In Marathon's timeline there were long running political struggles and ultimately wars on Mars between the United Earth Government and a political party called MIDA. MIDA briefly seized control of Mars before losing to the UEG's vastly superior military and space forces. Following their defeat MIDA then became a well funded terrorist organization operating in the Sol system. If you're curious, you can read the full history of MIDA over at MBO. (I linked as close as I could... Just search for MIDA.) Of course, there's no evidence that the gun and the terrorist group have anything to do with each other, but it is a fun sly reference to a game that first came out some twenty year ago. Too bad the meaning of MIDA was never revealed. :(

Amusingly, this little piece of Marathon history was from a computer terminal in a level section called The Rose which links it back to Destiny via the always important Alpha Lupi mini-ARG whose planet descriptions showed back up in Destiny itself as Grimoire cards of The Traveler dreaming.

Fun stuff! :)