Most bolts up to 3/4" begin to snap at between 30-50 Foot Pounds of torque.
If the hex head of the bolt is what is meant, maybe. Bolts are not sized by whatever head is on them, it's the actual diameter of the threads (and we don't need root diameter vs shank or all that.)
A 3/4 bolt is huge, and can take torque values in multiple hundreds of foot pounds. The problem is how torque is measured - it's really a value of resistance to turning. All lube, it suddenly changes, use a different style of thread, say, rollled vs. cut, more changes.
The high end of the torque value for the M16 at the barrel nut is given at about 85 foot pounds, using the tool and a torque wrench measure it, and it's assembled in a straight line, not clocked at a different angle. When using extensions on torque wrenches, you either go thru more math to get a real value - or do like the Army does and use the one they already calculated.
It's not rocket science, the directions are that - directives, do it this way. But that is the nature of the organization and it's infrastructure. For someone who's been involved in tech and mechanics for over 40 years, the underlying issues are what are important. You are twisting a steel nut down on an aluminum threaded pipe end of the receiver capturing the barrel extension.
It's not rocket science, same as torquing lug nuts on a car. Go no more than the value given - or live with the results. You torque it three times to work burrs off the threads of both, taking out the slack, and then set it to let the gas tube pass. 80 foot pounds give a lot of lee way.
The factory has the advantage that if something isn't working out, they can change things around and make it work out, they have a lot to choose from. They do that in advance by measuring and sorting all the parts and then putting the most compatible combinations together. We don't - neither does the armorer in the field. That has been taken into account.
One thing mentioned can and does give more room to get the fit right - squaring the nose on the upper. If things are going together too tightly, back off, hold the nose down against some high number grit wet and dry sandpaper, and keeping it absolutely vertical, give it some swirls. Knocking of a few thousandths of anodizing can make a huge difference. Go slow and check frequently, if you go too far, you are back to square one.
Sending parts back is really unnecessary. Understanding the mechanical principles will get you better results than slavishly following rules meant to be explained in a long course over weeks. We're just looking over their shoulders and reading somebody else's mail, so to speak.
