I know that there is a difference (I broke out the calipers) between where the FP actual is able to stick out as the bolt goes into battery. I checked 4 different M16 BCG and 9 different AR15 BCG of different types such as the 1/2 moon, hybrid, etc. All 4 M16 BCG allowed the bolt to rotate totally into battery before the FP was exposed. Only 1 of the AR15 worked the same and it was a “hybrid” that my brother had purchased at a local gun show.
Update: I did speak with an Armorer (Army) that is still AD. He measured, checked, and then read up on it. It appears that per military specs the M16 BCG are designed to never fire out of battery. There is a .001-.002 difference in the length of the tube the FP goes through. This gives just enough space to ensure the bolt fully turns and locks into battery. They do this so that wrongly timed full auto rifles will not accidentally detonate out of battery and thus leave a soldier injured or without a weapon. I still have not found any reference to this, but it does make sense. He said that he referenced the Master Armorer’s book, whatever the hell that is.
My whole point with this discussion was based around the safety of having detonation out of battery taken off the table as a possible failure when building your own AR. Let’s be realistic: lots of crap can happen when you are working on your rifle. If you knew that you wouldn’t accidentally detonate out of battery by swapping a BCG, wouldn’t you consider it during test fires? Say you swapped triggers and were first testing it? ETC….
I found my answer through other channels, but it is good information to have. I know I will be picking up a few more just for the initial firing of my rifles after a build/upgrade.