Does M1917 index same on throat wear gauge? Getting 11+ TE

[Col. Colt]

Here is a copy of my response to the same question on another forum - I hope it is helpful here.

Well, you guys got me to wondering - so I did some checks. I am an LE Style Armorer on modern police weapons, and help out occasionally at a local gunsmithy. We were able to assemble the following tools and the following guns for checking.

Two C-3940 GI throat erosion gauges (one Canadian made/Broadarrow marked!). Both in new/like new condition. (I got the canadian one in cosmoline wrap, and and the US GI one has no signs of wear.)
Checks done as follows:

M1917 Eddystone, early arsenal (post WWI) rebuild with Winchester 3-19 barrel. MW = 2, Throat Erosion 2.5, sighting across reciever ring edge. This came to us with at least six layers of fouling in the bore (powder/copper/powder/etc.) so it was definitely used after rebuild, and more than a few rounds. If it started it's new life as a "1.0" throat, that might mean at least 1500 rounds fired since overhaul, or a little more assuming you take the gauge as accurate in both guns. If we assume the gauge is dimensionally incorrect for an M1917 "Enfield" by about one graduation, then perhaps five hundred rounds with really Lousy Cleaning!! The weapon's overall finish condition seems to indicate the smaller number of rounds. (Perhaps during civilian ownership!)

M1903 Rock Island, rebarreled with SA 10-42 barrel, (possible Remington 1945 rebuild contract w/scant stock) obviously used after rebuild. MW=1.5, Throat Erosion 1.0 plus a hair. Note that this is tighter than the post 42 "Official" new measurement of around 2.5 from the factory. Perhaps Remington (or whoever installed the SA barrel) had returned to the original 1.0 Springfield throat measurement during barrel install.

M1903A3 Remington (Sporterized, with Redfield Jr. Base, being reconverted by owner into 03A4), probable rebarrel, dated 8-44 MW=1, Throat Erosion =1. Once again, we are lead to believe that wartime .30 caliber barrels should all measure 2.0 and above due to an Ordnance Change Order. But this is probably not the original barrel, due to it's date. Whoever installed it was, again, using the older 0-1 specification for the throat. (Is that how a standard .30-06 Reamer cuts the throat? That would explain a lot, if only the WWII installed barrels were throat reamed to 2.5 as new production.)

M1903 SA 10-44 Replacement Barrel Assembly, new, never installed, with front and rear sight bases. MW=1.5, plus a hair, Throat Erosion is a -5. (That's right, Negative Five, short chambered, comes up to 5 on the Machine Gun scale on both throat erosion gauges.)

From this, (only a small sample of guns, I admit - all we could pull together at the time) I would say that the C-3940 "M1903A1/.30 Browning Machine Gun Breech Bore Gage is "close enough for government work" when used in the M1917 "American Enfield". At most, I doubt if there is more than one graduation difference between using it in the M1903 and the M1917. And by the time that last, single graduation matters, the barrel is junk, either way. That is the same reasoning as to why they did not remake the C-3940 Gage when the new throat erosion/breech bore standard became 2.5 (to avoid pulled bullets due to inconsistent mass produced ammo dimensions) during 1942. They even explained it in an Ordnance publication, stating that the "shot out" measurement remains the same, so the intermediate measurement is not that significant.

Hope this is helpful - it is just one take, with a limited sample size. I would be happy to hear of any supporting or dissenting views, with evidence either way. CC