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rsterne · Moderator

Over the past few weeks I have been collecting the necessary parts for this new project.... Here is what I have purchased as a starting point.... I bought a Disco main tube and valve, the hammer and gauge block from a PRod, a trigger and guard from an MRod, a Disco Double kit and MRod trigger shim from Lloyd at Air-Gun-Lab, and a Boyd's Blaster stock in Camo from Norm at Discos-R-Us inletted for the Disco Double kit and MRod trigger.... I wanted to start from the best possible components to achieve my goal....

The main purpose of this project is as a test bed for the new .30 cal barrels that SMP (Sean) and I are having made for the new JSB .30 cal pellets.... These pellets are smaller than the 0.308" standard used by Powderburners, having a head size of only 0.300", so we have designed a new barrel to match.... They will be available in a 25" length to fit on the Disco and allow enough room for a muzzle brake or air stripper to be attached.... The barrels will be 0.50" OD, which will allow their use in a .25 cal MRod as well as a new repeater breech that I have designed and Sean will be making.... The breech will also accept a .25 cal barrel with a 0.50" OD, and make use of the 8-shot .25 cal MRod magazines, both stock for the quarterbore version, and enlarged to .30 cal (with 7 shots) for the bigger caliber.... The magazines will accept pellets or cast bullets up to 0.48" long.... Those can be stabilized in a .30 cal barrel with a 26" twist, which is ideal for the JSB pellets, so that was our choice.... The mandrel had been made, and we will be placing an initial order shortly....

In terms of potential performance, we're pretty much flying blind here at the moment.... My initial goal was ~850 fps with the 46 gr. JSBs (74 FPE), and while that remains a realistic performance level to get a decent shot count, I'm hoping that during development I may see that power level in .25 cal and about 88 FPE in the .30 cal, but at what pressure remains unknown.... Ideally, I would like to achieve 14 shots (2 clips) with the .30 cal using the Disco Double tube setup.... which would mean 7 shots (1 clip) on a standard Disco.... As the market for quarterbore rifles is likely way larger than for a .30 cal, we are trying to reach a compromise between the two, allowing as many components to be shared as possible.... and also for the minimum changes to Disco components.... It's a given that the valve needs to breathe a lot better, so that will be the place I start.... I will try the PRod hammer (without the extended striker) and my heavy Disco hammer spring, but I doubt they will be up to the job, so we anticipate making a heavier hammer with longer travel, and using a QB hammer spring to move enough air, particularly for the .30 cal.... An RVA will be part of the package, of course....

The new 7/8" wide, 6" long breech will be designed around the .25 cal MRod magazine, and will incorporate an additional mounting band at the front, located between the trigger and gauge.... This will provide exceptional rigidity for a floating barrel, although a barrel band may also be used if desired.... The top of the breech will be grooved full length for dovetail mounts.... It will be significantly taller than a Disco breech (to allow for the magazine), so for appearance it will incorporate a mount for a 7/8" OD Carbon Fibre shroud and a Hatsan-style air stripper.... The end of the barrel will be threaded, and when the air stripper is tightened against the end of the shroud, it will put it in compression, putting the barrel in tension.... The muzzle blast will be vented out through the stripper, and not into the shroud, so that legality will not be a question in Canada.... For those allowed to have quieter airguns, there will be clearance for a longer 1" OD shroud.... although the barrel tensioning feature will be lost of course.... It is hoped that 0.59" OD barrels will be available for QBs and Sean may even work on a repeater breech for them as well at some future date....

While we're on the subject, I want to make it clear that I will not be involved in the marketing of these barrels, breeches, and other components other than in doing the design work and prototyping.... Manufacturing things and selling them is Sean's bag, not mine.... so I ask that any inquiries in that regard be addressed to him.... My compensation is solely the satisfaction I get from making things work they way they should.... at which point I move on to the next project.... I have no interest in repetitive operations, nor do I have the time to do them....

Bob

Alstone · Moderator & Site Supporter

A great Project Bob and starting with quality parts the combined efforts of you and Sean should finish you with a unique rifle, it’s also nice to see a lot of forethought going into the project. I do sympathise with your altitude to repetitive operations my outlook also, make one and walk away from it.

Say hello to Sean

Al

rsterne · Moderator

As I said, the first thing that needs to be done is to improve the flow through the valve.... A stock Disco has the following port dimensions, starting at the barrel and working backwards against the flow....

