My first Ordnance Officers assignment after Viet-Nam and then OrdOff School at Quantico was with 1st Maint. Bn at Camp Pendleton.
I worked with CWO Gordon Kampen at the Small Arms Shop at Las Pulgas.
During this period we re-barreled every M16 in the 1stMArDiv to cull out the non-chromed chamber and barrels. So like we did thousands or re-barrels over a period of months.
I don't remember who in the shop started doing it, but the two pre-torques were found to help get you to the next notch if you initial 35 or so was way too short of the next notch.
We did not have some of the improved barrel nut wrenches and PeaceRiver Arms receiver blocks available today. So anything past 40 or so caused the barrel to rotate in the original aluminum vice jaws. Often, another Marine would resist the torque with a breaker bar through the front sight "A" frame, often resulting in the wrench slipping off the nut and damage occuring. So we were confident that we were putting enough torque to the assembly.
Unfortunately, "lessons learned" from shop practices like this don't end up in reprints of the TM.
Years later when I had charge of more than 2,600 M16A1's at Quantico which had seen years of hard use, lose barrels were a problem. You could actually feel that the barrel nut was lose, and how it "loosened" with a gas tube passing through it is still bit of a mystery. However, I remembered the two pre-torques from Pendleton, so we tried that to repair the rifles with the problem using that method and it worked fine.
But very often, when this rifle went to the range, it was found to have "excessive windage". We eventualy discovered a relationship to rifles with "loose" barrels, and upper receivers with alingment slots way over the .125" spec.
So I think there were two forces at work here, (1)the bullet tearing down the barrel about 6,000 times at 50,000 psi, tends to pull the barrel to the front, streching the upper aluminum receiver threads via the steel barrel nut, and (2) the right-hand twist transmitted to the barrel alignmnent pin (steel)in the upper receiver slot (alum) tends to widen this slot, causing the front sight to cant...so excessive windage.
When I had the oportunity to revise both the A2's Operator and Technical Manuals, I tried to put in as many of these lessoned learned as possible in the text.
Compare an original A1 Army Operator's Manual with the USMC's M16A2 (camo cover) and you will see lots of these from cleaning techniques to loading alternating tracers.
ColdBlue sends...