well, yup, you bought a pro mag, theres your problem.
ive tried that before too, never again. not worth it. (i dont have a gsg5, but ive tried them for other firearms.)
also, fyi, before you go screwing with that fake suppressor, there IS a recall of one of them, and from what i hear they are close to recalling others too. the atf has declared them illegal suppressors.