Good advice, thanks!
Also, uh, completely unrelated, but on the topic of "smack down," kindly refrain from literally giving your mags a smack down. I lost my temper with the sweet-ass smokey translucent mag my JG P90 came with, and smacked it down into place, and now my favourite mag doesn't feed at all. The spare mags I got are all solid black, and after contacting Toronto Airsoft, it seems they don't even sell the translucent mags on their own. Huh.
I'll just swap some parts around, so I'll have the internals of a solid black mag with the see-through body of the OEM mag. Make sure the mags click into place properly when swapping them. Push down on the round end; don't smack it like it owes you money and/or is an HK, you know?