocking pin is 0.10" further from the front.... The front half is drilled and tapped 3/8"-24 NF for the adjuster, which is made from a piece of bolt, and it is 0.40" long.... The rear half of the hammer is drilled 11/32" to clear a QB hammer spring.... The adjuster acts as the front spring seat, so that when you change the hammer stroke you don't change the preload.... and it carries an integral spring guide which doubles as a cocking indicator, and will be slotted to allow adjustment from the back with a screwdriver.... The slot milled on the top of the hammer is to allow clearance for the rearward position of the 4-48 breech screw used on the Disco, and is longer to compensate for the material removed from the back of the valve which will allow the hammer to move further forward.... The 8-32 hole in the side of the hammer is for a setscrew and plastic plug to act as a "brake" so that the adjuster can't self-adjust.... The RVA is longer to accomodate the longer QB hammer spring, and the adjusting screw is drilled through for the spring guide.... The guide will be shortened so that it is only visible when the gun is cocked.... A longer guide could also be used to carry a rear cocking knob if necessary.... The hammer/adjuster/guide assembly weighs 70gr, about 23% more than a stock Disco hammer....

These parts are now ready to be installed, but I have some things that need to be done to the gun to ready it first.... It needs a larger transfer port hole in the tube and breech, some milling done on the breech slots and tube cocking slot, and some work done on the barrel port and chamber.... Those are the next things to address....

Bob

rsterne · Moderator

I worked on the breech and barrel today.... I decided to add a breech mounting band located between the gauge and trigger group.... I have made several scratch-built breeches this way before, but it never occurred to me that there was room in a stock Crosman breech to incorporate the same feature.... I have Lloyd to thank for that.... The trick is to use 4-40 screws, and to drill the holes for them in EXACTLY the right place so that the holes don't break out into the barrel hole but are in as far as possible.... That works out to using a #33 drill (0.113") and locating the holes 0.285" either side of the centerline.... A 3/16" mill is used to create a counterbore for the heads, which are sunk 0.120" below the scope rail.... If you go too deep, you will again break out into the barrel hole.... The screws are visible from the side, but plenty strong enough.... There is a 1/4" wide slot milled in the bottom of the breech for the breech band, and it is milled 0.025" deep measured from the recess where the top of the main tube fits.... The breech band is milled down to only 0.020" thick at that point to insure the breech tightens down hard against the main tube.... The centerline of the band is 0.75" from the front of the Disco breech....

The two orange circles in the photo show where I lengthened the slots.... The bottom slot is lengthened to clear the cocking pin, and the side slot is lengthened to allow the bolt to be pulled back further to increase loading room and allow the gun to be cocked because the bolt is shortened and the cocking pin is further back in the hammer.... You can also see the massive transfer port recess.... The port will be made from a piece of 3/8" Teflon rod, drilled 0.219".... This almost caused me a problem, check out the photo below....

I've used a 5/16" OD transfer port before, but you really can't go much bigger than 3/16" ID.... I wanted a larger port, so I decided to make the port 3/8" OD.... I had already milled a 3/8" flat on the valve, and drilled the hole out in the main tube, and I was about to drill out the breech when I realized that the barrel stub is, of course, only 7/16" OD.... I've used a 5/16" flat on that before (barely), but there isn't much barrel wall left on a quarter bore.... If I milled a 3/8" flat, I'd likely break right through.... My solution was to install the barrel into the breech, line up the port, and tighten the barrel setscrew.... I then mounted the assembly in my milling attachement and milled a pocket until I just removed the 1/4" OD recess that was already in the Mountain Air LW barrel.... That meant that part of the 3/8" recess was in the breech, and part in the barrel, hence the rather strange shape.... The barrel port itself is 3/16" wide by 1/4" long at the bore, tapering up to a 0.219" hole at the port face (same area).... The oval barrel port was so that I didn't have to make the port wider than 75% of the bore which can cause pellet loading problems....

