Air Gun Home :: View topic - BAM B-51

rsterne · Moderator

About 2 years ago I bought a BAM B-51 in .22 cal from sniper when he got a crate of the last of them.... I knew they should be cleaned before shooting them, and I had other projects on the go, so hung it on the wall until "later".... Well, "later" finally arrived this week, so I read "The Book" (which I had previously downloaded), and stripped the barrel and breech off and pulled the hammer out to clean and deburr them and the tube.... The barrel was filthy, and the bolt was very stiff, so I removed the bolt, deburred all the holes and slots in the breech, and cleaned the barrel thoroughly, and gave the breech bore and the barrel a polish with Bore Brite.... The bolt was still stiff and I suspected it was bent, so over to the lathe, chucked the front, and spun it around to watch the handle wobble around nearly 1/16" off center.... No wonder it was stiff.... I spent quite a bit of time straightening it, and that freed it up nicely.... A thin smear of Moly paste and a new O-ring (the original was shredded from the sharp edges I had to remove) and it was ready to reinstall....

I had read about the hammer, and decided that the most suitable mod from "The Book" was to remove the threaded striker, cut a slot in the back end so that it could be adjusted with a long thin screwdriver through the rear plug, and reinstall it, being careful to put it back to the factory length.... I also put a small piece of plastic under the locking screw for the striker, and tightened the locking screw up to compress it against the threads to provide a "brake" so that the striker won't "self-adjust" and yet I can change the setting from the back.... The hammer is VERY heavy, and has a heavy steel spring shim and guide (top hat) inside, with the total weight being 148 grams, of which the top hat is 38.... This is extremely heavy, and is compensated for in the design by having a stiff spring and very short travel.... The normal method of tuning is to screw in the striker from the back (clockwise from the rear) which makes it protrude even further, shortening the stroke even more.... This moves the hammer back, increasing the preload on the spring, and making the cocking stroke shorter but even harder.... I wasn't too sure about the whole concept, but decided that I needed to test it in stock form and make the normal adjustments to get a baseline for further experiments....

I reinstalled the hammer, leaving the safety off so that I could adjust the hammer travel through the end plug with a long thin screwdriver.... The first tests were conducted at the stock travel setting, filling to 1000 psi for a shot, then 1500, then 2000 (velocity increasing), then 2500, 2800, and finally 3000, which resulted in the highest velocity (938 fps) with JSB Express 14.3 gr. pellets.... So, I knew I was in for a falling velocity shot string, at least with the lighter pellets.... I hooked up my Chrony to my Netbook using Chrony Connect, and starter shooting strings.... I then saved them into Excel to create the following graphs....

The initial tests, at stock hammer travel, showed a pretty powerful gun, 27-30 FPE (heavy pellets giving more FPE, as usual), but only 17-19 shots within my normal 4% ES.... The efficiency wasn't bad, considering the way the gun was tuned, from 0.89-0.95 FPE/CI, with the heaviest pellet having the best efficiency.... So far, no surprises, including the excess hammer strike, I expected that from what I had read about the gun, it was tuned for maximum velocity from the factory, not a proper bell-curve.... I removed the stock and cranked the striker in three turns, grabbed the middle-weight JSB 15.9 gr. Exact pellets, and found out I had gone too far.... The velocity at 3000 psi fell to the mid 700s, and took a dozen shots before it got to within 4% of what ended up being the peak at 874 fps.... After recording the string, I turned the striker out a turn, repeated the process, then once more, and the four strings with the 15.9 gr Exacts are all shown on the second graph above.... Here is a summary....

Stock setting.... decreasing string from 909 fps, averaging 894 fps (28.2 FPE), 17 shots at 0.90 FPE/CI, 3000 psi down to 2320....
In 1 turn.... normal bell-curve, 884-909-873, averaging 895 fps (28.3 FPE), 24 shots at 1.19 FPE/CI, 3000 psi down to 2280....
In 2 turns.... normal bell-curve, 862-896-861, averaging 878 fps (27.2 FPE), 25 shots at 1.20 FPE/CI, 2900 psi down to 2180....
In 3 turns.... normal bell-curve, 840-872-842, averaging 861 fps (26.2 FPE), 28 shots at 1.36 FPE/CI, 2700 psi down to 1980....

This confirmed that the factory hammer strike was too much, turning it out a turn gave more power and more shots at greater efficiency.... After that, turning the adjuster in more just dropped the pressure range and power.... The "proper" setting would be about 2 turns out from stock, using the factory rated 200 bar (2900 psi) fill pressure.... My gun was leaking down slowly during testing, so in fact the efficiency is actually better than what I recorded above.... I did NOT like the feel of cocking the gun at 3 turns out on the striker, and it was barely acceptable at 2.... The bolt pulls back a long way before engaging the hammer, and the cocking stroke is short and requires a very hard pull at that setting.... I think I will concentrate on lengthening the hammer stroke, reducing the preload, and reducing the hammer weight for the first round of mods.... The latter two will be done by replacing the top hat with a shorter unit (to reduce the preload) made from Delrin (to lighten it).... I will likely also fit a preload adjuster into the end cap to revert the adjustment to what I consider to be superior to the current setup.... Here is a photo of the gun, with a Leapers 3-9 x 32AO scope installed....

