The magazines are pretty easy to store an almost absurd number of on your person in fabric loops, and they hold 50 rounds each.

They're extremely effective weapons when facing any alien threat, even Kull warriors.

you can retvrn and go back to WW2 submachine gun pouches, those magazines were also long rectangles (although the P90's are still a tad longer)

I think these pouches

Man there is like no good way unfortunately. Best I've found is in a conventional top loading chest rig but worn lower on the body so as not to have clearance problems when drawing them. Retaining them is an issue as well, they don't fit in normal dump pouches and reindexing is a two hand job.