One of the interesting features of military surplus rifles are the unique bores each has. It’s entirely possible to have one Enfield with a bore measuring at .312” in the grooves while the next of the same model measures at .315”. Both are quite usable and probably capable of surprising accuracy. A handloader seeking the best accuracy needs some data regarding the rifle, and chief on the list is the actual diameter of the bore.
The accepted method of learning true bore diameter involves ‘slugging’ the barrel. Boiled down, the process involves pushing a soft lead slug through the bore with some type of rod. With any luck, and a moderate bit of skill, what’s gained is an exact sample of the rifles grooves and lands impressed on the lead, ready for easy external measurement.
I must admit the thought of pounding anything through the bore of one of my precious mil-surp rifles leaves me shaken. For some reason the violence of firing one doesn’t bother me at all, yet slowly pounding a misshapen lead slug down the bore is met with fear and trepidation.
While reading an article on loading double 0 buck in the 8x57mm as a backyard load, the author mentioned the easy procedure of simply pushing the lead ball into a case with thumb pressure. Hmm….. that didn’t seem too hard! I had a new-to-me Yugo M-48a mauser in 8x57mm, and suddenly also had a burning desire to know the actual dimensions of it’s pristine bore.
A quick trip to the work shop found some Sellior and Bellot 12 gauge 00 buckshot shells ready to be sacrificed. A slice of the crimp allowed a small palm full of soft leads balls to roll out onto the loading bench.
Using a digital micrometer to check the balls, they appear to have been formed by random pounding with large rocks. The ‘round’ slugs were actually misshapen and varied in dimension by over .010” from round. That aside, the most important measurement had the slimmest diameter still at .327”, while the stated groove on my Mauser is .323”. That left plenty of lead to give a sample while still being quite easy to push through the barrel. Sampling several left me with two mostly round balls of buckshot just right.
I intended to use two slugs to check my own technique, and be assured of making a stable measurement.
As with any firearms work, the first step is always to make the weapon safe. In this case I removed the bolt as I wished to push my ‘slug’ from the breach to the muzzle. Knowing the rifle would be muzzle down , the barrel was padded with
multiple layers of clean folded paper towels. This also served as a soft bed for the test slug to land on without being damaged. There is some force involved with pushing the lead ball through the bore. It’s best to take no chance and prepare ahead.
While steadying the rifle against my leg, the lead ball was dropped into the chamber. It came to rest in the throat of the barrel, ready to begin it’s long bore journey. Next, a heavy cleaning patch was placed against the chamber as padding for the rod and a way to center the rod in the bore.
The rod is now pushed into the chamber till the folded patch stops against the lead ball. Force is slowly applied against the ball, with perhaps a light strike on the cleaning rod handle to get it started into the bore. Once the buckshot is slid into the bore it should get much easier to push. In fact, it can usually be slid right through the bore with very little pressure at all.
Care should be taken to not force the slug into the padding at the muzzle. The lead is soft and will deform enough to give a false measurement. As long as the slug is pushing freely, it’s best to lift the rifle a few inches and allow the lead ball to fall free onto the padding.
Once the ball has suffered it’s long slide and landed on the paper towel, remove the rod and patch and check the bore. It should be clean and clear of obstruction. In the case of my Mauser the bore looked polished after slugging.
The product of our labor should be A simple round ball with a clear and reasonably precise imprint of the lands and grooves from the bore. Now we have something we can measure! The measuring tools consist of an outside micrometer for the groove diameter and a vernior caliper for the land diameter. The caliper can be used for both, but the micrometer will typically give a more accurate measurement and the groove diameter is the one regarded as necessary for accurate projectile choice.
The slug is carefully closed between the anvils of the micrometer. It’s best to use the ‘clutch’ on the mic if it has one. This will allow a more consistent reading and avoid compressing the soft lead sample. Care must be taken to measure the flattest part of the groove, not the edge of the lands (seen in reverse as indents in the lead sample). In the case of my mauser we can see the actual groove diameter is 0.3235”, or just .0005” over nominal. For all intents the rifle appears unfired.
To measure the lands (or indents in the sample slug) we must switch to the caliper. This tool has measuring jaws that narrow to near knife edge and should easily reach the depth of the indent where the micrometer cannot go. Again, care must be taken to not force the tool into the lead. A light touch is called for. There should be just enough friction of the tool on the lead to prevent the ball falling free of the jaws. No more should be needed.
The lands of the mauser measured at .313”. Again, nominal for this rifle. While the rifle appears unfired and the bore is pristine, knowing the actual dimensions will allow choosing the exact best projectile for accurate shooting. Brought together with careful loading technique this rifle will have every chance of being an outstanding and accurate rifle.
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