Both true, stonesean and mindme--and it's not something that deeply bothered me, just something I noticed because of my familiarity with that particular gun. The plastic top-mounted mag was present on the guns they used, and beardface checks the chamber (bonus points for writers there) in the same manner is the gun's current design. Due to the weird magazine, I think this would be one of the hardest guns to possibly retrofit for different ammo.
The full-auto version carries 50 rounds, which is pretty respectable, but would still be gone in a heartbeat with the long bursts of fire they used in the show.
Like I said, really small gripe here, I just like picking at that kind of stuff.