The reason/purpose for/of the M14/M1A N.M. flash suppressor mod is to give greater clearance to the bullet in the flash suppressor. It was well known that bullets did indeed hit the flash suppressor on many original contract M14's. In fact, it's a rather common problem. (The original give-away was the jacket-metal smears on the inside of the suppressor! --editor) The original flash suppressor bore diameter was kept at an absolute minimum in order to achieve the maximum compensator effect of the suppressor.
You see, the M14 was a machine gun & it behaved very badly as a machine gun (uncontrollable), so the suppressor was designed (badly) to help reduce muzzle climb (that's why the solid "web" area of the suppressor favors a reduction in muzzle climb). Later, a stabilizer assembly was designed to try to further control the muzzle climb, as were pistol grip stocks, bipods, flip up butt plates, forward grips, etc. None of it worked. That's why the world went to 22 caliber 'sheenguns.
The clearance in a standard suppressor is very slight, given all the variables (squareness of barrel face, squareness of the suppressor seat, straightness of suppressor, etc. There is a precise gauge (appropriately called the Flash Suppressor Alignment Gauge) that we use on every M14/M1A we tech inspect. You would be very surprised how many fail this gauge.
You will hear that the NM suppressor keeps rain drops from slamming the bullet into the suppressor, expanding gases, etc. This all my be true, but as "Big John" points out, the NM suppressor does work. In fact, all M14/M1A's should have NM suppressors, unless they are collectibles.