I just thought of this and wouldn't it be fun to have the more advanced weapons' parts lying around and then you can build them up into one weapon?
Like for example: G36C receiver, G36C barrel, and G36C stock combined together makes a gun.
I would also suggest:
Being able to use MP5 magazines with the MP5SD5 (I guess you guys could do what ACE does where it converts the magazine into "suppressed" ones once loaded into the gun) since the MP5SD5 doesn't need special subsonic ammunition.
Being able to use any 12 gauge shell with any of the in-game shotguns. I find it strange that you can't use 12 gauge 1866 ammo with the Remington 870 and M1014. Maybe swap the 15 round 12 gauge into an 8 round buckshot/slug item when you load it into the 870 or M1014? (Kinda like the ACE mag swap)
It's a crude workaround but it's better than not being able to use 12 gauge with all the shotguns.