I suspect the bullet diameter thing has two causes, one being caution on the part of the manufacturers, the other being that there is a bit too much "common knowledge" as to the barrel diminsions. You will see many references saying 308 or 309 but slug the barrel, there is a lot of variation. In actual fact DWM held very close tolerances and the "variation" is the difference between what everybody "knows" it is and what it actually is. The nominal groove diameter is 0.3110, max 0.3114, min 0.3098. This is from the DWM factory prints. SAAMI specs call for a minumum groove diameter of 0.3090 inch. I have measurements of 17 barrels of various makes including Ruger, Colt, Browning, Mauser, DWM. They run pretty close to the DWM specs. It's the bullets that are so often off size.
In any case, it never hurts to slug your barrel and know what you have. After you slug it send me the measurements for my data base.
unspellable
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