Getting more bang out of a P17

[RCnMo]

I posted earlier on this piston mod for the P17 and after some mild port and valve work, I only gained another 5-7 fps. I've come to the conclusion that with it's current design and without totally rebuilding the pump mechanism, 460fps is the best I can squeeze out of it. The adjustable piston gave me a huge increase in performance. 87fps or a 23% increase in velocity with the same effort. I found that if I bottom the piston out, it's opening stroke is too short to allow it to suck the air (intake stroke). If I open it to the intake stroke and adjust the piston so it bottoms on the closing stroke, the valve will open and bleed some of the air out. I was really hoping to hit 475 and wishing for 500fps. I don't think that's going to happen with my pistol unless I re-design the valve for a stronger spring and increase the stroke with a compound linkage. Here's some pics if anyone wants an idea of what I did.

very nice Ralph,
i know on the 13xx if the barrel is longer the fps goes up.
if you can add a longer barrel and cover it with a compensator/muzzlebreak maybe you can get out of it what you want that way?

[RCnMo]

I thought about that too Craig. Unfortunately, the way that it is hinged won't let it open all the way with anything protruding out of the front of the gun. I was thinking of a barrel three inches longer and adding a cool looking compensator. It just won't work unless I carve out the bottom of the frame. I may give that a shot down the road though. Thanks.

to bad that won't work, three inches would probably get you over 500.
maybe without the compensator if it doesn't look to honkey?

[RCnMo]

Yeah, I just got done measuring it over and it looks like that if I add any more than about an inch of extra barrel, I would have to mill back into the 'meat' of the frame. If I knew for sure that I would get a good gain out of it, I would probably give it a try. Maybe I'll get another wild hair and try it sometime.

that is to bad, i used to own the marksman 2004.
i got rid of it because it was to slow.

[SDale]

I've been thinking about adding some extra length to the barrel on mine and looking at it, not that much frame grinding is needed. All that would be needed would be a small channel about 1/4" wide or so and about 1/4" long with a depth of about  1/8 to 3/16" . I don't think that would compromise the structural integrity at all.


On a side note, I swapped the barrel on mine for one I made from a Crosman 2250 barrel. Same length etc. But I got some crappy results. I was only shooting in the low 300's. So the original .177 barrel went back in.

Now, if I opened up the transfer port a bit , did some more work on the valve  and made the barrel longer I'd be happy with 400-450. But I think the barrel would have to be all of 18 inches for that.

[WVscott]

What pellet were you using?  And what exactly did you do to the piston?  Is the piston rod threaded so that you can screw the piston down?  It looks like you have a nut on the rear of the piston.  Anyways your work looks really nice.  What would you charge to mod one of these?  I've had both the P17 and the German P3.  I got right around 400 fps out of the p3 with cpl's.  I ended up selling both of them and bought a P1 which gets almost 500 with cpl's.  If my P17 was getting 460 I'd be fairly happy with that.  Oh, btw, did the shot report get louder?
        Regards, Scott

[RCnMo]

The piston was made from scratch out of 6061 aluminum. I made a steel adjustable rod that was threaded 7/16-20 that allowed me to get a bit more compression by being able to adjust it's compressible length. I gave the piston a viton front seal and a nitrile rear seal. I used two seals for piece of mind and to help keep some lube in the cylinder rather than pushing it forward or to the rear. The pellets that you saw on the chrony were 7.9g cphp's.I got 460 using gamo hunters, which are just a tad lighter. The only mod that really made a difference on my pistol was the piston. The only reason I did this was because I was only getting 373fps out of mine and that was below the advertised velocity. The shot report difference is barely noticeable. If you are interested in an adjustable piston, send me a pm and we can discuss it. Thanks

[normal]

Like the pix on the mill table.  Spoze you could get a little more pressure/energy with a rubber face on your piston?  Maybe there's room in the air/valve chamber for some material to displace some air.  I'm thinking a length of tubing that fits outside the valve stem.  Not so pretty might be to fill some of the spaces with grease.

[RCnMo]

I thought about making a cup much like a parachute seal for the front of the piston. The problem I ran into was the valve will open partially and leak out air when too much air is compressed in the chamber. This piston mod will create a bit more compression than the valve will hold. Without re designing the valve with a heavier spring, I'm still stuck in the same boat. This was a definite improvement to what I had to begin with though.

[normal]

Oh, yeah, like you said in the original post.  I'm new here.  Got pretty fired up seeing right away people who can do things.  Got sloppy.  Heavier valve spring and perhaps a harder O ring, then you need a stronger hammer spring and on it goes.  Maybe redesigning the valve nose has some potential.  Different type of seal.  Thinking out loud....

[normal]

Thought I'd mention that I did all the assembly on one of my P17s with CRC Synthetic Brake Caliper Grease, expensive black stuff I use on almost anything that moves.  It's a little stiff, and very black (all that graphite and molybdenum sulfide), and almost as expensive as Molykote 111,  but seems to be the ticket for happy O rings, sears, pins, and on and on.