While suppressors are nice, there should also be muzzle devices ranging from flash hiders such as the Smith Industries Vortex which almost completely removes the flash, to muzzle brakes and compensator which help direct the gas away from you and or reduce recoil.
Because some guns allow for easy muzzle device changing, such as the AK-47, and some do not, such as the AR-15 where the muzzle device might need a wrench to break free of the tight threads, I suggest that for guns which you cannot easily replace the muzzle device, but theoretically you could replace the barrel, you should be able to have various barrels with muzzle devices already attached.
This would work directly with my other feature mentioned, where you would be able to replace barrels on the respective weapons.