Thread Timing
The Coup De Grâce will have a timed barrel thread. However, that does not mean that pre-fit barrels will time to within a degree or two. We cut a 1.0625-20 UN 2B female thread into the receivers. Accordingly we also specify a 1.0625-20 UN 2A male thread for the barrel. For those of you that are unfamiliar with thread specifications, the 2A and 2B denote the tolerance class of the thread, A for male and B for the female. Class 1 has the loosest tolerance, class 3 has the tightest, and class 2, you guessed it, is somewhere in between and is the most commonly used class in industrial and commercial applications.
The tolerance applies to the pitch diameter of each thread, which is the average of the major and minor diameters of the thread form when drawing with sharp roots and crests. Actual threads have truncated roots and crests for durability. For the specified barrel thread, the pitch diameters male and female threads can be as much as 0.0105 inches apart above the nominal clearance between the two. And since these are v-threads with a 60° included angle, the axial movement necessary to move the threads through the tolerance and into contact with one another is just the 0.0105" max tolerance x tangent of 30° = 0.006 inches. Given that there are 20 threads per inch, the barrel will move axially by 0.050" for every revolution, we can see now that an axial move of 0.006" requires .006/.050 = .12 revolutions = about 43 degrees. I'll bet that's more than most of you thought it would be. I know it surprised me the first time I looked into this.
So yes, we time the threads. But the thread mill used to cut the thread is constantly wearing out and therefore the pitch diameter of the thread within the receiver will be somewhere between the high and low limits of the tolerance which introduces considerable variability in the final position of the barrel's orientation with respect to the receiver. We try to hold the pitch diameter closer to the high limit simply because we want barrels to screw on every time, but we use most of the tolerance. Likewise, I think the barrel should be held closer to the low limit for the same reason. Clearance is good, especially in threads. There is no accuracy advantage to running tight threads. The 60° thread angle of both loose threads and tight threads both centralize the male within the female extremely well, and so long as the barrel is properly torqued, at least 100 lb-ft, the barrel will not move with respect to the receiver. I know, lots of people running lower torques, but I don't like do so.
I hope that helps.
Ted