Barrel Port - 0.133"
Transfer Port - 0.140"
Exhaust Post 0.146" (90* corner)
Throat ID - 0.219" : Stem OD - 0.156" - {Equivalent diameter - 0.154"}
Valve ID - 0.514" : Poppet OD - 0.428" - {Equivalent diameter - 0.285"} (radial gap 0.043")
Inlet ID - 0.250"
Gauge Port ID - 0.125" (PRod Gauge Port ID - 0.343")

As you can see, in a stock Disco, the narrowest restriction is the gauge port on the inlet side of the valve.... Replacing this with a PRod gauge port makes the inlet side virtually unrestricted compared to the outlet, with the smallest diameter being the inlet of the valve itself at 0.250".... The porting on the outlet side is relatively well balanced, with the smallest point being the barrel port itself, which is 0.134" in both .177 and .22 cal.... I find that making the barrel port larger than 75% of the bore size can lead to loading problems or damage to the pellet.... The 0.134" barrel port is therefore the maximum width that can realistically be used in a .177.... In all other calibers, making it (and the other ports feeding it) larger can provide increased airflow, and hence offers the potential for more FPE.... These "75%" port diameters are as follows:

.20 cal - 0.150"
.22 cal - 0.163"
.25 cal - 0.188"
.30 cal - 0.225"
.35 cal (9mm) - 0.268"

From past experiments on Disco valves, I have determined that there is enough meat around the exhaust port to run a 3/16" mill in at a 20* angle (although in .22 cal, I use a 5/32" mill).... greatly increasing the area and easing the 90* corner at the throat.... The flat where the transfer port seals can be increased slightly to a 5/16" diameter, and I have had great success making transfer ports from 5/16" OD Teflon rod with a 3/16" or fractionally larger hole.... Taking into account various production considerations, and the fact that we're not trying to throw 100+ gr. cast bullets downrange in the .30 cal, we decided to design around the optimum 3/16" barrel port in .25 cal.... I basically started at that barrel port and worked my way back from there, making sure there was more flow at each stage, and ended up with the following dimensions....

Barrel Port - 0.188" (double the area)
Transfer Port - 0.188"
Exhaust Port - 0.188" x 0.202" - {Equivalent diameter - 0.195"} (70* angle with rounded corner)
Throat ID - 0.234" : Stem OD 0.115" - {Equivalent diameter - 0.204"}
Valve ID - 0.625" : Poppet OD 0.428" - {Equivalent diameter - 0.455"} (radial gap 0.098")
Inlet ID - 0.266"
Gauge Port ID - 0.375"

I decided that rather than use the PRod gauge port I would make a new extended valve front end with integral gauge mount.... One of the things that is problematic with the Disco design is that you have to slide three O-rings past the 1/2" diameter hole in the tube where the gauge is installed.... This large hole tends to act like a cheese grater and often damages the O-rings.... By making the gauge port and the valve front end in one piece, the O-rings are in front of that hole and don't get shredded.... The only downside is that each front end has to me made to match the valve back end it is used with because of "thread clocking".... However, that's Sean's problem, not mine, I only have to make one....

In the above photo you can see the slimmed down valve stem, and then extended valve front with 1/8" NPT gauge mount.... The small recess aft of that is for the front trigger mounting screw, which protrudes inside the tube when installed.... Notice that the O-rings are in front of the gauge mount....

In this photo, you can see that the front of the gauge portion is opened out to 1/2" for a bit of increased volume, and you can see the huge 3/8" hole through it for most of its length.... Note that the valve rear half is bored out to 5/8" from the seat to within 0.3" of the front, vastly increasing the volume inside the valve, but that was mostly done to ease the flow around the head of the poppet.... Although from an area point of view there wasn't a problem, I have a gut feeling that trying to cram that much air through a gap not much more than a millimeter wide and then forcing it to turn twice through 90* has to produce extra friction.... Therefore I bore out my valves to ease the flow past the poppet....

This last photo shows the most restricted point on the valve inlet and outlet.... On the inlet side, the hole is the same diameter as the inside of the valve spring, and the area is twice as large as the transfer and barrel port.... If you look at the exhaust port compared to a stock Disco valve you will see a huge difference.... When you run a 3/16" mill in at a 20* angle, where it meets the transfer port flat it no longer apprears round, but is elongated fore and aft because of the angle.... To make that port round, I move the mill sideways 0.007" each way, which rounds out the hole at the transfer port, and enlarges it slighty in area.... (When I do this in .22 cal using a 5/32" mill, I get an equivalent diameter of 0.162", which matches the porting in a .22 cal).... In this case, the resulting port is 0.188" x 0.202", which is the equivalent of a 0.195" hole.... Not only is the area 78% larger, the 20* angle of the port and the rounded lower edge (cleaned up with a Dremel) must at least double the flow at the same pressure.... I'm very pleased with the results, and while these mods are not of the simple "home brew" variety, Sean should have no difficulty in replicating them on a production basis.... I'm about ready to install the valve and the Disco Double tube kit and test for leaks....

