TE is only one difference between Service and Rack Grades. Muzzle Wear (MW) is the OTHER big difference.
On a Service Grade you are guaranteed a TE of 5 or less and a MW of 3 or less. On a Rack Grade the TE will be between 5 and 10, the MW...... who knows.
I've seen Rack Grades come out with muzzles reading over 6 on my gauge.
Trust me... no meaningful accuracy comes from a MW reading of 6 or more. Looks like a shotgun. NO lands at all in the first 1/2 inch of the bore.
Before anyone asks, counter boring an M1 is not a fruitful endeavor due to the very short distance between the muzzle and the gas port. Most shop manuals and GI info states that the M1 muzzle can be trimmed back no more than about .100" before you start to get FTF's due to low gas port pressure.
When the muzzle on an M1 has been roached by cleaning rod abuse, it's time to re-barrel.
BTW, there is NO correlation between TE and MW.
TE bears a direct relationship to rounds fired. Conventional wisdom states anywhere from 1k to 2k rounds per single number on TE gauge. USGI armory practice was to make sure any rifle being issued to troops about to "go in harms way" was at TE of 5 or less (same as CMP Service Grade). Barrels with TE over 5 but less than 10 were reserved for stateside use. When TE reached 10 the rifle was downehecked and sent back for a new tube.
MW is caused by cleaning rod abuse, not shooting.
A barrel can show a TE reading of 10 (ready for replacement according to GI practice) and have a perfectly crisp, new looking muzzle if it was properly cleaned. On the other hand, a relatively new barrel with a TE of 2 can have a completely hosed muzzle if the party to whom the rifle was issued took no care with the GI cleaning rod.
This relationship (or lack thereof) is born out by simply observing the rifles that come out of the CMP......
Just my thoughts....
Best regards,
Swampy
Garands forever