As a regular After Effects user, couple of things I noticed that you could improve on if serious on pursing your craft. Step one, gunshots while probably accurate for that size probably want something a little flashier. For a whole selection of muzzle flashes try checking out detonation films. Also, for the gunshot sound, you need a better balance of room sound to gunshot sound, I can atleast tell you were using a .wav which is good for a sound effect, just make sure to balance the levels. Also, a little smoke at the end would have been a nice touch, no cheesy smoke though. Keep it up.
Cheers,
Kensai
PS: This shouldn't have been posted in Halo Universe. General discussion mate.