Field Engineering: You're doing it right.

Here's an article on the development, design, and deployment of the Army's "Ironman" system. Which is basically putting a belt of ammunition in a backpack and connecting it to a machine gun via a flexible linkage.

All modular, all off the shelf, and very effective and cool. Bravo.

Update: Here's some more photos and a claim that the field developed version is a copy of an existing prototype.

If true undercuts the point, though the first link seems to point more to parallel development, as it has been a commonly seen idea.