Crosman Pell Master 700 .22

[jackssmirkingrevenge]

The latest addition to the airgun menagerie that looks like it could be a good candidate for a balanced valve and HPA conversion

[jackssmirkingrevenge]

Balanced valve draft design

[avator]

I have one in stock form that still works great.. had to replace an oring in the piercing cap.

[Goose]

Wow.  That's as fine an example as I've ever seen! 

How does it shoot?  Does it group well?

Cheers,

J~

[jackssmirkingrevenge]

Wow.  That's as fine an example as I've ever seen!

 

It is indeed in pretty good nick!

How does it shoot?  Does it group well?

Unfortunately it was leaking when I acquired it so I haven't tested it in stock condition, however I have completed the HPA conversion with the piercing assembly replaced with a chamber extension and fill nipple, and I have aslo made the balanced valve in the meantime:





Here is an updated diagram to reflect how the valve was actually made approximately to scale:



The base of the valve is retained by the original circlip and the valve spring is also the standard one.  There is a groove in the pin in order to equalize the pressure in the balanced portion so it will reclose on firing.  The sealing face is UHMWPE in order to facilitate sealing as there is less pressure keeping the valve shut, at 800 psi there is around 40 lbs pressing on it but with a 5mm stem that drops to around 15 lbs, less than half.

Testing at "CO2 on a hot day" air pressure of 1200 psi I got the following results with 14.3 grain Crosman pellets:

1 - 730 fps - 16.9 ft lbs
2 - 699 fps - 15.5 ft lbs
3 - 654 fps - 13.5 ft lbs
4 - 605 fps - 11.6 ft lbs
5 - 537 fps - 9.2 ft lbs
6 - 485 fps - 7.5 ft lbs
7 - 424 fps - 5.7 ft lbs 

That's quite a boost over the approximately 6 ft lbs these seem to generate out of the box with CO2 but of course nowhere near the 40-50 shots and it's not a usable power curve, a less powerful hammer spring should help flatten it out.

There are also no doubt considerable losses due to unsealed parts in this design.  The hammer that also acts as a transfer port is metal on metal in both the valve body and the breech, and the tap loading breech itself is also unsealed, so I may go through the bother of addressing these issues too.

 

[OTmachine]

Looks Good!  Thanks for sharing!  I will probably have to start on balanced valves myself for some big bores in the making.

[JPSAXNC]

That's a beauty, I made a custom hammer for mine and put an o-ring in the tap and one in the front of the valve to seal the hammer.

[19Sheridan57]

   I remembered someone else who was modifying these rifles & did a search. This is what I found ;

https://www.gatewaytoairguns.org/GTA/index.php?topic=199277.0

[jackssmirkingrevenge]

I remembered someone else who was modifying these rifles & did a search. This is what I found

I believe that is the rifle referred to by JPSAXNC above, interesting work!

I've carried out a variety of experiments with this in the meantime by putting spacers to limit hammer impact, for example with a 2mm thick spacer I got the following velocity results in feet per second with 14.3 grain pellets starting at 1200 psi:

457, 380, 419, 368, 393, 412, 372, 353, 338, 328

Ultimately I decided less shots with more power was preferable and I made a new valve with the stem protruding around half the original from the seal face.  Here is the valve setup animated approximately to scale to show the firing cycle:

   

Video showing the general setup and some testing, five good shots before it needs pumping and "minute of soup tin" accuracy makes it a fun little plinker with a bit more "oomph" than the original:



Shot 1 - 762 fps - 18.4 ft lbs
Shot 2 - 733 fps - 17.1 ft lbs
Shot 3 - 657 fps - 13.7 ft lbs
Shot 4 - 605 fps - 11.6 ft lbs
Shot 5 - 563 fps - 10.1 ft lbs

Average muzzle energy is around 14 ft lbs which is more than double what the original could do.

[Motorhead]

The descending shot string is very typical with a balanced valve ... in that, The velocity with such valves is VERY tied to the pressure within plenum if regulated, or in HP storage if not.
In all my fiddling and making balanced valves, the primary applications have near always been regulated PCP's.  Once hammers weight, stroke and strike buffering are figured out the miens to speed up or slow down is simply altering pressure the valve is processing.

[jackssmirkingrevenge]

I think that makes sense because in a conventional valve, the chamber pressure is a much greater factor in the force keeping the valve closed, so as that pressure drops, the valve opens faster and for a longer interval, a larger burst of air compensates for this pressure drop.  In a balanced valve that closing force is a lot more constant so that self-regulation disappears.