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pumpmaster 760 velocity increase?

July 21 2005 at 11:30 PM
  (Login HermesTrismegistus)
from IP address

Several years ago, a buddy of mine demonstrated a 760 pumpmaster he had modified to substantially increase the velocity of the pellet leaving the barrell.

He was a real nerdy type of guy, and while sitting around in his bedroom smoking some...anyway, in Ottawa, he showed me his pellet rifle.

I wasn't too impressed, having been given the exact same model at the tender age of 7 by my dad at christmas, but he swore up and down that he had modified it to shoot almost as powerfully as a real gun.

I didn't believe him, but as it was his...um...tobacco we were smoking, I humored him.

So there I was, feeling pretty...um..."happy"...sitting on his bed with the tip of the barrel about four inches from my girlfriends left ear, with him sitting on the floor looking at his ...um....parsley plant growing under a floodlight in his bedroom.

I was absentmindedly fingering the trigger and saftey, when POW!!! and it went off.

My girlfriend stormed out almost in tears, and wouldn't have sex with me until about four days later when the ringing in her ear subsided and would'nt shut up about it for at least two weeks.

I didn't much care, but when the nerd and I went to investigate, we found that the pellet had gone through his bedroom wall, down the hallway, through the kitchen wall into the side of a cupboard, through the thin wooden cupboard door, and ended up putting a dent roughly the size and shape of a dogs nipple in the steel fume hood over his stove.

I beleived his claim of modifying the pellet gun after that boy!

The problem is that I only vaguely recollect how to do it.
I guess all that um, "parsley" fogged up the old bean a little.

He said something about re-inforcing the waste valve. Something about putting the right size metal washer behind a rubber gasket.

Something about this rubber gasket limiting the pressure that the resevoir would hold and leaking(by design) after a certain limit was reached.

So that the number of pumps reached a point where the pressure wouldn't go any higher.

But he said that adding the metal washer increased the rigidity of the rubber gasket so that you could just keep pumping the damn thing up....and up....and up....etc.

He said that that little pellet I deafened my girlfriend with had over two hundred pumps behind it, after which time he got scared that the resevoir would burst in his hands and he quit pumping.

I've looked on the net and couldn't find anything about this procedure on the net though?

Is this secret gone forever with some idjit ten years in my past in eastern canada?

Because I sure am curious about that now that I've taken up airgunning as a hobby again at the ripe old age of 28.

(For academic and informational purposes only, of course)

Hermes Trismegistus

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(Login Skillet)
Crosman Forum Member

Well, I have altered an old-style 760 for increased velocity...

July 22 2005, 9:40 AM 

But after reading about your accident, I'm not sure I want to tell you!

The older-style 'blow-off valve' type 760 that I have was shooting about 530 fps, stock. It was also leaking pressure around the bolt when fired... the little blast of air was obvious and the stock bolt has no o-ring to seal it... I've seen this in the other, older 760 that I've fired as well.

I modded a Sheridan bolt to fit and altered the valve thus: I removed the aluminum spacer from the valve assy and replaced it with an extra valve spring (now you have TWO springs, end-to-end) this increases valve capacity.

Gun now does 570 with 10 pumps and continues to gather velocity up to 20 pumps, at which it manages 698 fps but at the expense of a harsher trigger pull.

I rarely pump it up to the max velocity, preferring to use this gun as a plinker.

Other mods include adding a chopped, rifled barrel (gun is VERY accurate now) and a little heat-shrink tubing over the trigger (acts like a shoe) to reduce felt trigger pull.

I can provide more details on the bolt mod for those who might be interested, but you will need access to a lathe, drill press and a torch for brazing. The end resilt is a bolt that seals properly.

Good luck and watch where the muzzle is pointed!


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