Let me throw another measurement into the mix.
The barrel length is measured from the bolt face to the end of the barrel (with no device attached). On a non-blow back operated AR (i.e. with a rotating bolt and locking lugs) the bolt face is about .3" behind the face of the upper receiver when the bolt is in battery.
In other words, if your barrel was 10.5" long and hand guard was an honest 10" long and was flush with the face of the receiver, there would be only .2" of barrel extending past the end of the hand guard.
It gets even better in a blow back operated ARs as they don't have locking lugs and don't have a barrel extension. Thus the bolt face is slightly aft of where the end of the bolt carrier is in a lugged bolt AR.
In this case, the barrel on this 10.5" RRA 9mm is actually 11.125" long. The hand guard butts up against the upper receiver and it is an honest 10" in length. However, because the bolt face is much farther behind the face of the receiver in a blow back operated AR, there is only a half inch of barrel extending past the hand guard.
It's notable that RRA calls this a 10 1/2" barrel as the equivalent length in front of the receiver is the same as a 10 1/2" .223 barrel, rather than the 11 1/8" barrel that it really is.
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