I would definitely add the M110 Marksmen Rifle
For it being my favorite rifle.
I would also have a wide variety of scopes like 8x 16x and maybe 4x and the compatibility with the Acog,M68,Reflex, and the rv1 with it's own back-up sight like the m4a1 has (BUIS)
I would make it be chambered in 7.62x51 mm (mag cap would be lowered to 20 on this variant) or 5.56 mm (mag cap would be 30)
It is compatible with the Atlas Bipod and the suppressor could be the Nato on the 5.56 variant and east on the 7.62 (since the nato suppressor group does not have a 7.62 weapon)
This would be the only group of things i would add.