I was reviewing various AR15 videos, and saw that many of the uppers were being built with shims going over the chamber-end of the barrel and then being slide into the receiver. This would be used to slightly extend the barrel out of the receiver, affecting the alignment of the notches in the barrel nut for the gas tube.
Another installation video showed shims going over the muzzle end of the barrel and staying between the receiver and the barrel nut. The purpose was the same: slightly set back the barrel nut so that the gas tube could be aligned correctly. However, the barrel is fully in the upper receiver.
So,
1. How does the first method not affect head space?
2. Is one method better than the other?
3. Is this at all dependent on the rest of the system being used, e.g. M-Lok vs GI hand guards?
Another installation video showed shims going over the muzzle end of the barrel and staying between the receiver and the barrel nut. The purpose was the same: slightly set back the barrel nut so that the gas tube could be aligned correctly. However, the barrel is fully in the upper receiver.
So,
1. How does the first method not affect head space?
2. Is one method better than the other?
3. Is this at all dependent on the rest of the system being used, e.g. M-Lok vs GI hand guards?