Big Bore Pistol Build

[rsterne]

For those of you that have been following my development thread on the 2240 - 2550, with the Extended Valve, and have been wondering where it was leading.... here is the answer.... Now that I have a much larger valve, and the ability to develop enough power, I can get on with the project I had in mind originally.... a non-PAL pistol (ie under 500 fps) in .25 cal, followed by .300 cal and .357 cal until I run out of available power (which may very well be at .25 cal, only time will tell).... I wanted to incorporate the MRod self-indexing .25 cal magazine, which thanks to Lloyd at AirGunLab, is also available in .30 cal.... and because I'm right handed I wanted a left-hand bolt action so that I can maintain my grip while cocking the gun and feeding another pellet.... I also wanted to use a Center-Point 32mm Reflex Sight, but they fit on a Weaver rail, so having a Weaver mount on the breech was part of the project as well....

I spent about half a day designing and making the Weaver mount, which bolts on top of the forward section of the breech, and then two full days machining the breech and bolt.... Here is the result....



The two holes right at the edges of the Weaver rail are to access the 4-40 screws that locate the front of the breech, and the hole behind them is to access one of the two barrel setscrews, so I never have to remove the Weaver rail from the breech.... The breech is made for a 1/2" OD barrel, and will accept a .25 ACP barrel, plus the .300 cal and .357 cal barrels that Sean Pero and I came up with and Sean is selling.... All the barrels will carry an internal O-ring, sealing on the caliber-sized straight nose of the bolt, which seats the pellet past the transfer port and then retracts into the "J" slot so that the transfer port is completely unobstructed.... I used this setup on my Disco Double, and I really like it, so I decided to incorporate it into this pistol build....

The main tube is from a Crosman 2250, using the Extended Valve and lightweight hammer, driven by a Disco hammer spring adjusted by a Challenger RVA.... The valve was opened up to a 1/4" throat, and the 5/16" OD Teflon transfer port is 7/32" ID, with barrel porting to match.... The main tube and the breech are drilled 5/16" to accept the transfer port, and I added a 6-32 tapped hole between the two 4-40 holes in the main tube for a larger central mounting screw to make the breech 100% rigid.... The MRod magazine will need a small clearance notch in the left side ridge on the bottom to clear the head of the screw which protrudes slightly.... Here is what the gun looks like, assembled but without a barrel....



I'm pretty excited to see how far this project can proceed before running out of power just using a 12 gr. CO2 cartridge, and staying within the 500 fps Canadian non-PAL limits....

Bob

[rsterne]

I got my barrels from Sean last week, so it was time to get back to this project.... I ordered the barrels before the project even started, and I got three 12.5" barrels, in .250 cal, .300 cal, and .357 cal.... With the data I collected using a 24" barrel in .25 cal I now have my doubts if anything larger than the .25 cal makes sense on CO2, but it was time to fit the quarterbore barrel and see what happens.... I spent the day machining and fitting the barrel to the repeater breech I made and then proceeded to test the gun.... Here is a photo of the completed 2550 pistol....



The breech functions flawlessly, I really like cocking the gun with my left hand, which the MRod magazine makes easy.... I wound the Disco spring all the way to coil bind, shot 2 shots through the Chrony, backed the adjuster out a turn and kept going, shooting pairs of shots until the adjuster was 12 turns out.... Here is the result....



Instead of the velocity plateau only being above about 3 turns out (between there and coil bind), it was twice that long as a result of the shorter barrel.... In other words, you could reduce the preload a lot more without losing any velocity compared to the 24" barrel.... The maximum velocity was in the 470s with JSB Kings, just about perfect for a non-PAL pistol.... I adjusted the preload to 8 turns out, just on the knee of the curve, and put in a new cartridge and shot a string, taking 1 shot per minute, with the results as shown above.... The magazines are 8 shots, and the velocity started at 475 fps and gradually dropped to 461 fps at shot 17, staying above 450 at shot 21.... and the end of the 3rd magazine, at shot 24, was on the steep drop off but still above 400 fps....

