Non-PAL 1740 Build

I just received a new 2240 from AirGunEric with the intention of converting it to .177 cal.... I already had a 10" barrel from a 1377 and a Crosman steel breech, so the major parts were looked after.... With our wonderful rules in Canada, it is critical to keep the gun under 500 fps as I have no intention of ending up with a Restricted Firearm that I can only use at an approved pistol range.... I have previously built a 1750 with an 18" barrel and carbine stock that shoots just under 500, so I already have some ideas to try.... Recently, however, I got a tiny 0.070" ID transfer port from Eric, so I thought that would be my first trial.... Eventually I hope to get the gun tuned so that the valve is pretty much self-regulating so that the velocity is more or less independant of temperature and the velocity right in the 490's....

I disassembled the 2240, deburred the tube, polished up the trigger and replace the spring with a lighter one and fitted a piece of brass tubing for a spring guide.... I put the gun back together with the .177 barrel and breech, loaded up a CO2 cartridge, and fired a couple of shots, confirming that the velocity was too high.... I tried some smaller transfer ports, and even one that was only 3/32" ID (0.094") wasn't restricted enough.... The 0.070" port worked just fine, however, producing velocities in the 460ish fps range at 70*F.... I loaded a fresh cartridge and shot a full string, firing one shot every 30 seconds.... By the time I had fired about 20 shots the velocity settled in at about 430 fps due to the cooling effect of shooting, and then it plateaued, falling to about 420 fps by shot 60 and then taking a dive over the next 15 shots.... Anyway, at least I have a baseline for further work....

Bob

I've got this thing pretty much sorted now.... I ended up with a 0.109" transfer port, a hammer trimmed to 50% of it's original weight, and a bit over 3 coils trimmed off the hammer spring.... First of all a photo....

http://i378.photobucket.com/albums/o...on-PAL1740.jpg

I know, boring.... just a 2240 with a 10" long .177 barrel, a red dot and a muzzle brake.... However, the good part is that instead of blowing all the CO2 out the barrel, I'm sipping it very frugally....

http://i378.photobucket.com/albums/o...LCO2Pistol.jpg

The red line is the results with the 0.070" transfer port and the gun otherwise stock.... Just restricting the velocity probably doubled the shot count from a stock gun.... but you can see two things.... First, the effect of cooling on the velocity, even though I only shot every 30 seconds.... Second, the short green line shows how much velocity was lost with a 20 degree drop in temperature, about 40 fps.... I got about 60 shots at an average of 430 fps, and the total energy over 71 shots was 217 FPE....

With the light hammer and the trimmed spring, the hammer strike is so weak it as marginal on piercing the CO2 cartridge.... To look after that I drilled a 1/8" hole in the end cap through which I can insert a brass rod to rest against the back of the hammer.... Pull the trigger, give the rod a whack to pierce the cartridge, and you're good to go.... I had to go back up about half way to stock on the transfer port to get the velocity I wanted.... This seemed the best compromise I could obtain with this gun.... The blue line was shot at room temperature, shooting 1 shot every 30 seconds as before.... I tried temperatures from about 50 to 80 degrees, and the velocities stay right in the same range, all that happens is the number of shots change in proportion to the temperature.... I even stuck the gun in the fridge for an hour, and when I pulled it out the velocity was still 460 fps and increased over 10 shots to 480 fps.... at which point the gun still felt quite cold.... This means I have acheived a high degree of self-regulation in the valve.... I got about 85 shots averaging 490 fps before the velocity started to drop.... and the total energy over 102 shots was 407 FPE, nearly double what I had before.... While this is a long way short of what I got with my 1750, I think it's pretty good with only a 10" barrel....

All in all, I'm pretty pleased with the results.... The gun shoots well (a heck of a lot better than I can shoot with a pistol) and the velocity is very stable over a wide temperature range.... certainly anything I will shoot in with it.... In addition, I've got my 4-reticle Red Dot sight back on something worthy of it....

Bob

Detuning is much easier than "performance" tuning. Valve volume reduction is another way of achieving the same ends...

Small bushings seem to work well on the 22-40 valve, and increase shot count- just a possible work around for that overly light hammer strike- at a hefty increase in shot count. Vagrant's your man to talk to as he has more than a bit of experience with taming down the 22-XX in .177 with the brass bushings...

Regards,

Doc Sharptail

Restricting the volume and/or the transfer port can achieve the reduction in FPE and increase the shot count over stock.... note what I achieved in the first tests with the tiny 0.070" transfer port.... That's only half of what I was trying to get, however.... You can't get the valve to be self-regulating without reducing the hammer strike.... The key to getting CO2 to be relatively insensitive to temperature is to make it operate like a PCP at the top of it's bell-curve.... When the temperature and pressure go up, the valve can't open as far, and the velocity will eventually drop.... Taken far enough, it will cause the valve to enter lock at a lower temperature, of course.... When the temperature and pressure drop, the valve actually opens more, compensating for the pressure drop with an increase in flow.... You can actually see (and hear) this occurring with this gun when you first take the gun out of the fridge.... you can see a bit of a CO2 cloud until the gun warms up.... However, the velocity loss is kept to a minimum....

Bob