Fiddling around with a P17

[jackssmirkingrevenge]

Picked up a used Beeman P17 and trying various modifications to up the power output:

1) cleaned up the valve stem, installed a harder o-ring and drilled out the transfer port to 3.8mm from 2.5mm

2) added a Delrin sleeve to the chamber to reduce volume and increase compression

3) machined an old Crosman barrel to fit at 9.75" 





Here are the results:



Incremental increases in power except for the increased compression that actually reduced performance, contrary to some results reported elsewhere.

My original intention was to convert it to 0.22" and rather embarrassingly that's what I thought I was doing with the new barrel, in my mind it was a 0.22" I had in the scrap box and I didn't bother checking

I do have a spare 2240 barrel though so will give that another go.

[Frank in Fairfield]

I know it is fun to tinker but I was given a P17 by a fellow who then bought a HW40 (P3).
I shot the HW40 earlier this week.
I like the P17 better as stock.

[jackssmirkingrevenge]

It is a good pistol out of the box but at that price one can't help but change things

Here is the 2240 barrel machined to fit between the original and the long 0.177" barrels:



Test data:



25% more energy and pretty consistent.

[Offhand]

Thanks for reporting the results of your experiments!
Did you use a lathe to machine the barrel to fit, or something else?

[jackssmirkingrevenge]

Did you use a lathe to machine the barrel to fit, or something else?

Machined on lathe, I had wanted to do a similar project just over 6 years ago but on a small manual Sherline lathe it seemed like a lot of work, now I have a "proper" 10x18 lathe with powered feed and it was much less labor intensive

[Back_Roads]

 They can be turned on a drill press also, I did some messing with P-17 about a year or so ago.

 The bottom gun is a Zoraki.

[jackssmirkingrevenge]

Impressive, what velocities were you getting with that long barrel?

[Back_Roads]

 It dropped 30 fps with the longer barrel, the medium barreled one is at 11" the long is 20". 11 seems to be the sweet length for these.


 And some pellet test.

[WobblyHand]

I made the piston thicker on my P22.  Cast some JB weld on the top of the piston to fill the gap.  Then machined to exact fit.  Cut a small channel for the air to reach the valve.  Managed 362 fps average on 14.3 gr 22 cal pellets.

Also put in felt o-ring for oiling.  Wyoman had a great set of mods for the P17, adapted most of them. 

Gee, you are giving me the itch to mod another one.  I still have a few left from the great $7.50 sale on Amazon a while back.  What I'm missing are suitable barrels.  Also missing is a decent moderator approach.  Need to clear the trigger guard and still get enough air.

[jackssmirkingrevenge]

It dropped 30 fps with the longer barrel, the medium barreled one is at 11" the long is 20".

That's what I would have expected.

I made the piston thicker on my P22.  Cast some JB weld on the top of the piston to fill the gap.  Then machined to exact fit.  Cut a small channel for the air to reach the valve.  Managed 362 fps average on 14.3 gr 22 cal pellets.

Using the "crushed aluminum foil method" I determined there was 0.8mm clearance between the piston head and the cylinder head, so I machined a 0.6mm shim to see if it would make a difference:



(I also added a second o-ring to the piston just in case)

I got an average of 325 fps with the shim fitted, for some reason my attempts to increase compression with this thing have resulted only in reduced performance

With the shim removed I did a test comparing cocking slowly and quickly and there was a difference in both velocity and consistency:

[WobblyHand]

Could be the picture, but I'd check those o-rings carefully.  If they have any defects on the surface, they can leak.  Leaks lower your compression.  Just from the picture, I'd replace the o-rings with fresh ones.

Shims didn't really work for me.  The JB-Weld trick did work.  The stuff is easy to machine, too.  Clean the piston end.  Just wrap some scotch tape around the piston end (after removing the o-rings) and build up a 3 mm layer of adhesive.  Let it cure 24-48 hours.  Remove tape.  Machine flat.  Reassemble without o-rings and check for fit, removing the minimum, while allowing the piston to fully seat without camming over.  Add an air escape channel to the valve. 

While you have the gun open, check for any burs inside!  Remove the burs, no matter how small - because they will cut your o-ring(s).  Learned that the hard way.

[Offhand]

Some great information and advice here!

 I've tinkered with my P-17 but not nearly as much as you have. Would you mind explaining what you mean by cutting a small channel so the air could reach the valve? Is this because the added piston height would otherwise block the air intake?

Thanks!

[jackssmirkingrevenge]

Would you mind explaining what you mean by cutting a small channel so the air could reach the valve? Is this because the added piston height would otherwise block the air intake?

I believe he means something like this (from here):



What I don't understand is why this would work while a similar shim at the bottom of the piston stroke wouldn't.

Could be the picture, but I'd check those o-rings carefully.  If they have any defects on the surface, they can leak.  Leaks lower your compression.  Just from the picture, I'd replace the o-rings with fresh ones.

It must be the image (there is silicone grease on the o-rings) because they're free of defects, I've also polished the inside of the cylinder.

To finish this up, I threaded the barrel to take a 14mm CCW threaded muzzle adapter.





I milled out the slot in the lower frame so the adapter would fit when open but of course any sort of accessory would have to be removed for every shot, so it's mostly there to stabilize the barrel in the frame, and it adds a certain je ne sais quoi to the look too

[WobblyHand]

Very nice work! 

As for the channel, I meant more like this.
 
That's Gary's (WyoMan's) picture, but I did something similar.

[jackssmirkingrevenge]

I'm wondering if it's the fact that I bored out the transfer port that seems to negate the effects of increasing the compression on my example.

[subscriber]

I'm wondering if it's the fact that I bored out the transfer port that seems to negate the effects of increasing the compression on my example.

Probably.  Reduces starting pressure and reduces expansion ratio on firing.