I'm very pleased with the results, but it looks to me like there is little point in trying to go to a larger caliber.... No question it will work, but I think the velocity will drop significantly below the 500 fps goal.... With the Disco spring having lots of preload still available, generating the valve dwell for the larger calibers isn't the problem, but flowing enough CO2 to provide good velocity with only 850 psi likely is....

Bob

[rsterne]

I decided today to test this gun with my .300 and .357 cal rifle barrels from the Disco Double.... By using a mix of breeches and bolts from the two guns I was able to test the .30 cal normally, but the .35 cal I had to remove the breech each time to cock the gun.... The bottom line is that all the transfer ports and barrel ports are the same size (0.219") so the results are consistent across the calibers.... I tested both barrels to determine the velocity at various preload settings, and found that the .30 cal still had plenty of hammer strike available, the knee of the curve is about 3 turns out from coil bind.... but with the .35 cal the knee of the curve is very near the maximum hammer strike I can get with the Disco spring and the lightweight hammer.... Here are the results....



Anyway, the CO2 has enough power to drive the 44.8 gr. JSB .30 cal pellets at 495ish, and backed down to 470ish (21 FPE) I can get 16 shots before it drops below 450 and a total of 20 down to 325 fps.... The efficiency is 0.97 FPE/CI (403 FPE on 12.4 grams of CO2, I actually weighed the cartridge before and after).... So, you can make a .30 cal CO2 non-PAL rifle, but cutting the barrel in half would drop the velocity too far to make a decent pistol, IMO.... It would seem that the most logical thing is to stick to the .25 cal barrel for a CO2 pistol....

With the .357 barrel (28") I was just touching 390 fps and the velocity started to drop slightly as soon as I reduced the preload below coil bind, so a Disco spring will just barely max the gun out on CO2 in that caliber with the light hammer.... With the velocity under 400 fps, I didn't bother shooting a string.... For this to work, even as a rifle on CO2, would require larger ports, a larger valve throat, tranfer port, hammer, spring, and who knows what else, because once you start going that large the problems just snowball.... I don't even know if you could get a 22XX based rifle up close to 500 fps on CO2 with that big a caliber, but it is WAYYYYYY out of reach for this pistol build....

I then put the .25 cal pistol barrel back on, removed the RVA and reinstalled the fixed rear cap and added a spring guide with about a 0.050" spacer and ended up at 470-475 fps with the Kings.... I even heated the gun up with a hair dryer to over 100*F and the velocity dropped to ~400 fps and increased as it cooled back down to room temperature.... If I leave it sit to come to 68*F and then shoot a series of shots rapidly it starts at about 470, rised to nearly 480 at the 3rd shot (as the gun cools) and then starts to decline from there.... So, I've got the valve self-regulating with the velocity peaking at about 60*F.... How's THAT for kewl !?!

This has been an interesting build.... I now have a non-PAL repeating pistol that delivers nearly 24 shots (3 magazines) at ~12 FPE, and stays safely below the 500 fps limit over the entire range of temperatures.... and there are no "external adjustments" on the gun that can put it over 500.... I really like the functioning of the action with the left-handed bolt, and the Center-Point Holo-sight makes it easy to acquire the target.... All in all, it's a very usuable pistol.... The best thing is that, once again, I learned a lot.... It seems that if you tune a CO2 gun for the "knee" of the curve at room temperature, you are somewhere in the self-regulating zone for the valve.... What that means is that the velocity is stable over a wide range of temperatures, dropping only at extreme higher and lower temperatures.... I've seen this before with CO2, but now I know where to look on the tuning curve to find it, or at least get close.... The added benefit, of course, is that a gun tuned that way is very economical of CO2, getting more shots per cartridge, although the shot count will drop with temperature....

