Coupla things here, Mark.
The U.S. Army used a specific Ten(10) point scale correlating Throat Diameter, Erosion, and Service Life:
From Scott Duff:
A Comment on Throat Erosion from Scott Duff
It has come to my attention from discussions with many M1 Garand owners that a misunderstanding of an acceptable versus an unacceptable Throat Erosion (TE) reading exists. A note of clarification is in order. When discussing serviceability specifications of U.S. Martial Arms, it is best to review the primary source, the U.S. Army Ordnance Corps documents from when the M1 rifle was the issue rifle to front-line troops. I have in my files TB ORD 587 dated 20 December 1954 entitled Field Inspection and Serviceability Standards for Small Arms Material. A chart on page four outlines the serviceability specifications for the M1, M1C, and M1D rifles “To Accompany Troops Overseas.” The specification given for “Breech Bore Rejection” is “Over 0.306” or a TE reading of 6 on a gage. No specification is given for muzzle wear. If combat troops in the 1950s were issued rifles with a TE of 6 or under, then the same is good enough for a rifle in my collection. I would not go to the John C. Garand Match at Camp Perry with a rifle with a TE of 6, but for general collecting, in my opinion, it matters little.
This is also a good time to note that based upon my observations the TE of a new production SA barrel during World War II ran anywhere from approximately 1.0 to 3.5. Pre World War II SA barrels were usually in the 0.0 to 1.0 range. Winchester barrels were generally in the -1.0 to 2.0 range, but I have seen new WRA barrels with higher and lower readings. An expanded version of this including information on post WII barrels will be published in a future issue of the Garand Collectors Association Journal (GCA).
Given: Bore Dia. = 30 Caliber = 0.300"
1. Throat Erosion - generally occurs at a rate of 0.001" (or one gauge number) for every
2,000 rounds fired.
2. Muzzle Erosion - generally occurs at 1/3 the rate of Throat Erosion.
(GCA empirical test data)
Given: A new, post-war barrel with average gauge readings of ME=0.0/TE=1.0
Given: Maximum barrel service life is TE=10.0 (0.310")
The Bbl. with a ME=1.0/TE=4.0 should have:
~ 6,000 rounds through it. [(TE=4.0-1.0=3.0) X 2,000].
and a Muzzle Erosion of (3.0)/3 = 1.0, or simply ME=1
~ 12,000 rounds of service life left. (10.0-4.0) X 2000.
although accuracy may start falling off in ~ 8,000 rounds (TE=4.0+4.0=8.0 or 0.308")
Red