You mean like this?
The "bumper" on the bottom is a bumper. It gives you something extra to push on when you're seating the mag and something to hit the ground when you drop your mag.
On TM Hicapa type mags...the big bumper is just a piece of plastic...it covers part of the actual mag body. That style of mag, in real steel, is used for competition (the Hicapa 5.1 is modeled after the STI 2011 series).