90% is the barrel. All the other factors make up the last 10%. Of course the shooter is always a wildcard though.
Kevin, the MK262 Mod1 ammo is regularly tested by Black Hills to make sure it's meeting their specs. They're looking for 2" at 300 yards. I'm sure there are a few M4s that will do that but not many. Of course that isn't what is expected of them either.
But that's the reason for the DMR SAM's, RECCE's, etc. Better accuracy. If you are trying to wring the last 10% out of a particular rig, the carrier can play a very small part of that. Mostly they help with things like easier cleaning and smoother functioning. Not important things in a fighting gun where reliability is king, but worth considering if precision is the main goal. Still, purely as an accuracy enhancer, the carrier is far down the list of must have upgrades.