Machine Gun Scavenger and Reserves are bugged. Firstly, they no longer stack and if you double up on either of these mods they cancel each other out. For example Xenophage has a base of 28 ammo with one reserve mod it goes to 32 but with a second reserve mod it goes back to 28. Also, Machine Gun Scavenger picks up between 10 and 15 ammo if only ONE mod is equipped but if you double up it only picks up 5 or 6. This seems to be exclusive to Machine Guns as my tests on Swords and Grenade Launchers showed the Scavenger and Reserves mods still stack. Also, not exclusive to Xenophage, I tested it with Bane of Sorrow and Heir Apparent and got the same result.