I plan on ordering the mystic (750) with a booster threaded 1/2x28 (100) for when its on a pistol and a fixed barrel mount threaded 1/2x28 (87.50) for shooting it on 22 rifles and whatever other rifles i can make work with it. So thats 2 mounts that cover pretty much anything, assuming everything you want to put it on in each configuration is threaded the same. Say you wanted to put it on a glock 17 that was threaded metric, and a M&P 9mm threaded 1/2x28, then you'd need the one booster, but a piston for each thread. Kinda confusing, but pretty much you need a mount for each different thread you plan on using.