Trigger: Blowgun
It wont be enough to throw your paintballs out
Semi-Auto...hmm..
Well you'll have to have a modded sprinkler valve and a air tank.
Too bad sprinklers are rated to 150 psi =/.
ORR
Hammer Valve, as stated before.
Might take some time to machine one, or improvise parts-As ant did-.
I suggest hammer
My soon to be new UAPP paintball gun!!
Alright thanks, if a Blowgun wont throw my paintballs out what do i use then? 
And hammer valve seems complicated but ill try to put that in the blueprints still dont really know how they work either BUT I WILL TRY lol.
.:EDIT:.
Hmmm how would a water hose trigger work?
And hammer valve seems complicated but ill try to put that in the blueprints still dont really know how they work either BUT I WILL TRY lol.
.:EDIT:.
Hmmm how would a water hose trigger work?