Concepts made by Mars and Barrett for the US Army Precision Grenadier System (PGS) program. Or as some say, its preparation for creating real life Warhammer Bolter...
There is only few images of that prototype are available, so not much details are there on how it work(that prototype didnt even had mag release so I added it).

Model is rigged, but there are also files with all parts separted.
It have 3 PBR Materials in 4K. Black and FDE colors are included.
Verts: 13.5K
Tris: 26.8K

NOTE: Model is not made to be 3D printed. Do not ask for conversion or scale for it.

UPDATE(file: barrett ssrs UPDATE 1 zip):
-Added... Charging handle. As previous images of that prototype didnt had that visible and I guess I was to blind to even notice that gun need that part -_-