Bob

Bob La Londe · Veteran Member

Ok, I may be reading that wrong, but it sounds like a lighter hammer spring might get you close to the curve you think it should shoot and be easier to cock.

rsterne · Moderator

In this photo you can see the stock hammer, guide, spring, and end cap.... Notice that the striker is sticking out about 2 turns from the nose of the hammer (stock setting was 0.75mm), and also notice the distance from the front of the hammer to the lock screw....

I shortened the front of the hammer by 2mm and made a flush striker to increase the hammer throw by 2.75mm (0.108") over stock.... I also made a very light plastic spring guide that adds only 1/4" of preload to the spring instead of 1/2".... Here are those parts....

I also removed the screw that prevents the hammer from rotating.... It is only necessary if you want to adjust the striker (I don't) and with it removed the hammer can be withdrawn out the back of the tube by lifting up the trigger sear without having to remove the breech.... I reassembled the gun, hoping that the extra hammer travel would mean it would still work about the same with less preload.... It didn't.... I had to put 4 washers under the back end of the hammer spring, and shorten the spring guide because it hit them.... I ended up making the guide the same length as the guide portion on the original metal one because I ended up needing more than stock preload with the much lighter guide.... In other words, I lost more hammer strike by reducing the weight than I gained by increasing the travel.... I shot strings with both 4 and 5 washers, and then put back the original metal guide with no washers, so I had stock preload minus 2.75mm but the spring was still at the same length when cocked as a stock gun.... This combination actually increased the power over stock.... The shot strings, using 15.9 gr. JSB Exacts, are shown below....

The interesting thing is that I gained in efficiency and shot count with the additional hammer travel.... In addition, I ended up with more power when I went back to the stock metal guide (top hat) but with the extra 2.75mm of hammer travel.... Strangely, I also ended up with a bell-curve with that setup, even though the hammer strike was greater than before (when I didn't have one).... Here is a summary of the three strings....

Light Guide, 4 washers.... normal bell-curve, 856-892-856, averaging 878 fps (27.2 FPE), 32 shots at 1.41 FPE/CI, 2900 psi down to 2140....
Light Guide, 5 washers.... normal bell-curve, 870-905-871, averaging 889 fps (27.9 FPE), 28 shots at 1.29 FPE/CI, 3000 psi down to 2270....
Heavy Guide, stock preload.... bell-curve missing some shots at the beginning, 903-924-887, averaging 908 fps (29.1 FPE), 20 shots at 1.27 FPE/CI, 3000 psi down to 2420....

The first string is directly comparable to the first tests at 2 turns out, same power (27.2 FPE), but now 32 shots instead of 25, 1.41 FPE/CI instead of 1.20.... It looks like the additional hammer travel is worthwhile.... I need to fit a preload adjuster to the rear end cap and do some more tests, so that is next....

Bob

Bob La Londe · Veteran Member

I need to go look at my B50. I believe in having safeties, but I wonder if a safety lever could be made with a through hole in the middle so an adjuster could stick through it. At the same time I might be tempted to put a slightly longer lever on it to make it easier to engage and disengage the safety.

In general I've been so happy with my B50 that I just haven't messed with it. The only thing I've done to it so far is make a compensator out of aluminum tube using Marauder baffles.

rsterne · Moderator

You are correct about the lighter spring.... It will also move the fill and refill pressures down and therefore reduce the power.... Most of the mods in "The Book" do exactly that, because they are aimed at .177 cal for FT use(either 12 or 20 FPE)....

Bob

Bob La Londe · Veteran Member

rsterne · Moderator

This afternoon I made an RVA (rear velocity adjuster / power adjuster / preload adjuster) and shortened the spring guide to compensate for it.... Here are the parts....

On the left is the guide, shortened 1/4".... In the middle is the B-51 end cap.... I milled a 3/8" diameter recess into it leaving 1/4" of the threads remaining, and then milled a 1/8" slot on the bottom side of the spring seat area.... On the right is the 8-32 adjusting screw, with a thinned nut and the adjuster with pin to prevent rotation.... You insert the screw through the threaded hole in the end cap (it just clears and can rotate) and then install the nut and tighten it against the shank of the screw, which leaves just enough space the screw can rotate but only have about 0.010" end float.... Then screw on the adjuster with the pin the slot in the end cap so it can't rotate....

