How to measure rifle bore diameter

[Bill Daniel]

I have a new 45-70 and will shoot only lead for target and deer. Many recommend measuring the rifle bore by using a lead slug. Bear Tooth bullets has a kit for this but I was wondering if a lead fishing sinker of a certain size could be used to save on money? If so do you know what size sinker would be best?
Thanks, Bill

[Dixie Gunsmithing]

It's best to use Cerrosafe, as it's shrinkage is known, which is a very small amount, and it is easier to do. You can get it from Brownell's.

To use it, you have to oil and plug the bore about an inch or so back from the muzzle with a patch, and pour this in the muzzle. Then, using a cleaning rod, you tap it out the front. It's the same process as casting the chamber, but from the other end.

[jimbob86]

I slugged my Nagant Revolver with a .314" lead bullet, some pentrating oil, and a dowel.

[ceh47]

I have slugged guns with lead fishing sinkers that are just a little larger than the dia. of the bore. The sinker is very soft and can be pushed through with a wooden dowel fairly easily. And it only costs pennies.

[PetahW]

.

I would suggest temporarily blocking the bore while the sinker is struck from the opposite side, so the soft lead will completely upset, filling the grooves, prior to pushing it out for measurement.



.

[iraiam]

I'll usually make a pure lead slug a little larger than the bore. I just make a mold out of a couple scrap chunks of steel and drill or machine a hole of the desired diameter.

[Bill Daniel]

I took a half inch lead egg sinker which unfortunately only measures 0.445" and hammered it to a diameter of 0.465". Tonight after work I will oil the barrel with Slip 2000 and tap it through with a wooden dowel.
Thanks for your help!
Bill

[Bill Daniel]

Well, just to finish up the bore groove diameter on the Henry 010 measured 0.456". This should be well obturated by the Oregon Trail 405 grain as it measured 0.459"
Thanks to all
Bill

[Bart B.]

Bore diameter's the distance across the tops of the lands.

Groove diameter's the distance across the bottom of the grooves.

Bore diameters are smaller than groove diameters.

Did you mean groove diameter? That's what bullet diameters should be; typically .001" bigger for lead bullets.

[Unclenick]

Also, 0.002" oversize shoots better in some lever guns, but 0.003" over often shoots worse. So you may be in for some experimenting or sizing work before those bullets do their best.

The last time I tried to get fishing sinkers, they were from Wally World and were made of something a lot harder than pure lead. I found the same at the local Gander Mountain. It seems the "get the lead out" movement has got to the sinker suppliers to this area, at least. As a result, I've either had to cast my own or I buy the pure lead Hornady round balls oversize and roll them down to a few thousandths over groove diameter for slugging. There are also kits and supplied available from Beartooth Bullets, Missouri Bullets, and NECO.

Anything other than pure lead is too springy to do a good slugging job. I forced a cast bullet (BHN 16 claimed) down a bore before and couldn't feel any features of the bore surface with it at all. It was just a hard-to-push slug that came out about half a thousandth bigger than actual groove diameter.

I recommend slugging the whole length of the bore and, where possible, doing it from both directions to feel for rough spots or constrictions that may need polishing or even firelapping. I find a barrel can feel significantly different pushing the slug through it, depending which end you start from. Constrictions easily felt from one end can be glided over from the other.

Also, don't rely on a caliper for measurements, as I've seen done in a YouTube video. They are just not precise enough. If you don't have one, get a $20 OD thimble micrometer that resolves 0.0001". If you have a gun with an odd number of lands and grooves and don't know how to make a measurement for that, Missouri Bullets measures them for free.

[Bill Daniel]

Thank's Bart and Unclenick! I did mean the groove diameter. I was thinking of the groove diameter of my rifles bore and combined the two terms Unclenick the lead weight was hard and was from the World of Wallace. I will order BearTooth's kit and see if the bore is really 0.456" tight or if the lead sinker was so hard as to spring back a thousandth or more.
Thanks again,
Bill

[F. Guffey]

I no longer suggest different methods and or techniques for determining the diameter of a hole/bore, seems the strong got weak and the weak passed out, then there are the two different diameters, how is it possible to do one without the other when using a soft type lead?



F. Guffey

[Old Stony]

I've used cerrosafe to cast chambers, but I usually just oil up the bore and tap a muzzleloader round ball into the muzzle with a small plastic mallet and it seems to form the soft lead into the grooves just fine.

[Bart B.]

.... there are the two different diameters, how is it possible to do one without the other when using a soft type lead?

I've measured a .35 Whelen's bore diameter pushing through a .355" diameter lead bullet made for 9mm Luger pistols. It never touched the bottom of a groove; that diameter was .3573".

