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I slugged the barrel of my LB No.4 Mk1 last night and came up with this. The bore diameter was .3047...less than .305 on my caliper. And the groove diameter was .3167...less than .317.
THe bore look to be in real good condition BUT should I be concerned about the groove diameter?
If bore, throat and muzzle are OK, then don't worry.
0.311 bullets should work, but flat-base may group better than boat-tails.
And if 311s are disappointing, just get the 0.312 / 174gn round-nose Hornady Interlock bullets (No. 3130) - the long cylindrical section gives better guidance in loose bores.
:wave:
I am assuming that your barrel was a 5 groove. I am curious how you came up with your measurement. I purchased a 1942 Long Branch some months ago, and had a time slugging it. After some research I came up with a v-block idea (by a fellow with more knowledge than I), and said block had to be 108 degrees in order to work. I came up with a groove reading of .314. Being a lead bullet shooter, I failed with the largest bullets I had, and went to a paper patch which gave me .315. They shot quite well. 7/16th at 50 yards, and 1.5 inches at 100. I am having some fun with this rifle. I guess if we came home with one that shot them all through the same hole, it wouldn't be much fun at all...............