This is an addon that tweaks objects and weapons from the player in the game to go through friendly marines, and reducing the damage dealt to them. This helps preserve these friendly marines for a longer period of time.
Just load it after Brutal Doom. All credit goes Sergeant Mark IV.
Key note: So long as the species of the entity in question is "Marines", all hitscan weapons, not just projectile weapons, should either pass through them and not blow them up. The railgun is still something I am unable to tweak, however, and friendlies may still take damage.
This complements other ally addons if they are based upon Mark's own Marines species.