My own life size, customized and customizable version of the MA5K Carbine from Halo: Ghosts of Onyx.
About this creation
The "B" variant of the MA5K Carbine saw limited use during the Human-Covenant War. It used elements of the M392 Designated Marksman Rifle from the Army and the BR55HB SR from the Marine Corps in its functional and aesthetic upgrade.
This is my own custom design of the MA5K Carbine from Halo: Ghosts of Onyx with extra flashlight and scope attachments. I like the look and feel of compact guns, so that's why I ended up building the MA5K. It is also inspired by the FAMAS, FN P90, Halo 3 Battle Rifle, Halo 3 Assault Rifle, and Halo: Reach DMR.
Masterful! If you've got a Halo-related costume lying around your home, this is faithful enough to go along with it to any function at which you would normally dress up like a fictional character (Halloween, birthdays, local television news interviews, etc.).