www.pyramidair.com/blog/2008/04/shimming-diana-breech-seal.html
I am footnoting a link here to the blog of the famed B.B.Pelletier who in this instance had a guest writer for the purpose of discussing shimming out the breech seal of the Diana Model 60, in order to increase modest pellet velocity. The salient point, of course, that will occur to everyone, is that this applies to other airguns as well.
The article speaks for itself and is obviously the chronicling of a very capable machinist, which I am not, but I know someone who is and that is all that matters.
The only caveat I would add is that more velocity is not necessarily better; there is a limit to which increasing the speed of the pellet leaving the barrel improves accuracy. This limit is, of course, the speed of sound, about 1100 feet per second. When a projectile crosses, or even gets close to this barrier, turbulence starts to play a big part in its path and the shooter hears a loud crack or snap as well. Up to this point, more speed means a flatter parabola in the trajectory of the pellet and less time for intervening disturbances to affect accuracy. Now we have arrived at what makes airgun shooting fun and interesting and for that matter, all types of shooting.
Anyhow, the Diana 60 apparently is a little on the low end for muzzle velocity and these experimenters are always trying to tune things closer to the sound barrier and crank things up a bit. Wonderful fun. When I get mine repaired and on the firing line again, it has a pretty fat, new breech seal in it and I am thinking that will do things well. My problem is, on the other hand, the piston seals, plural because this is a recoilless rifle.
I tried shimming the breech seal, not using the highly technical methods employed here, but by cutting shim material from thin plastic. It was a cumbersome job and ultimately very frustrating, particularly since I was using O-rings purchased from Ace Hardware. I then ran across a source on the internet, Umarex, whereby I placed a dollar and a SASE in an envelope and they returned me two O-rings of the exact size needed for the Diana. Unfortunately, although the ring fit very well, the exercise only peeled away the first layer of the onion...the piston seals are dry-rotted and need to be replaced, as mentioned in my original post. That now becomes my next issue with this rifle.
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