Bob

[rsterne]

I had an idea for mounting a piercing pin on the front of a Disco valve so that I could eliminate the internal pin in the valve and open up the front end for more flow.... This would require sealing the front of the tube and pinning the valve (or using a Disco valve, mounted with three screws), but it would drastically increase the valve volume which would be desireable for trying larger calibers.... Here are the parts....



The Disco valve front end is drilled out to 3/8" ID back to just shy of the spring seat, and that throat is drilled out to 17/64", the same as the spring ID.... Then the very front of the valve where the O-rings are, is drilled out to 1/2" for a depth of 3/8".... The piercing cap is machined from steel, with the pin mounted on a spider formed by drilling three 3/16" holes so that they are just inside the 1/2" OD portion of the cap which will be pressed into the front of the valve.... The cap is drilled from the back to 3/8" ID to match the bore of the drilled out Disco valve front end.... Therefore way more CO2 can enter the valve than can exit through the throat, effectively making the area around the neck of the CO2 cylinder plus the inside of the Disco valve front end, part of the valve volume.... If that still isn't enough volume, then I can remove the CO2 cartridge and go to a bulk fill by adding a male Foster to the front cap.... Here is what it looks like from the front....



The cap still has to be pressed into place with a bit of green loctite for good measure to make it permanent.... If a two cartridge setup was desired, another piercing pin would be installed in the front cap and one cartridge loaded each way, as in a QB78.... using a longer tube of course....

Bob

[rsterne]

I redesigned the piercing cap and front plug today to increase the volume of the "valve" section....ie from the cartridge to the valve seat.... I have now doubled the volume from the extended valve to 10 cc....



Basically I extended the piercing pin forward of the valve to move the cartridge forward in the tube, increasing the volume between it and the front of the valve.... I also made a new rotating plug for the front of the tube.... The brass section which carries the O-ring is free to rotate relative to the threaded cap but is fastened to it with an 8-32 screw through the center, which is in turn prevented from unscrewing by the setscrew.... Tightening the cap drives the cartridge onto the piercing pin, releasing the CO2 into the tube.... I'm hoping that by doubling the volume available for the shot I can get close to 500 fps with the .30 cal barrel, or maybe even the .35 cal....

Bob

[rsterne]

I bored out the 2240 valve back end to 0.62" ID, so my total valve volume is now about 11 cc including the area around the neck of the cartridge.... I assembled the bottom end and tested for leaks.... It was good to go, so I bolted on the breech from my Disco Double with the .357 barrel (28") and proceeded to test the velocity at various preloads.... It got close to 500 fps at coil bind, but there was no plateau, so I needed more hammer strike to find the maximum possible with this combination of valve volume and porting.... All I had to do was drop in a stock Disco hammer to go along with the Disco spring I was already using, and that provided enough to show a plateau in the power curve, at 515 fps using the 78 gr. JSB pellets (46 FPE).... Here is the way the gun reacts to changing the preload, followed by a shot string at 5 turns out from coil bind on the preload....



As you can see, you don't get very many shots from a cartridge at these power levels.... The 6 good shots averaged 485 fpe (42 FPE), and I tested the gun with a 67 gr. roundball and it shot 555 fps, so the gun would have to be detuned to be a non-PAL rifle.... To be at all practical, it would need to have a longer tube with two cartridges, or be bulk fill, which I can do with the same valve, by just installing a Disco fill fitting.... At least now there is the possibility of making a workable CO2 pistol in .35 cal shooting roundball somewhere in the (mid?) 400s so I wil try that at a later date....

In the meantime I'm convinced I now have enough power to make the .30 cal pistol a viable project, so the next stage is to fit my 12.5" barrel and make a new bolt (or at least bolt nose) so that I can find out how that will perform.... It would appear that my earlier concerns about not enough power for a pistol larger than .25 cal shooting close to the 500 fps non-PAL limit may have disappeared by having more CO2 volume available for the shot....