I created another problem with my 3/8" recess.... It was so close to the end of the barrel I couldn't machine in 0.065" for the O-ring that will seal the bolt without breaking through.... My solution was to only machine the O-ring groove in 0.050" and to drill a shallow 3/8" recess in the front of the breech to gain the extra thickness I needed for the O-ring.... I assembled it with the O-ring in place, and a piece of 1/4" rod is a nice snug fit in the O-ring, so the bolt should seal perfectly....

This whole project is becoming a matter of figuring out how to make things bigger and better than the Disco was ever designed for.... I just have to be careful not to box myself into a corner doing it....

Bob

rsterne · Moderator

No photos today, not really anything significant to show.... except for the preliminary results.... How does 101 FPE at 2800 psi with 42.4 gr. EunJins sound?.... Time to top up the SCUBA tank, the ShoeBox is ticking away right now....

Today I finished up all the little jobs that seem to take forever.... I made a new bolt and bolt handle, a transfer port, and carved a notch in the stock to clear the new breech band.... Careful deburring of the tube was necessary as two of the O-rings have to slide past the gauge hole and front trigger hole in the tube.... I deburred the chamber and polished the barrel with JB Bore Paste, followed by JB Bore Bright.... Then came careful cleaning and assembly, and it paid off, there were no leaks.... The gun looks so stock I can't believe it....

I only had time (and air) for a few shots, enough to find out that the 25.4 gr. JSB Kings were wayyyyyyyyyy too light at anywhere near maximum power, so I switched to the only tin of EunJin Points I have.... I'm going to run out of pellets and testing will come to an abrupt halt shortly.... Actually, the first shot was a shocker, I filled the gun, loaded it, pulled the trigger, and.... NOTHING.... Well the hammer went clunk and I thought WTF, valve lock?.... Then I remembered that I assembled it with the hammer face fulled recessed and it can't open the valve like that.... Well, at least I knew I had enough striker travel....

I tried a few adjustments on the travel, all with the hammer spring preload maxed out (I need more travel).... and it turns out that from 3 to 4 turns CW on the striker from fully retracted all shot the same velocity, 1037 fps at 2800-2900 psi.... I set the gun to 3.5 turns, filled the gun to 2800, and shot 3 shots.... 1037, 1005, and 965 fps.... That's 101, 95, and 88 FPE, and the pressure after the 3rd shot was 2000 psi.... YUP, it's an air hog when maxed out.... I haven't done any testing on the hammer spring preload, but I assume I can dial it down to a pretty decent (although short) string....

Anyway, I would have to call this project a success already.... I broke the 100 FPE barrier, and I did it at 2800 psi with 42.4 gr. pellets.... This humble Disco has turned into a BEAST !!!

Bob

rsterne · Moderator

I spent the day today figuring out how what makes this thing tick.... With the hammer travel / valve lift adjustment I now have another variable to deal with.... I had already discovered that I have enough adjustment on the hammer striker to go from zero valve lift (with a 0.4" recess on the striker) to just over 0.20" of potential valve lift at 5 turns (the adjuster is 24 TPI, or 0.042" per turn).... The first few shots I fired yesterday told me that from 3-4 turns from maximum travel (and minimum lift) is the sweet spot.... ie a travel of about 0.9" and a maximum lift of 0.15".... I also discovered that as I increase the hammer travel, since the preload doesn't change with this adjuster design, the chance of going coil bound on the spring increases.... I knew that 2 turns was about as little travel as I would likely need, so I found a preload that allowed that much lift without binding.... Using that preload, and using 3000 psi for every shot, I checked the velocity for each turn of hammer travel, and using an O-ring on the spring guide, I measured the valve lift.... I then set the travel in the middle of the sweet spot, and then I repeated the procedure, checking the velocity at coil bind, and then backing out the RVA a turn at a time.... Here are the results....

