1750 PCP Target Pistol

[Fastair]

Looks great .I am sure it will be a pleasure to shoot.

[sholo]

How are you cutting threads in a hole that shallow? That's always been a problem for me, even with a "bottoming" tap. I've found that I need a few "modified" taps for threading holes like that in those tight clearance situations....

I have one full length tap and bought a second tap which I cut the bottom 1/3 off (basically the tapered portion used to start threads). Use the full length one as far as it will go, then switch to the shorter one to finish. Only way I know how to get nice, well-defined threads in a shallow recess. I use the same procedure when pinning a 22xx valve, or upgrading to 10-32 on a Disco valve.

Interesting location for the bbl. band. Looking forward to seeing some groups.

It does look a little goofy, doesn't it?

Believe it or not, there was a method to my madness! When I switched to .177 a while back I added a couple more barrel set screws to the breech and free floated the barrel. My groups tightened up (plus I liked the look) so I left it. Well, when I tore it down last night I noticed a bit of flex happening at the front of breech. Breech screws were tight, but with very little pressure applied to the barrel, I noticed the front of the breech would separate from the tube (very slightly mind you, but still movement nonetheless), so this is my quick fix.... With the barrel band tight against the breech I basically have an extra long breech with three attachment points...


Todd

[sholo]

Nice job Todd. I like the clean look of it.

Al

Looks great .I am sure it will be a pleasure to shoot.

Thanks guys!

Not really much of a build, but more of an "upgrade" to an existing piece!

Truth be told, I've pretty much run out of things to do on my other guns for the time being and my next project won't happen for a while yet as I just started gathering pieces for it, so hopefully this little adventure will temporarily satisfy my tinkerin' urges!

Todd

[SeanMP]

It does look a little goofy, doesn't it?

Todd

No I dont think it's goofy at all. In fact I think your on the right track. Moving it forward creates an extra node. One node is single ended and one node is trapped between two fixed points. This creates two totally different waves cycling at the same time.

[sholo]

Well I managed to sneak away to the basement long enough to finish the first round of testing. Still have a bit of fiddling to do yet, but I'm getting close...at least I think I am.

I knew that in order to keep a light (7.56 gr) pellet under 500 fps on a 2000 psi fill would require choking the air flow off substantially, so I'm using a bone-stock Disco valve, lighter hammer (45 grams vs 60), clipped 2240 hammer spring and a TP from a Crosman Challenger which measures .050". Gun currently has a 14" barrel because that's all I have on hand at the moment in .177, but plan on eventually getting a 10.1" LW barrel from Eric.

It was a PITA to tear gun apart to get the rear cap off in order to clip hammer spring a bit at a time, so I drilled/tapped the cap 3/8-16 and made up a rudimentary RVA of sorts using a couple of 6-32 knurled nuts (the kind used to hold a lampshade on), a piece of threaded rod 3/8" long drilled/tapped 6-32, and finally a piece of 3/16 aluminum rod for the guide which was also drilled/tapped 6-32. A 1-1/4" long 6-32 S/S screw (with red loctite applied) holds it all together. It ain't pretty, but does the job (stock 2240 spring above for comparison):
m_1750 PCP Target 010.jpg
I'm using the RVA more as a "removable plug" to access the spring rather than a velocity adjuster. It can't be turned in any farther than the first knurled nut, so I've made it (or should say in the process of making it) so that when it is maxed out (turned all the way in) a 7.56 gr pellet is travelling at 500 fps or less (4.18 fpe). I'm still getting 25-30 shots in the middle of the string though that are 501-506 fps (one at 511) so it looks like I need to remove another half coil or so...

Using Bob's "within 4% of peak velocity" guideline, I'm currently getting 60 shots from a 2000 psi fill shooting all the way down 650 psi, at which point the velocity drops to below 480 fps:

Shot#

1-10: H=486.3, L=482.6, AVG=484.5

11-20: H=506.6, L=501.3, AVG=503.9

21-30: H=505.4, L=500.8, AVG=503.1

31-40: H=505.5, L=494.3, AVG=499.9

41-50: H=495.1, L=491.9, AVG=493.5

51-60: H=490.5, L=481.7, AVG=486.1

I then put up a target with 6 bull's on it (traced a dime for bull) and shot 10 shots at each bull from 10 yards to see how the POA/POI correlated with the falling pressure in tube. I now understand why Bob uses the 4% guideline - very little deviation across all 60 shots!

