Why is it such an issue? Is ammunition shortages really a critical element for the game? What's to stop them from just using other weapons (other than the pesky -10 modifier, perhaps the idea that they might be violating the "spirit" of their weapons, etc.)? Thus why not just give them unlimited ammunition until it can become a significant story element, such as through a Turning Point in one of those Horde battle thingy-ma-bobs?
A bit of judicious cutting and any weapon would be usable by a Marine in Power Armour.
Just a thought.
Kage