what is the average throat/muzzle wear of a service grade M1 and how do those translate to inherent accuracy compared to a new bore?
Here is what I have been told. It may or may not be true, but it sounds reasonable.
Muzzles and throats are gauged on a continuous scale between 0 and 5. When you go to one of the CMP storefronts, they will loan you a gauge to check throats and muzzles.
Throat wear represents number of rounds fired. IOW, if 2000 rounds have been fired through the gun, then the throat gauge would read as a '2'. At 5000 rounds, the gauge would read as a '5' and it would be time to send the rifle back to the arsenal for refurb.
Muzzle wear is more of a statement of cleaning practices. As we all know, Mr. Cleaning Rod is not our friend, and particularly on the Garand and M14/M1A because they cannot be cleaned from the breech end. Again, I have been told that much of the muzzle wear on a Garand can be attributed to the cleaning rod abrading the muzzle. This is the major reason that most European countries freebored their muzzles up until the Cold War.
So, I have never bothered to verify this, because, frankly, it does not really matter to me. I have one Garand where the throat gauges 4.3, and it is a 2.5 inch rifle at 100 yards. That's not in the same category as my K-31, but it is better than one of my Mausers, and a lot better than the Carcano.
To answer your question, on the last 2 Service Grade Garands that I bought, one was about 1.7 throat and 2 muzzle, and the other was a 1.5 throat and a 1.7 muzzle. I was in the Anniston store and picked through the racks for these rifles. I was not buying them for the gauge values, but more for overall condition and manufacturer.
