As far as mods, I think removing the piston/push rod/linkage and removing the blowback barrel tip vent holes would make it much quieter with a moderator. From below thread they said semi auto functioned without the piston.
I get 2 shots back to back. Fast enough you can't register them apart, can only tell by mag count. After the 2nd round it does not go to battery with the third. It requires it to be re-cocked for the next round. It's not doubling up in the barrel. Not too sure what's going on with it.
So you manually cock it, and engage the sear.
-Fire it, it cycles the bolt back but not back enough to engage the sear,
-It slamfires the next round at a lesser power/fps because the hammer spring never fully compressed last time
-Third shot power is even lower, etc.. until it stops going full auto.
Trigger or maybe this issue someone reported?:
The hatsan velox , Air guns, Airgun Forum
www.gatewaytoairguns.org
>I finally realized that the piston was compressing air as it went back and that the air had nowhere to vent and that's why the bolt probe would not go back all the way. The piston was stopping on a cushion of air