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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
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Some of the information Steve Mathew provides is that tolerance of the rifle barrels (1903s) was .2995 to .3015. Ergo a new bore could be a 2 (or less than 1).
There is a contention that the 1917s were higher spec under the assumption they used the 303 rifling values, but I do not believe that. I do not believe the tolerance were any less strict than the 1903 (the barrels may not have been as good shooters but tolerance is only part of the capability to shoot accurately)
I have not used the other gauges but we have used the SM gauge across a big range of 1903s and I think up to 5 1917s. There is at least one 0 MW in the 1903 group, the TE has not been that. More typical is 1 or a bit better.
The best value I have seen on the 1917 so far is .75 and a 1 (MW and TE respectively). That was an Eddystone and if they were that good, Remington s and Winchester would have been no less (those were both 2s though being shot or just an mfg tolerance is unknown though I suspect tolerance)
There is no doubt in my mind that any of the gauges are going to be close and SM gauge states that clearly as his mfg is .005 (half a TE or MW number)
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I understand the very early M1917's were made with left-over P-14 parts. They had piles of raw forgings and semi-finished parts in all 3 factories. But heard that Eddystone had many .303 unfinished barrels still in house. So to hurry-up the production they used .303 barre banks but finished them into .30-'06 caliber. They were terrible shooters and gave Eddystone a bad reputation that lasted for years. None ot the rumors ever were tied to Winchester or Remington. Lots of WWI rumors/stories still floating around.
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I don't think that rumor, using stocks of British parts up first, holds water, at all. Raw English Contract excesss forgings might have worked, if not already processed too far. We did use some leftover British bayonet blades - but they were not critical.
First of all, the US Ordnance Department wanted all of the parts by all three makers to be interchangable. I think they achieved very close to that, probably in the high ninety percentage rate. (100% was a problem whenever hand fitting was needed.) The British Orders had enough differences in each factory to require selective fitting and selection of many of the parts, which was acceptable to the British. Not good enough for the US Army!
And in order for the M1917 to have Interchangeable Parts, they asked that all three makers wait for a standard set of drawings, so all parts would be built to the exact same dimensions. Eddystone and Remington did - Winchester did not wait and went ahead and produced a small quantity of M1917's that were NOT INTERCHANGABLE! (This ended up having an interesting cost for Winchester. General Pershing (WWI USAArmy) when problems with Winchesters due to lack of interchangable parts, occured requested that ONLY EDDYSTONE and REMINGTOn built rifles be sent to Europe!!)
Eddystone/Remington did not start until they had the exact specs in hand - thus I doubt if they "muddied the waters" of their quality control with old M1914/Pattern 14 parts that would not meet the specification that they had WAITED FOR!
According to General Julian Hatcher, US Army Ordnance, the Enfield type of rifling produced a tighter bore, not a more loose one.
As you said, a lot of RUMORS - from almost 100 years ago. CC
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I think CC has it nailed.
One of the reasons for the need for the Model of 1917 was that they could not produce the1903s in quantities needed. They loosened up no standards on those and I find it implausible they did with the 1917s that were going to be front line service rifles.
They finally allowed a variant of the 1903 barrel tolerance latter in in WWII when they were second line issue weapons (and I thin HS refused to not do that or change from 4 grove)