The blue lines show what happens to the velocity and lift as I vary the hammer travel adjustment.... You will notice that the lift reaches a maximum of 0.095" about 2 turns from maximum, and that it drops off rapidly once the face of the hammer starts striking the bumper on the back of the valve.... The velocity follows the lift, except that there is a slight drop in velocity when the hammer stroke is maxed out.... I suspect that is from the hammer rebounding off the bumper and shortening the dwell slightly.... The red lines show the relationship between velocity and lift and spring preload.... They show the normal trend, and are approaching a plateau at maximum, but there is probably still a few fps remaining in the gun if I fitted a stronger hammer spring.... Incidently, I'm using a set QB hammer spring, that is 2.3" long and made of 0.047" wire.... When you look at how much more lift was required to pick up 24 fps, any further gains would be at the expense of a LOT of air.... so I don't think I'll bother looking for another spring.... I should mention that this testing was done by loading the gun with two 23.1 gr. roundballs (#3 buckshot) at the same time, as I was running low on pellets.... It's not the perfect scenario for testing, as the shot varies slightly in diameter.... but the trends are clear, so I feel it presents a fair representation of what is going on....

I then wanted to find out how the FPE varied with pellet weight.... Having a limited range of weights, I used a simple trick and doubled up on pellets (even three at a time) to produce heavier weights for testing.... When you see how smooth the curves are, I think you will agree that the method appears to give valid results....

A few highlights to note.... The 17.3 gr. Lasers flew across the Chrony at 1328 fps.... JSB 25.4 gr. Kings hit 1219 fps (84 FPE), H&N Baracudas did 1158 fps, and EunJin Domes were just shy of Mach 1 at 1115 fps.... The 42.4 gr. EunJin Points did 1055 fps (105 FPE).... I used doubled up pellets next, and then three at a time to get weights of over 90 grains.... The FPE peaked using 70-85 grains of pellets at 115 FPE, but the curve was pretty flat from 62-106 gr.... All pellets heavier than 42 gr. produced over 100 FPE at the 3000 psi test pressure.... Because of this, I have ordered some 50, 53, and 58 gr. pellets from Mr.Hollowpoint.... His 73 gr. pellets, at 0.651" long, would be unlikely to stabilize in the LW barrel, according to the Border Barrels Twist Calculator.... Besides, pellets in the 50-60 gr. weight range should end up in the 900s for velocity....

Previous experience has showed me that heavier pellets can stand a bit more hammer strike to develop maximum FPE, while lighter ones are optimized with a lighter hammer strike.... That trend would actually shift the peak of the FPE curve in favour of heavier pellets, and increase the velocity and energy the gun could develop using them.... However, trying to optimize the gun for such heavy pellets would require a faster twist because of their length.... Again, I don't think it's worth the effort, and I'm willing to leave a little power on the table.... After all, it's very likely I will detune the gun significantly from these power levels anyway....

Bob

gicos · Member

You are a gun building beast and the Yoda of the airgun world, Bob. Beautiful work. YouAreAGod

ZipSnipe · Veteran Member

I second gicos comment YouAreAGod

rsterne · Moderator

Lots of testing today.... I used my new adjustable inline regulator and tethered the gun to my SCUBA tank.... I started out at 2400 psi, with the hammer travel set at 0.9", which gives me about 0.15" of lift available before the hammer hits the bumper on the back of the valve.... I tried four different pellets at every hammer spring preload setting from coil bind to 8 turns out.... Here are the results....

The velocity curve with the JSB Kings is exactly what I have come to expect.... a plateau (at just above Mach 1), and then rolling over the knee of the curve into a nearly linear decline.... As the pellet weight increased, the knee moved slightly to the left (to higher preload), and eventually at maximum preload it started to drop again.... I had concluded previously that this is because at high preload the hammer is bouncing off the bumper on the back of the valve, shortening the dwell.... Why this seems more pronounced with heavier pellets I am not sure.... To check this, I reduced the hammer stroke 0.1", which increases the maximum lift that can occur by 0.042" (so the hammer doesn't get to the bumper).... I retested the lightest and heaviest pellets and the drop in velocity with the heavy pellets at maximum preload was gone.... I then reduced the pressure to 2000 psi, and then to 1600, and repeated all the preload settings at each pressure for the JSB Kings and EunJin Points....

I don't recall ever doing a chart quite like this before.... It actually is pretty interesting.... The point at which the curves cross over is where the velocity is the same at two different pressures.... Where the 1600 and 2400 psi curves are crossing, and the 2000 psi curve is just above where they cross, is a potential tuning point for a bell-curve.... I set the gun up for that point with the Kings, fine tuned the RVA and the fill pressures slightly, and got my first shot string....

