The pouch thingies for a P90 are very similar to these. I've seen ones that are fully enclosed like this but also ones that are open on both ends and use lycra or some shit to be a lil stretchy.

Typically they're held horizontally rather than vertically. All around, just kind of a pain in the ass but there aren't really that many firearms where you can carry 500 or even 1000 rounds on your person with relative convenience.