The gun uses a stripper clip by opening the top and putting them in, the bottom I'm pretty sure is to get rid of the bullets that are in the gun by opening it
that thing you're talking about is actually just an extruding part of the integral magazine the gun has for a bit extra capacity and also to accomodate for the round it shoots, which is also why the AK-47's magazine is so curved
it works similarly to the M1 Garand except that that gun's magazine exists only inside of the gun because it was just designed that way
both guns eject normally though