The MA5D Assault Rifle is among the main weapons used in the 5th Marine Expeditionary Unit. The Assault rifle, alongside the M6G, are the only weapons non-donator players can have unless they apply for a faction that allows other weapons in use or they donate. The weapon also automatically becomes equipped when a player with flags for the rifle connects to the server. Up until October 10-11, 2015, the Halo 4 variation of the weapon was used until it was replaced by it's variation seen in Halo 5: Guardians.
The MA5D behaves similar to it's portrayal in Halo 4 (and later Halo 5: Guardians) but was recreated for use in the Source Engine and thus has some restrictions due to a different engine. Players can also lower or raise their weapon by holding the reload button for a short moment (the R key by default). Like any other Halo-themed weapon in Garry's Mod, the rounds of the assault rifle are capable of causing much damage a NPC or another player and can shoot through surfaces such as glass.
- The MA5D saw temporary replacement with the MA5C but was later returned as the standard-issue weapon for canonical reasons.
- There is a server glitch that causes the weapon to make the screen spin for some players. This glitch can be corrected by looking down at the ground.