Once assembled, there is 8 turns (1/4") of adjustment, and the only modification necessary on the stock was to file a small angled notch to clear the ball-end hex driver used to adjust it.... In the photo, it is roughly in the middle of its adjustment range.... When fully out, the adjuster takes up 0.10" of room in the spring seat, so I shortened the guide 1/4" to allow adjustment in both directions from the stock preload.... It is at the stock setting when 3 turns out from maximum preload.... I then tethered the gun to a tank through a regulator set to 200 bar (2900 psi) and shot 5 shots at each setting (1 turn increments) and plotted the average velocity as shown in the top graph below....

Since the stock preload was 3 turns out, and it was on the lower end of the knee of the velocity curve at 200 bar, I thought that would be a good place to shoot my first string.... The results are in the bottom graph above.... I was extremely pleased, as the first shot at 3000 psi ended up exactly 4% below the peak of 904 fps, and I got 32 shots averaging 888 fps (27.9 FPE) from 3000 psi down to 2220, which works out to 1.38 FPE/CI.... That is a great number at that power, one that few PCPs can match in my experience.... The question is why is this gun so efficient?....

There are a few things I noticed that are consistent with many of the things in "The Book".... First of all the initial, upwards part of the shot strings are rather ragged, with significant shot to shot variation (although within the 4% ES), compared to the downward, low pressure side of the string.... Secondly, the pressure drop is almost perfectly linear instead of curved upwards in the middle, which is normal for unregulated PCPs (they usually use more air per shot at low pressure).... Third, I noticed no "burping" and very little increase in the volume of the muzzle blast towards the end of the string.... Fourth, the sear has quite a strong spring so that it drags on the hammer to a significant degree.... Here is what I think is happening.... I think the heavily sprung sear is dragging so hard on the heavy, slow moving hammer, that that friction prevents it from bouncing and wasting air at the low pressure end of the string, and indeed you can't hear any resulting "burrpppp" from hammer bounce.... However, this drag, being not completely identical shot to shot, causes the velocity swings in the first part of the string, exactly where you would expect to see that effect exaggerated.... On the latter part of the string, even large changes in hammer strike will not change the velocity, and we don't see any.... The (almost complete?) lack of hammer bounce shows up in reduced air usage and greater efficiency towards the low pressure end of the string.... I think a lot of that is due to the (what we would think was excessive) sear drag on the hammer....

Now I have a problem.... Should I leave well enough alone and be happy with a fairly powerful and very efficient gun?.... or should I hog out the tiny ports and see if there is a beast sleeping inside this gentle giant?.... I think we all know the answer to that.... *grin*....

Bob

robert w · Senior Member

Rstearn you amaze me with your fine gun work.looks like your wakin that b 50 up

AirGunEric · Site Admin

Bob La Londe · Veteran Member

garththomas · Veteran Member

So what you've done actually is lower the preload on the factory spring and increased the hammer travel by 2mm, no other TP work or polishing to achieve that string and your filling to 2900psi with no valve lock.

rsterne · Moderator

That last string was with stock preload but 2.75mm more travel and a 15 gram lighter spring guide (so hammer).... Stock ports and 3000 psi fill....

Bob

rsterne · Moderator

Today I removed the breech and drilled out the barrel port and milled the transfer port recess deeper, completely through the breech to put the flat that it seals against in the barrel itself.... That allowed me to change to a 5/16" OD x 5/16" long Teflon Rod transfer port.... The stock porting (valve, transfer port and barrel) was a very stingy 0.114" (likely made for a .177) and I went to my usual 75% of the bore for the barrel port, which is a #20 drill (0.162").... The transfer port was the next size up (0.166"), and when I did the valve exhaust port later in the day, I used an 11/64" drill (0.172").... Here is a photo of the breech after machining, and the new transfer port sitting on top of it.... Note the beautiful job on the leade where it tapers into the rifling, visible through the barrel port.... The stock bolt probe seats JSB pellets nicely ahead of the (now larger) port, so no problem there....

At this point I still had not modified the valve, but I wanted to see what I had gained (if anything) so I filled the gun and shot a string with the 15.9 gr. Exacts using the same 3 turns out on preload that I did on the last string for comparison.... It was shooting hotter, and I figured if I gained more when I drill out the valve port (and I should) I might be overpowering the Exacts, so I shot another string with the 18.1 gr. Heavies as well, same preload setting.... Here are the results....

With the 15.9 gr. pellets I gained a solid 30 fps, lost a few shots, and the first shot at 3000 psi was more than 4% below the peak velocity, which had moved down from 2700 psi to 2600, but was now 935 fps.... I had 27 shots within my 4% ES, averaging 921 fps (30.0 FPE) at 1.35 FPE/CI.... With the 18.1 gr. pellets, the first two shots were too slow, and there was a nice long plateau from 2700 psi down to 2400 at just under 890 fps.... This was an exceptional string, I got 33 shots averaging 876 fps (30.8 FPE) from 2900 psi down to 2100, which works out to 1.45 FPE/CI.... That's pretty amazing in .22 cal at over 30 FPE, IMO....