The following target doesn't really show the full potential of the gun as I shot from a standing position with the front of gun resting on a rolled up towel on top of a 4-1/2' step ladder - not the best configuration for accuracy testing... . Center two targets are a bit of a disaster...not the gun's fault though - I lost my "groove" and took too long between shots developing a case of the shakes, which the target painfully reflects (that "shiny thing" in the center is a dime):
m_1750 PCP Target 015.jpg

Started with a full 2000 psi fill and this is what the gauge read after shot 60 (about 650 psi ...)
m_1750 PCP Target 002.jpg

Next step, I think I will get the JBWeld out and "fill-n-drill" an extra .109" TP I have to .040" and see what that does for shot count/velocity. If that doesn't work I'll go back to the .050" TP and just clip a half coil or so to make sure it stays under the 500 fps/4.2 fpe limit which should gain me another few shots in the process...

Hope I didn't bore you all with the long-winded post! Thanks for reading,

Todd

[rsterne]

Very nice job!.... How much air volume do you think you have?.... It seems pretty efficient, a lot of shots from a small volume.... I do have a measurement on a 2250 tube with a Disco valve and fill fitting (nothing extra or drilled out) and I have 24 cc.... Adding 2.5 cc for the valve volume give a total of 1.62 CI.... If that is the case for your gun, and you dropped 1350 psi (93 bar) you used a total of 1.62 x 93= 151 CI of air.... Using 500 fps average (4.2 FPE ) for 60 shots that is 252 FPE total, which works out to 252/151 = 1.67 FPE/CI... Like I said very efficient.... Using that 0.050" Challenger transfer port was genius....

PS, you can add a tiny amount of volume by drilling the front of the Disco valve out to 3/8" ID almost down to the spring seat.... You can also drill the end where the O-rings are to 1/2" OD stopping at least 1/16" short of the shoulder where the groove for the trigger screw is.... If you want even a bit more volume, cut a couple of threads off the front of the valve inside.... Every little bit of volume you gain will mean either more shots or a narrower velocity spread over the same number of shots....

Bob

[sholo]

Thanks for the kudo's and tips on increasing the volume. I'll do just that next time it's apart!

I fully planned on measuring how much volume I ended up with, but got going on a roll and had it back together and was in the process of pumping it up before I gave it a second thought..., but your measurements should be dang close as I have the same configuration you just described.

Although 1.67 FPE/CI is a little generous (almost half the string is between 482 - 495 FPS), it is still pretty respectable. Much better than I thought it would be...


Todd

[rsterne]

Amazing what efficiency you can get with minimum dwell and a tiny transfer port.... I've hit 1.93 FPE/CI so far, and I'm sure over 2.0 is in reach....

Bob

[Doc Sharptail]

2) I could take the male Foster off the adapter, install a street "T" and then re-install Foster as well as gauge (Foster straight out, gauge pointing down). This would be the easiest way, but I would like a cleaner look (i.e - not look like a plumbing job).

Cheers,
Todd

(Insert audio clip of dog laughing through teeth HERE)

From the hydraulics dept at Princess Auto. They sometimes need to be that small for multiple hydraulic control box applications. I'm hoping they're stainless, and at least hydraulics rated- I'll be putting them up to a bit less than the hydraulic requirement.

2000 PSI? I'd never even considered that. (tells you how much I know) That would make a nice back up pistol on the trail. By the looks of it, it would almost fit into one of the Cr 13-XX holsters- not so sure the gauge would clear. The ideas I get perusing this forum are surprising to me.

Dang! As if I didn't have enough projects! BUT! That pistol is surely worth me thinking about...

Nice groups! Looks like it will "shoot-in" a bit too.

Regards,

Doc Sharptail

[sholo]

If you are looking for really good, heavy duty, high quality fittings (not Crappy Tire quality) then check these guys out. https://mail-attachment.googleuserco...wc14noDdwucgvI Sean clued me in to these guys some time ago when I was looking for an adapter for my Hill pump...it's the only place I buy my fittings now.

Nice groups! Looks like it will "shoot-in" a bit too.

Hehehe....I was at Sean's place a couple weekend's ago and he choked/lapped my 14"Crosman barrel - AWESOME job!!!

Will post an update shortly, but I can tell you it is MUCH more accurate now!

Dang! As if I didn't have enough projects!

Welcome to my world...


Cheers,
Todd