Originally Posted by
NickG
True enough john, though at 21mm its on the limit at full stroke , and if you add too much spring you get into dieseling, I am struggling with a pro sport at the moment. the other thing with the sleeved down tunes is if you don't have the sleeve and the end cap sealed dead on they are very unforgiving, all the normal tests show its good( ie finger over the end and pull the piston out , and immersed in water ). I think any voids in the loctite cause lost volume ruining any chance of making power.
I have my own 21mm that I cant get bellow 11.5fpe, and the prosport I cant get above 10.5 fpe . both have the same spec pistons, tubes , spring &pre load . I have been on the prosport for months , even had a tube made up with the end welded , its getting boring now
Agree on sealing, I machine a taper to the end of the sleave, so that the outer edge is about 0.3mm further back than the inner edge.. the void is filled with loctite / bond and it ensures that the inside bore is really tight flush to the end of the comp tube. On a TX, where the endcap is remoaveable, I machine a recess around the outside of the face face such that the unmachined inner of the end cap protrudes slightly into the sleave (around 1mm), which makes for a really good seal.
As for your 21, just stick with 22 or 23, you have so much more scope for tuning... quite why it's short on power, no idea besides what you have suggested, but that won't stop me guessing
- surface finish of the bore holding lube or not (which provides a little power through combustion)
- pellet seat / fit in the breech affecting release pressure
- leaks around the tp and the end plug
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.