Air Gun Home :: View topic - Non-PAL 1740 Build

rsterne · Moderator

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

rsterne · Moderator

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....

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....

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

Mark5043 · Silver Status Member

Thanks for posting this Bob...great efficiency there... Thumb Up!

rsterne · Moderator

It just shows how important it is to efficiency to not have too hard a hammer hit.... Open the valve too far and you just waste air/CO2....

Bob

Mark5043 · Silver Status Member

True indeed.I've played around with several combinations,as well.I put this information in my air gun file.It will be useful in a target pistol/plinker.I'm fortunate enough to not have any restrictions,so I tend to gravitate towards power.However I'm learning how to balance that power with a reasonable degree of efficiency.One thing became apparent to me rather quickly,along the way.There is not much difference in power between setting an RVA to near coil bind,or backing off the adjustment enough to provide very close to that power,but a significantly greater shot count...which you already knew of course.I haven't even come close to utilizing,or trying out all the different things that can be done...that's what keeps it interesting imo..

rsterne · Moderator

If you want to do an interesting experiment.... set your RVA as close to coil bind (where the gun won't cock) as you can safely, shoot 5 shots, record the average, and then back it down a turn and repeat.... Graph the velocities and providing the hammer spring is strong enough you will get a curve with a plateau (or nearly so) with the RVA near maximum and hardly any drop in velocity as you back it off initially.... Then the curve will roll over and the velocity will start to drop.... Assuming you are interested in power, but not at the expense of shot count, explore the "shoulder" area of the curve in further detail (maybe 1/4 turn adjustment on the RVA, depending on the threads).... looking at how shot count relates to velocity.... This applies to CO2, regulated HPAs and even to non-regulated PCPs.... You might be surprised at how a small change in hammer spring preload can make a big difference in air consumption with hardly any difference in velocity....

Bob

Mark5043 · Silver Status Member

Thanks,that explains things quite nicely.I will surely do this when the weather gets better.It's still cold,and rainy here in Northern California.

Mark