Bob

rsterne · Moderator

Got the valve installed and the Disco Double tube kit assembled and trial fitted to the stock.... Norm did a beautiful job with the inletting, it just dropped right in....

It seems to be holding pressure just fine.... I made a band to hold the two tubes lined up securely and installed it just in front of the stock.... Once I'm all finished the development process I'll strip it all apart and blue the second tube.... Normally Lloyd supplies them finished, but I requested it in natural.... Next week I hope to get started on the breech....

Bob

rsterne · Moderator

Here is a photo of the roughed out breech.... all the longitudinal machining is done and it's bored 1/2" end to end.... This basically took me all day....

Lots of work yet to finish it; loading port, bolt slot, cocking slot, transfer port, and all the mounting holes including the slot for the front band....

Bob

toadmyster223 · Senior Member

I thought doubles had two tubes; what makes this a double? Very cool, anyway! Great work as always

rsterne · Moderator

You're looking at the two tubes in that photo.... There is no barrel and breech on the gun yet....

Bob

toadmyster223 · Senior Member

:B Thanks Bob Hammer Head

rsterne · Moderator

Update on these barrels.... The mandrel is made, and the barrels will be produced to the following specs:

.30 cal Airgun barrel intended for the new JSB 46 & 50 gr. pellets
Made from CrMoly steel, hammer forged
Groove 0.300"
Land 0.294"
Twist 1 turn in 26"
6 Lands
OD 0.500" (0.590" also available)
Standard length 25" (12.5" for pistols)

What they are NOT: They are not intended to use 0.308" cast bullets, nor are they intended to use any bullet over 0.48" in length, as those will not fit into a modified .25 cal MRod magazine, and may not stabilize in the 26" twist....

Sean will be placing an initial order as soon as we can collect pre-orders and determine a price.... For THIS ORDER ONLY, and only for BARREL BLANKS with no machining, we will accept orders for lengths other than the above, but be advised there may be a small surcharge from the manufacturer for barrels over 29" long....

I will be contacting those who have expressed an interest in pre-ordering barrel blanks.... Anyone else who wants to get in on this initial order, either for blanks or for machined barrels.... please contact me ASAP.... Sean will be finalizing the list within a week, getting prices, and then contacting you to arrange payment.... Sean will be offering all machining services (chambering, porting, and crowning, and can even add a choke) at extra cost if you desire, please ask him for details directly....

Bob

rsterne · Moderator

I was able to get back to machining the breech today, and finished everything except the bolt slot.... The breech accepts a .25 MRod self-indexing magazine, and Lloyd at AirGunLab is tooling up to supply them in a 7-shot .30 cal version.... The breech accepts a 1/2" OD barrel, and the plan is for Sean to supply them in both .25 cal and .30 cal versions.... The boss on the front is 3/4" diameter, to accept a 7/8" OD Carbon Fibre barrel shroud, allowing the barrel to be tensioned by the screw-on Hatsan style air stripper.... Here is what it looks like at the present time....

The breech mounts in three places, at the rear to the RVA (end plug), in the middle to one of the two positions of the 4-48 screw (allowing it to be used on a Disco or 22XX tube), and at the front with a band around the main tube between the trigger and gauge location.... The barrel is solidly mounted with three 8-32 setscrews.... The rigidity of this breech and barrel assembly should be superior to any other breech I have seen on a Disco/22XX, eliminating accuracy robbing flexing and bending....

There are three holes in the middle of this breech because the holes drilled in the Disco tube I have were misaligned, so I had to drill and tap another hole in between them and use that.... Only that hole is counterbored in this breech.... Normally that hole would be absent, and both the other locations would be counterbored.... One difference in this breech is that I left enough material under the magazine to allow a standard 4-48 SHCS to be used instead of the custom low-profile one Crosman uses that accepts that tiny 0.050" allen key.... The thicker head on a standard 4-48 SHCS accepts a 3/32" allen key, so no more stripping out the head of the screw.... The 4-40 screws in the breech band will accept the same size tool....

I have passed along the names of everyone who has asked for a barrel from the initial production run to Sean.... He's pretty busy right now, but will contact all of you with a price as soon as he can.... In the meantime, I'll continue building parts so that when my barrel arrives hopefully it will the the last thing that needs machining....

Bob

robert w · Senior Member

bob, this is becoming REAL INTERESTING . your gonna make me want to shoot it when its done. looks greatso far and i bet its fine when done sir Mr. Green

rsterne · Moderator

Over the last couple of days I made the two bolts (.25 and .30 cal), the bolt handle, and machined the bolt slot in the breech.... The bolt design is quite unique, thanks to Sean.... at least I have never seen it before.... It is probeless.... The sealing O-ring is in a groove in the end of the barrel....

