Hey i know i am a little late to this discussion.
I had thought to do something similar what i was doing, what i was thinking is to re purpose alien weapons in order to build Terran.
So all weapon will be divided into 3 main components
Power-cell, alien alloy, and (i don't remember the other part
), maybe reaction chamber or plasma containment field or something.
In any case when a weapon gets capture from the aliens, X-COM engineers would be able to dismantle and extract these component and later use those components to build Terran version. However you will not recover as much as it was used to build it. This way engineers play a very important role.
Clearly there would need to be a balance, for example using power cells
extracted power-cell from pistol 1 from rifle 3 and heavy 5.
to build 1 pistol clip you need 3 power-cells, for a rifle clip 5 and a heavy 7 or whatever number works.
just ideas.