like TheRedHorseman said, bump-fire (or rapid fire) a drum and you'll see that the fun level is unmatched over all otherr mags. drums are also a good investment in that they will at the very least hold their value, so if you buy one and don't like it, you can always get your money back on it. drums also have a lower profile than the 40's (and the 30's as well) and like you said, if you shoot prone/use a bipod, it's clear to see the advantage over the 40's. 40's also tend to wobble quite a bit (i never had one that fit as snug as a 30 rounder or a drum) in the magwell and the wobble in conjunction with the higer weight and length of the 40 creates more torque on the magwell in that if the 40 is caught up on something (like the ground or a branch or you drop it etc), it has the potential to stress the magwell and this is something that always subconsciously bothered me about the 40's. Also, unlike 2 40's jungle clipped, with a drum you dont have to stop and flip them which is especially useful when outnumbered & confronted by a mob circling 360 degrees around you