[hdbiker]

For my Mosin , I took a .45 cal. musket ball , lighty hammered it into over size shape , then drove into the muzzle with a plastic hammer and followed up with an old brass cleaning rod . worked good and measures .312.hdbiker

[Bill Daniel]

Well to finish my story. The Henry 010 45-70 rifle groove diameter is 0.456" in diameter (0.4558"-0.4562") in range after re-measuring with the Bear Tooth soft lead balls. Bear Tooth bullets measure 0.460" and Oregon Trail 0.459". I will order some Rim Rock bullets which measure at 0.458" to see if there is a difference in accuracy with the Oregon Trail Laser Cast that I have.
Thanks to all again.
Bill

[Bill Daniel]

Sorry to resurrect this post but the finish to my story was incorrect. My technique on slugging the bore on my 45-70 was at fault. The lead even with the kit from Bear Tooth Bullets was too hard to fully fill the groves in my rifle(again I did not push the slug in both directions as recommended by some of my TFLers). When I cast the bore with Cerrosafe alloy it measured 0.4571" to 0.4573" not the 0.4558" to 0.4562" I got by my slug.
All the best,
Bill

[Dixie Gunsmithing]

Bill, remember that Cerosafe shrinks while first cooling, and you have to measure it in an hours time, allowing it to cool for it to show the correct OD. By the time 200 hours has passed, it supposed to expand 0.0025". Anyhow, measure it at 60 minutes of cooling.

[Bill Daniel]

Thanks Dixie GS! That's what I did. Remove it from the bore at 30 minutes after poring and measuring it at 60 minutes.
Thanks an all the best,
Bill

[Unclenick]

I'm curious what made the lead slugs come out too small? Generally, if you try to use anything but pure lead, it is hard to drive in and makes a tight fit because it is a little springy, and you get an oversize result. It also ruins the feel. A pure lead slug with a little oil should be possible to push through the bore, letting you feel for loose spots and constrictions. If one gets loose for some reason, you can run a brass rod in from the other side and tap with a mallet to bump the lead back up to a tight fit.

[Bill Daniel]

Unclenick, I don't know. I thought that the lead might have been too hard to fully fill the grooves that and that I only made a single pass with the slug. The lead in the Bear Basin kit was not 0.458" so I used a hammer to flatten it to a diameter of 0.475 before I made my pass. The Walmart lead sinker and the Bear Basin kit both yielded the 0.4558" to 0.4562" diameter. I am only a gunsmith wannabe so if you or any of our gunsmiths think I should repeat the measurements I would be great full to hear.
Thanks,
Bill

[NoSecondBest]

Meister Bullet Company sells a kit for slugging barrels. You get enough pure lead slugs to do about five guns. The kit contains about nine slugs (in five groups) of varying diameters. They also give you a lubricant and hard wood dowels. You use progressivly larger diameter slugs until you fully fill the grooves in the barrel. They provide detailed instructions on how to use the kit. It works very well. google Meisterbullets.com and you'll see what they have and the prices.

[Goatwhiskers]

A word to the wise: Never use a wood dowel to drive the slug down the barrel. Use a steel or brass rod, you can wrap it with tape at intervals if you're worried about damaging the bore. A wood dowel can split and drive in next to the slug wedging the whole mess in the bore, quite interesting to get out. GW

[Unclenick]

Bill,

Earlier you said it was a Beartooth Bullets kit and later a Bear Basin kit. I am unfamiliar with the latter. If it was Beartooth and you found it too hard, I would call Beartooth and mention it. His sinker supplier may have changed alloys on him without saying so.

Walmart sinkers are definitely not pure lead. At least, they aren't here in Ohio. Instead, they seem to be some kind of pretty hard lead-free alloy and are not suitable for slugging, IMHO.

I mostly use Hornady round lead balls. They are pure lead. I take one that is oversize and roll it between two pieces of steel flat until I have an ovoid that is roughly two to five thousandths bigger than I expect groove diameter to be. I run an oily patch through the bore and wipe the surface of the lead with it, then tap the lead in with a short brass rod.

Goatwhiskers is right that wood is risky to use to try to budge anything inside a bore that's not moving easily. But just to start lead in at the muzzle, it's OK. Brass is nicer and quicker, though, as it's harder and doesn't cushion the mallet blow. Once the slug is in the oiled bore, pure lead can be pushed through with a longer piece of brass rod (best to guarantee you don't wear the muzzle crown) or with a cleaning rod using hand pressure. Some hard alloys cannot be pushed that easily and need hammering through the bore. Don't ever use wood for that, as what Goatwhiskers described can happen. If the bore is oiled and you have to hammer the slug through, though, it's too hard an alloy.

McMaster is at the reasonable end of the price spectrum for 353 brass (hard enough) and I got 3' long 1/4", 5/16", and 3/8" rods from them and as well as a 5 mm by 1 meter long rod in 385 brass for 22's and 6 mm.

For oil I've always used a light mineral gun oil, like Hoppe's, but the next time around I'm going to try the Ultra Lube 4X penetrating/lubricating oil sold at Lowe's. I've been getting good results using that in locks and on other odds and ends, where it really does seem to be four times more slippery than the usual suspects. It may interfere with bore feel, though, and that's what I need to check. Being vegetable-based, its safe on all plastics and finishes, and that's another reason to prefer it if it works out.

[Bill Daniel]

My error!! The kit was from Bear Tooth Bullets. Sorry to confuse.
Bill