Not wanting to stop there, I bled off the pressure and pulled the valve.... I was shocked when I saw the size of the valve spring, and how much preload it has, partly from the 1/8" thick brass spacer on the poppet.... The preload is so much I don't think the valve can even lift 0.1" without the spring going coil bound, which is likely the cause for the known poppet failures from shooting the gun at low pressure or dry-firing it.... The poppet was an interesting design, with a highly concave face that seals right around the outside edge, making the seal 0.41" OD.... At 3000 psi, that takes 396 lbs. to crack the valve, plus the force of that massive spring (another 20 lbs?).... so no wonder the hammer weighs a ton.... The throat of the valve is a generous 0.240", which seems totally out of place compared to the tiny 0.114" exhaust port.... The port is quite near the seat, but I determined there was lots of room to go to the size I wanted (in fact you could go to 3/16" for a .25 cal.), so I carefully positioned the valve in my milling attachment on my old Atlas lathe and drilled the exhaust port out, in 3 steps, to 11/64" (0.172").... I then eased the corners at the bottom to remove any sharp edges with a small spherical burr in the Dremel, cleaned everything up, (including the inside of the tube, it was filthy), replaced all the O-rings, added a small washer to prevent the check valve from damaging the brass filter in the fill fitting, and reassembled the gun.... I'm pretty sure I cured the leak, at least it's a LOT better.... Here is a photo of the drilled out valve and the poppet, valve spring, and spacer....

I am in the process of topping off my Great White tank with my Shoebox compressor, so shooting strings will have to wait, but I had enough air in my SCUBA tank to get one fill to 3000 psi.... I still had the preload at 3 turns out, grabbed the 18.1 gr. JSBs and took a shot.... Hmmmmmmmmm.... 772 fps.... Valve lock?.... I continued shooting and the gun came alive at about 2700 psi and peaked at 2400 at.... are you ready for this?.... 1013 fps (41.3 FPE).... I refilled to 2900 and cranked the preload in a turn at a time, and it was still increasing at 2900 psi even at maximum preload, where it was doing about 980 fps.... I will be shooting some strings when I have my tank topped up, but it looks like I will eventually need more hammer strike to find out what the maximum is at 3000 psi.... After the next round of tests I will either increase the hammer travel more, remove some of the preload on the valve spring (or use a weaker one), or both, as the gun is already hard to cock at maximum preload.... Who would have thought that a gun known for too much hammer strike would need even more once the beast inside was unleashed?....

Bob

rsterne · Moderator

I took some time this evening to shoot some strings with the modded valve at several preload settings.... I used the JSB Exact Heavy 18.1 gr. pellets for all tests, and only those shots within a 4% ES were recorded.... Here are the results....

To say this thing came alive when I drilled the valve out would be an understatement.... The highest velocity recorded was 1028 fps (42.5 FPE) at about 2500 psi with the preload at maximum.... The string is near ideal for a 200 bar (2900 psi) fill, but I can't max. the gun out at that pressure, there isn't enough hammer strike to find out what it would do with the JSB Heavies, let alone heavier pellets.... That string averaged 1012 fps (41.2 FPE) for 22 shots from 2900 psi down to 2120, for an amazing 1.40 FPE/CI.... At 3 turns out, I got 24 shots at 982 fps (38.7 FPE) average, from 2700 psi down to 1930 for 1.46 FPE/CI.... At 5 turns out the fill pressure was 2500 psi, returning 25 shots at 960 fps average (37.1 FPE), down to 1760 psi, for an efficiency of 1.51 FPE/CI.... At the minimum preload, 8 turns out, I still got an average of 931 fps (34.8 FPE) for 23 shots from 2200 psi down to just 1600, with an astounding 1.61 FPE/CI, it only used 600 psi to do that....

I particularly like the string at 5 turns out, from 941-973-941 using a 2500 psi fill to get 25 shots, the longest string.... I know the JSB Heavies are accurate at that velocity in other guns, and they were shooting one-hole groups during the Chrony session, which certainly looks promising.... However, it seems I can never leave well enough alone, so I'm going to pull the valve apart and remove some of the preload by thinning out that brass collar on the poppet.... I can't shake the thought that I can get the same performance without beating the valve to death with such a heavy hammer strike, I would really like to reduce that....

Bob

Bob La Londe · Veteran Member

You are inspiring me to want to open up my B50, and put the .22 barrel and breech on it. I just haven't wanted to mess with the gun because it shoots cheap Crosman Premier 10.5/.177 so well.