There are only 7 shots, but they average over 60 FPE, are within a 4% ES, and only use 750 psi of air.... That works out to an efficiency of 1.00 FPE/CI, not bad for a first effort.... Before I started this round of changes to the gun, I used to get 11 shots within a 5% ES, but at only 45 FPE, and using 900 psi of air, for 0.96 FPE/CI....

Here is a summary of the results to date....

3000 psi:
25.4 gr. JSB Kings - 1219 fps (83.8 FPE)
30.9 gr. Baracudas - 1158 fps (92.0 FPE)
35.2 gr. EunJin Domes - 1115 fps (97.2 FPE)
42.4 gr. EunJin Points - 1055 fps (104.8 FPE)

2400 psi:
25.4 gr. Kings - 1160 fps (75.9 FPE)
30.9 gr. Baracudas - 1100 fps (83.0 FPE)
35.5 gr. EJ Domes - 1045 fps (86.1 FPE)
42.4 gr. EJ Points - 986 fps (91.6 FPE)

2000 psi:
25.4 gr. Kings - 1087 fps (66.7 FPE)
42.4 gr. EJ Points - 933 fps (82.0 FPE)

1600 psi:
25.4 gr. Kings - 1004 fps (56.9 FPE)
42.4 gr. EJ Points - 854 fps (68.7 FPE)

I'm very pleased with these results, particularly at the lower pressures.... The increase in performance over the previous version is very satisfying....

Bob

rsterne · Moderator

I did a bit of pellet testing today.... I was shooting inside at only 7 yards, and all the groups were pencil sized holes and I couldn't tell any difference in accuracy.... The good news is that if pellets are atrocious they generally open up even at that range.... I set the gun with the hammer travel at 4 turns in and the preload at 5 turns out.... and tethered it to my SCUBA tank with my inline regulator set at 2000 psi.... The first few shots were a little hot, so I backed the preload out another turn and the gun was noticeably quieter, so I know from that and the velocity it's operating on the knee of the velocity curve, not on the plateau.... I used several different pellets, from a 23.3 gr. #3 buckshot up to the 42.4 gr. EunJin Pointed.... and then I doubled up on the lighter ones to get up to just over 60 gr.... I wanted to find out the relationship between pellet weight and velocity and energy, and here's what I found out....

The right hand side of the curve is where the double loaded pellets are, so it's only a rough idea of what might happen with pellets heavier than 42.4 gr.... I'll have to wait until I get my Mr.Hollowpoints to add them to this data....

Bob

gicos · Member

That rifle is a BEAST Bob! 104 FPE equals a .22 Long Rifle standard velocity. The coolest part isn't that it'll be shot at that power level all the time, but that it CAN go there. And probably higher! You're really making me reconsider .25 caliber with this one and your last beauty, damn it.

rsterne · Moderator

I took some time today to experiment with the hammer travel / valve lift settings to see what effect it had on shot strings and efficiency.... I chose a power level that is at the upper end of what would be practical with JSB Kings, around 1000 fps average.... I started with a fill pressure of about 2000 psi and 4 turns in on the hammer travel and found I got a decent 7 shot string with the RVA set at 7 turns out filling to 1900 psi.... I then turned the hammer travel out 1 turn, the RVA out another half turn, and got another 7 shot string starting from 1950 psi.... One more turn in on the hammer travel same RVA setting, and I got a good string starting from 2200 psi, but only 6 shots.... I had to use the higher fill pressure to overcome the limited lift imposed by that setting, however.... In each case, I played with the RVA and fill pressure to get the best (ie flattest) bell-curve.... Here are the results....

