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Slugging a bore, how I did mine.

Posted: Sun Jul 30, 2017 11:04 am
by Zeliard
Originally posted by carteach0.

Note: Images are missing, but the text describes the process well, I think.

How I slugged the bores on my rifles. No fuss, no muss, fairly easy.

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.

I used a cleaning rod for this test, as the OO slug was not an extremely tight fit in the 8x57 Mauser. For a .30 caliber
the slug might have to be started with a wooden dowel and a mallet. For my .30 caliber rifles I sized the buckshot to .314" before slugging the bore.

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.

Images missing

As the target shows, this rifle can shoot. The bore slug does not lie! Next will come
loading ammunition to explore the rifles ability to shoot gas checked cast bullets.
Knowing the true bore diameter will allow choosing the correct bullet and sizing for
best results.

If I can do this, anybody should be able to!

Re: Slugging a bore, how I did mine.

Posted: Sun Nov 18, 2018 9:47 am
by 72 usmc

Re: Slugging a bore, how I did mine.

Posted: Mon Dec 10, 2018 7:42 am
by slowbob2
A lot of bad info on how to slug a bore . It would explain why I have found about 90% of the people I have dealt with in over 30 years of working on military rifles can not slug a bore accurately . The main problems they have are : fouled bore , measuring tool not zeroed or do not know how to use the tool , used a round ball or fishing weight , did not lube the bore . Also the land dia is just as important as the groove for accurate shooting . They do not understand the total bore area to bullet type and diameter relationship , that is why Hornady's .268 Carcano bullet does not work .

Re: Slugging a bore, how I did mine.

Posted: Mon Dec 10, 2018 8:33 am
by Cattus Borealis

What is your procedure for slugging a bore?

Re: Slugging a bore, how I did mine.

Posted: Mon Dec 10, 2018 12:23 pm
by DaleH
TIP - Once one gets a good slug - however you obtained it - if/when you have a bore with an odd number of lands & grooves, use a locksmithing SS shim stock, one on each side at the micrometer anvils, to get a good reliable measurement. Subtract 2X the land height (or bore depth) to get the bore diameter from the overall diameter ... you get the idea.

Re: Slugging a bore, how I did mine.

Posted: Mon Dec 10, 2018 1:06 pm
Bore slugging – funny...Of course there would be a dozen and one different ways to slug a bore....some easier than others....some better than others. But no matter how it is done, there will be some that are critical of the way that it is done – just the nature of the beast. But I, thinking different than most realize that one of the easiest ways to learn in anything is ask for the criticism – every little nit-picken thing. And I have found that quite often this is hard to get because most people just think that it is not nice to unnecessarily criticize. And yes it is hard to ‘nicely’ criticize because usually one has to take more time to do this. Like taking this bore slugging thing...I think, I THINK, that I can properly slug any bore within reason, that one could bring here to my shop within 'one minute'. Mainly just because I happen to be prepared to do it. And I don’t think that one could be critical of any part of it.

Not for reasons unknown to me, but for reasons ‘KNOWN TO ME’, a lot of people will start out with a round soft lead ball. The reason that I wouldn’t do this is because I have done this...and found it not to be conducive, ‘FOR ME‘. I think the only reason that people will use this method is that they have read it so often that ‘round balls’ are used and they will just blindly do it this way...and that they are just learning the hard way, maybe, learning, that there are better and easier methods to use.

Re: Slugging a bore, how I did mine.

Posted: Mon Dec 10, 2018 7:50 pm
by 72 usmc
Yep, all sorts of ways, You can use kits like Meister bullets ... etails.asp
or Vollmer kits, some use cerrosafe. More ideas

Also consider, are you using cheep high school plastic calipers or a nice Starret caliper? I would assume you can zero it out and read it. Maybe a digital set if you got old eyes and can not see the little fine lines. :dance: O, and soft lead, not wheel weight or a harder lead alloy for the slug. Do it 4x and take the average of the measurements if using a cheep, round, lead pistol ball. And if the bore is dirty or pitted sewer pipe why bother to even slug the thing.
As a jar head, I inspect the bore, clean it, headspace it on a field gauge, and then fire the rifle first to see how it shoots with different ammo. If it's OK, I do not worry. If the bullet is hitting the target sideways, then more investigation is needed or trade the surplus junk off. :shhh: :lol: :lol: :lol:

Sort of like a bullet test on a muzzle vs an actual muzzle gauge. Heck, at my age and with my 4 eyes, I can not see both sights or a target at 500 yards. :violin: Almost any surplus rifle will put a bullet in the black with a CMP target at 100 yards. If not, it is a sorry piece of junk. All this talk reminds me of Mr. Science as the skeptical graduate student vs the R.M.A., the old timer set in his ways-the real mesoamerican archaeologist in Flannery's(1976) Early Mesoamerican Village. The debate goes on each arguing & criticizing each other as serious ideas/topics come about. In the process each is learning from the other. A great book from times past. We all learn from these interesting comments & criticisms to the posts in the forum. I am an old kid from the Mountain Dew generation- I am just going to shoot it and have fun with the time I have left. :geek: :shhh: