If you have any of the halo games you could, ya know, watch. Or you could go onto youtube and watch. Or you could look at the halopedia and look up the weapons and they usually give a good description.
actually its more like the magazine for a Belgian P90. only the P90 has the clip on the top inline with the firearms action. the bullets are stored perpendicular to the gun, so each round makes a 90 degree turn before it enters the chamber = more rounds per magazine. the mag is clear on the P90 as well so its very easy to tell how many rounds you have left. the front of the mag is locked into a type of notch an the rear of the mag snaps into place.
so when you eject the mag on the smg its more like it swings out rather than pulling straight back. as if its hinged in the front, and then just falls off. you dont even have to touch the mag to eject it, its button operated i beleive.