Rather than have either a hollow probe (which loads the pellet by pushing on the base, and the air flows through it).... or a thin extended probe (which loads the pellet by pushing inside the skirt and the air flows around it).... this bolt simply has a flat end.... The key is in the bolt slot.... The bolt pushes on the base of the pellet (or bullet) to load it, and stops with the base or skirt past the barrel port.... It is then withdrawn to clear the barrel port completely, leaving no obstruction to the airflow once in the firing position....

Sean's original bolt slot design was "T" shaped, the bolt moved past the battery position, was withdrawn and turned down to lock.... We then modified it to a triangular shape, where the bolt handle was pulled down and back on an angle to the locking position.... I then saw a photo of the Crosman Rogue which has a slot shaped like the one in the above photo, and uses the retraction of the bolt to the firing position as a safety interlock (as I understand it).... I adopted that rectangular slot design and Sean's probeless bolt for this rifle.... In addition to having no obstruction to the airflow in the barrel, the bolt handle, once in the firing position, can't pop up on firing.... The bolt in the second photo is shown in the firing position....

Bob

rsterne · Moderator

I got most of the remaining parts made today.... Here is the front breech mounting band, shown installed on the breech....

It fits over the tube between the trigger and the gauge, and securely clamps the front of the breech to the main tube.... I then made a new hammer and RVA assembly....

The hammer is the same length as a Disco / 22XX hammer, but it is cylindrical and will therefore work with a Disco, MRod, or PRod trigger group.... If an MRod trigger is used, it should be shimmed down 0.030" because it is designed for a thicker main tube.... The hammer is drilled to accept a QB 7X hammer spring, which is longer and larger in diameter, stiffer, and has more travel.... The cocking pin is moved back 0.065", allowing slightly more total hammer travel.... The hammer weighs 58 gr. the same as a Disco hammer, and has a notch to clear the rear breech screw location used on a Disco.... The spring guide is made from a piece of 7/32" drill rod with a nut threaded on, peened, and turned.... It is long enough to protrude through the RVA adjusting screw and serve as a cocking indicator.... It weighs 15 gr., so the total hammer weight with guide is 73 gr., 26% heavier than a Disco hammer....

You will notice that the hammer face is counterbored 0.070" to increase the hammer stroke and limit the valve lift.... In combination with the 0.070" shortened valve stem, the stroke has increased from 0.58" to 0.72" (an increase of 24%), and there is still 0.18" of valve lift available.... I have found that to be more lift than required on all other Disco platforms I have worked on, in fact it is seldom more than 0.12" in a properly tuned gun.... The RVA is made from aluminum, and carries an adjusting screw made from a 3/8"-24 NF bolt.... The spring and guide can be removed through the threaded hole, no need to remove the end plug once installed.... The guide slides easily through the adjusting screw, and protrudes when the gun is cocked, giving a visible cocking indicator.... There are two flats on the adjuster to allow it to be turned with a 5/16" wrench.... and it is locked in place with a thin (jam) nut once adjusted.... I have enough travel on the RVA to go from no preload to coil bind, about 16 turns....

I am now at the point where I can install the breech on the main tube and inlet the stock for the front breech band.... Once that is done, I can get on with sanding and finishing the stock.... Most of the gun should be done by the time the barrels show up....

Bob

rsterne · Moderator

I installed the cocking pins in the bolts today and machined out the breech to clear them....

When the bolt is in the "down" position (shown), the head of the 4-40 SHCS rotates out of the way of the hammer pin into the recess milled in the side of the breech.... When the bolt rotates to the "up" position to open and load a pellet, it engages the hammer pin, drawing it back to cock the hammer.... Nothing magic, but complicated to measure umpteen ways to make sure it all works properly and allows the bolt to retract far enough to remove the magazine when the gun is cocked....

I inletted the stock for the breech band, finish sanded it, and put on the first coat of Watco Danish Oil yesterday.... The stock will be done in about a week.... I wet sand each of the next six coats, two with 220 grit, two with 400, and tha last two with 600.... and then give it a final coat of Minwax.... I think the "Forest Camo" combination of black, brown, and green in the Boyd's Laminated Blaster stock should be quite attractive when done.... It is much darker than the first photo with it unfinished....

I think all the machining is done except for the barrel.... and a shoulder on the Hatsan air stripper to locate the carbon fibre shroud.... and then the last step will be to make a transfer port from a piece of 5/16" Teflon rod.... Basically, once the barrel arrives, the gun is only days (hours?) away from initial testing.... I've already pulled the trigger and heard a satisfying BANG when the hammer opened the valve for the first time.... I won't be bluing the Disco Double tube and any small parts until after testing....

Bob

lampy · Veteran Member

Very cool!