Where are the other machine gun weight mods?
There is only one weapon mod that lowers the weight of weapons, it's called "Ultralight Materials" and it's only available for SMG's.
It has 5 ranks, each decreasing the weight by an extra 10%, starting at 50%.
Rank 1: 50%
Rank 2: 60%
Rank 3: 70%
Rank 4: 80%
Rank 5: 90%