This set of files will bring M1's Alien Weapon to M2, in place of the SMG. Just put the .shpA and .sndA files in your M2 main data folder and the .phyA one in the Physics folder. Then boot up the game and, in Preferences -> Environment, click on each field (Shapes, Sounds, Physics) and replace the standard files with the ones from this package. You're good to go now.

WARNING: Only the Major Enforcers will drop it, for now. They will still shoot with their flame cannons, no worries.