Thank you!!!! Now we are getting somewhere.
My Norc M4 would have the carbine gas system, right?
I noticed that when I shot it the bolt seemed to be quite violent, as if it is over-gassed. i don't like the idea of beating up a rifle, especially when I am only shooting holes in paper 25 yards away (CQB).
For this reason I load my CQB ammo with fast powder (680) to drop the port pressure. Muzzle velocity is down to about 1500 fps. It is like shooting a BB gun, but it still cycles 100% I am guessing the pressure port is bigger than it should be.
So if I have the barrel cut from 12.5" to 10.5", the theory is that the reduced dwell time will reduce bolt energy. OK, because it looks like that would actually be a step in the right direction, since the bolt is now too violent with full power ammo.