Bore diameter question

[Carl Gustav]

What should the diameter measured across the lands be for a US military 30 caliber rifle barrel be? It should apply to both M-1 carbines and Garands I would think as well as Springfields. I've assumed 0.300". Carl

[John Kepler]

Every rifle barrel has two bore diameters....the Major and the Minor. The Minor diameter is measured land-to-land, the Major diameter from groove-to-groove. The Minor diameter for a .30 cal is......are you ready?......wait for it.....0.300" which is why it's called a ".30 cal."! The Major diameter is......ready again?.....wait for it.....0.308", just like it says on the box of .308 Win! Yep! Sometimes there really IS truth in advertising!

Oh....you missed one! The .30 Army and the M1892 Krag-Jorgensen....it's a .30 cal too!

[Carl Gustav]

That's what I've assumed but I wanted to make sure I wasn't missing something which I do from time to time. I have a Columbian Mauser in .30-06 that measures 0.298" across the lands. I received two rack M-1 carbines from the CMP today. I don't have muzzle wear guages but I do have a set of pin gauges in 0.001" increments. A 0.301" (-0.0002") gauge won't go into the muzzles and a 0.300" guage will go all the way in. I find it hard to believe they were classified as rack grade because of muzzle wear. Maybe throat erosion? Both bores look excellent. Both these carbines exceed by far the CMP's description of rack grade. One has a brand new birch stock with a new butt plate and the metal finish is excellent. The other stock is walnut and in very good condiiton with pronounced tiger striping and is a Bavarian Forest Police variation Inland. No complaints here!! Carl

[Jim K.]

Actually, the specs have tolerances. So the bore diameter is .2995"-.3015" and groove diameter as .3075"-3095".

All barrel bores, bullets, cartridges, chambers, cartridge making dies, reloading sizing dies, are made with tolerances to allow for wear of the reamers used to make them or make the dies for them. If tolerances were not allowed, a new reamer would be required every few uses and costs of both guns and ammunition would be astronomical.

And that is why chambers are measured with the well-known "GO" and "NO-GO" headspace gauges rather than with a single Pass/Fail gauge.

Jim