Bob

[rsterne]

I machined the .300 cal 12.5" barrel today and got it mounted in the breech.... I determined the velocity with various preload settings and graphed it, decided I would tune the gun to 7 turns out on the preload and shot a string.... I checked the velocity using roundball and it was JUST under 500 fps, and at 39.6 gr. those are lighter than any available pellet, so I replaced the RVA with a fixed cap with a custom fitted spring guide to set the velocity the way it was at 7 turns out.... Here are the results....



I'm getting 10 good shots between 458 and 470 fps, and the average is 466 fps (21.6 FPE).... The velocity falls off when I warm the gun over about 75*F and seems to peak at about 60*F, falling again slowly after that, so the valve is self-regulating very well, which results in the very flat shot string.... The gun is relatively insensitive to rate of fire for three quick shots before the velocity drops on the 4th about 10 fps.... This is one of the advantages of tuning a CO2 gun in this manner....

I plan to make a longer tube from a 2260 tube to hold two CO2 cartridges, and that should give me three full 7-shot magazines before the power drops off, and the tube will still be a couple of inches shorter than the barrel.... I'm VERY pleased with the development of this pistol, and now that I have about 11 cc available for the valve to draw from I have a working non-PAL pistol in .300 cal, and it can also be made into a non-PAL rifle in .357 cal.... with twice the power but half the shots.... I think I have reached the limits of this build, and I have no plans to bother setting the gun up with the .357 cal 12.5" barrel I have.... I'm so pleased with the way it shoot in .30 cal, and the projected shot count with two cartridges, I just don't see any point....

Bob

[rsterne]

Make those cans dance !!!



Exit hole on soup can, with entrance hole visible on the other side.... This thing sure packs a whallop !!!

Bob

[rsterne]

I finally got back to this build today.... Sean Pero shortened and threaded a 2260 tube for me so that I could use two CO2 cartridges or a bulk fill.... I made the piercing cap, and I already had a B&A Bulk Fill adapter kicking around, and there is enough clearance to rotate it under the barrel, so I can use either setup.... Here are the parts....



The valve incorportates a piercing cap as shown earlier in the thread, and the piercing pin in the front is sealed with a 5/32" ID O-ring on it in a recess in the brass plug which is held in by a drilled Crosman 2240 front cap.... The aluminum knob is set-screwed to the piercing pin.... You insert the two cartridges as shown (bottom to bottom) and screw in the plug until it pierces one cartridge, at which time the pressure stops it from rotating.... Then you simply push on the plunger to pierce the other cartridge.... Alternately, you can use the bulk-fill front end and fill from a CO2 tank.... You only get a few more shots with the bulk CO2, but it's a lot cheaper.... There doesn't seem to be any particular difference in performance, but you do get a larger cloud of CO2 blown out the barrel with the bulk fill setup....

After initial testing, I found that the velocity was about 440 fps, with either the 45 gr. JSB pellets or .30 cal roundball (40 gr.).... The roundball was not very consistent in diameter, and the ES was larger, so I decided to refit a stock hammer (instead of the 50% lighter one) to increase the velocity.... I'm using a set Disco spring, running in a stock, fixed 2240 end cap, with a spring guide that brings the length back up to that of a stock spring.... The velocity is now about 487 fps (nearly 24 FPE), and the fastest I saw over several clips was 495, for a good solid 14 shots (2 full clips) on two cartridges.... The increase in power burns up a bit more CO2, but it was worth it to get closer to the <500 fps non-PAL limit.... The velocity peaks at about 60-65*F, and the gun shoots a bit slower when either hotter or colder, so the valve is operating in the balance zone.... This also means that you can fire a whole clip as quickly as you can recock without the velocity dropping.... Here is the completed pistol....



The gun weighs 2 lb. 14 oz. ready to go including two cartridges, a 7-shot magazine, and the red/green multi-reticle holosight.... While doing the Chrony work inside at 20' I had several 7 shot groups that were just one ragged hole.... I love the way it cocks and reloads with the left handed bolt without having to remove my hand from the grip.... This has been a fun project, resulting in a very useful .30 cal CO2 Pistol....

Bob