In practical terms, there is little difference between the first two strings, and in fact they had the same efficiency, 1.08 FPE.CI.... At the greatest hammer travel, however (the black line), the valve can only open a maximum of 0.084" before the hammer hits the bumper on the back of the valve.... First of all, you can see that the string is shorter and steeper.... Instead of getting 7 shots within a 4% ES like in the first two strings, I only got 6 shots within a 6% ES.... However, the efficiency was quite a bit better, at 1.33 FPE/CI.... more power from less air used, but a shorter string.... Some of that extra efficiency, however, could be from the higher pressure required to overcome the severely limited lift at that setting.... I therefore wanted to try more settings in between 2-3 turns in on the hammer travel to explore what was happening.... I did several more strings, at 2.25, 2.5, and 2.75 turns in on the hammer travel, and played around with the RVA and fill pressure (in all cases between 1900-2000 psi) to optimize the shot strings at the same power levels.... Here are those results....

Now a clear trend is emerging.... as the maximum lift on the valve decreases, the shot string shortens and drops off very quickly after the peak.... The black line (0.095" max lift) shows that very clearly.... The red line (0.116" max lift) shows the flattest shot string, and the blue line (0.105" max lift) is in between, with the velocity just starting to turn down on shot 7.... All three of these settings produced better efficiency (1.14-1.16 FPE/CI) than the "unlimited lift" curves from the first chart, but not as good as the "severely limited" curve.... My conclusion was that once the hammer contacts the bumper (which seems to occur at about 0.105" of lift at roughly 1500 psi) the velocity then drops off on subsequent shots because the valve can no longer open further.... That very contact limits the amount of air wasted on those shots, increasing the efficiency.... but when carried too far causes the velocity to drop like a stone, shortening the shot string by a full shot (a big percentage out of 7 shots)....

I've never used the bstaley O-ring buffer mod, but from what I understand, these results would appear to contradict what he finds.... I can only assume that the energy absorbing qualities of the bumper is the reason.... The 70 durometer O-rings he usually uses may act a lot less like a lift "limiter" than a lift "cushioner", but that's just a guess.... In any case, the best shot strings seem to be as the bumper JUST starts to come into play.... When the hammer just starts to touch it on the last shot or two it seems to be increasing the efficiency slightly without truncating the shot string.... The roughly 6-7% increase in efficiency is small but measureable.... However, I'm not so sure that at these power levels it is worth the added complication of having the extra adjustment to worry about....

From previous experience, and from using Lloyd's spreadsheet to provide an estimate of how the efficiency is changing over the shot string.... I'm guessing that for the best setting the first shot is about 1.5 FPE/CI, and the last shot is about 0.8 FPE/CI.... That would mean that running the gun on a regulator at 2000 psi should return about 1.5 FPE/CI at 55 FPE, the practical limit for these pellets.... At some point I will tether it to a small tank to confirm that hypothesis....

Bob

rsterne · Moderator

I tethered the gun directly to my SCUBA tank today (no regulator), filled it to 2000 psi and bled the line and shot a few strings.... Then I did the same with the tank valve shut off but the line not bled, so that it's volume becomes part of the reservoir.... However, by doing that I can use the (larger, more accurate) gauge on the fill clamp to monitor the pressure during the shot string.... It appears than an 8 shot string gets stretched to 9 shots with the added volume of the hose, fill clamp, and fittings.... so that means the total volume is 9.26 CI instead of 8.23 (the reservoir by itself is 135 cc).... Here are the results of the 9 shot string, broken into three segments of 3 shots each....

Shots 1-3: average velocity 998 fps, 180 psi used.... 169 FPE from 115 CI of air.... 1.47 FPE/CI
Shots 4-6: average velocity 1022 fps, 230 psi used.... 177 FPE from 147 CI of air.... 1.20 FPE/CI
Shots 7-9: average velocity 998 fps, 270 psi used.... 169 FPE from 172 CI of air.... 0.99 FPE/CI
Shots 1-9: average velocity 1006 fps, 680 psi used.... 514 FPE from 434 CI of air.... 1.18 FPE/CI average

Using the gun tethered to a 2000 psi regulator, I can therefore expect to shoot at ~56 FPE at an efficiency of about 1.5 FPE/CI with the 25.4 gr JSB Kings.... That is very good efficiency with a relatively light bullet at those power levels.... so I'm very pleased.... I'm looking forward to trying it with heavier bullets....

Bob

radar · Silver Status Member