measuring odd number rifling diameters

[mdeland]

Wayne , I heard a simple method of measuring odd count rifling diameters today that I never thought off. With a caliper measure either land or groove from out side the muzzle (thickness) on opposite sides then add them together and subtract from muzzle OD.
Kind of a slap your forehead moment for me!

[TexasMac]

Mike,

Thanks. It’s also a slap your forehead moment for me. I had completely forgotten about that simple technique in our previous discussions and when I wrote up the article on the subject. I finally found my old notes and will update my website article with the details. Rather than calipers I used a tube micrometer (more accurate) to measure the land and groove thicknesses at the muzzle of my trapdoor carbine. The results were right on. BTW, it’s not necessary to measure opposite land and grooves but any land and groove thickness assuming, of course, that all the grooves are the same and all the lands are the same.

Wayne

[mdeland]

Wayne, the reason for measuring the opposite side as best you can is because no barrel has a perfectly centered bore with the OD. This is primarily for round barrels but I think It might also work on an octagon if either the land or groove lines up with a flat corner.

[TexasMac]

Wayne, the reason for measuring the opposite side as best you can is because no barrel has a perfectly centered bore with the OD. This is primarily for round barrels but I think It might also work on an octagon if either the land or groove lines up with a flat corner.

Good point Mike. But I'm not convinced that measuring opposite lands and grooves will compensate for a non-centered bore unless one made sure to measure at the thinnest point on one side, than the opposite side which should be the thickest point. Thinking about it some more it may be better to measure all the land thicknesses and average the results, and do the same for the groove thicknesses. BTW, it seems my Trapdoor barrel has a very uniform wall thickness at the muzzle. All the land and grooves were within 0.001" and I measured them all.

Concerning octagon barrels, my 1st thought is it would be just about impossible to use this technique with any degree of accuracy. I believe it's impossible for a 3, 5 or 7 groove bore to align as would be required with the flat sides of an octagon barrel.

BTW, I'm glad you brought non-centered bores & non-round barrels into the discussion.

Wayne

[mdeland]

Yes they will often be close at the muzzle and breech because that is were the dead center was when the muzzle OD was turned . The barrel hole though will not follow this concentricity over it's full length . It looks to me like they usually follow a slow spiral that worms out of center as the bore gets deeper in the barrel hole. I'm told you can often see this when mounting a barrel through the spindle of a lathe and then turn it on at a low RPM and look down the bore after both ends have been dial indicated on center.