Measuring bore size

[Snidely70431]

I have noticed several threads about measuring bore size of muzzle loaders that often seem to involve pounding lead into the bore. I remembered an old gunsmithing book has a section about measuring chambers, a technique which should work with bore sizing. This is from Clyde Baker's Modern Gunsmithing, copyright 1933:

MEASURING CHAMBERS. To measure the chamber of a rifle it is necessary to make a sulphur cast. The chamber, and about one inch of the rifling forward from it, should be thoroughly cleaned and then covered with a very thin film of light, clean oil. Take a cork the size of the bore of the rifle and drill a small hole through the exact center. In this hole place a piece of straight wire about .0625-inch in diameter. Press the cork into the chamber and up about half an inch into the rifling, so that the wire extends through the cork and back to a point several inches in rear of the breech. The wire functions as a handle for the cast as the cast is very brittle. The mixture of the cast is made of the following materials:

Sulphur ............................................2 ounces
Powdered lamp black.........................3 grains
Gum camphor dissolved in alcohol.....3 drops

Heat very slowly and stir continually. When the mixture arrives at a thin pouring consistency, pour it into the chamber quickly, and allow to cool thoroughly before removing. To remove, place a rod in the muzzle and shove lightly on the cork, letting the cast come out slowly, and handling it very gingerly as it is quite brittle. The cast can now be measured with a micrometer and scale, and will give the dimensions of the chamber as well as the shape. The mixture is almost shrink proof, but it is well to allow .0005-inch for shrinkage if measured at once, or .001-inch if measured a day after cast. It is important that the mixture be heated slowly, otherwise it becomes too thick to pour.

 

[Stubert]

How are you going to do this on a muzzle loader? They only have one opening at the muzzle, there is no way to do a chamber cast unless you remove the breech plug. What you are describing is a cast for a breech loader.

 

[PapaG]

Stick the cork down the muzzle with the wire coming out. Pour the mix, cool, pull it back out. Not that hard. I normally just drive a lead slug, a long one, partway in, pull out and measure. Less mess.

 

[arcticap]

Brownell's make a casting product named CERROSAFE:--->>> https://www.brownells.com/gunsmith-.../cerrosafe-chamber-casting-alloy-prod384.aspx

mykeal's post #11 describes how to use it to make a cast of a bore:--->>> https://www.thehighroad.org/index.php?threads/bore-slugging.424777/#post-5301430

 

[Catcar67]

I use my adjustable hole gauge. It goes about 3 inches down the muzzle. I mic the gauge and add 0.002 to adjust for groove depth. It may not be perfect but since you use a patch (except for revolvers) it's close enough.

 

[Snidely70431]

How are you going to do this on a muzzle loader? They only have one opening at the muzzle, there is no way to do a chamber cast unless you remove the breech plug. What you are describing is a cast for a breech loader.

Uh, you remove the breech plug? Since I mostly work with modern inline muzzle loaders it never occurred to me that someone would not remove the breech plug.

 

[Snidely70431]

Brownell's make a casting product named CERROSAFE:--->>> https://www.brownells.com/gunsmith-.../cerrosafe-chamber-casting-alloy-prod384.aspx

mykeal's post #11 describes how to use it to make a cast of a bore:--->>> https://www.thehighroad.org/index.php?threads/bore-slugging.424777/#post-5301430

Cerrosafe is a mixture of Bismuth, lead, tin and cadmium. Not exactly nice stuff to play with. On the plus side it does melt at lower temperatures than sulphur.
https://en.wikipedia.org/wiki/Cerrosafe

 

[Steel Horse Rider]

If you cast your own bullets or balls from lead I don't think the exposure to cerrosafe is particularly risky. But then I don't think casting lead is particularly risky as long as you use prudent practices and are not exposed to it on a daily basis, remember, moderation is a good and healthy practice in all realms of your life.

 

[arcticap]

Rotometals sells CERROSAFE ingots for the lower price of $9.99 per pound.--->>> https://www.rotometals.com/chamber-...MI6M-svfmu4AIVjYCfCh1nggosEAQYBCABEgIBOvD_BwE

Their product page also lists its contraction/expansion factors hour by hour.

 

[Catcar67]

The caliber should be stamped on the barrel or somewhere, if it's a revolver. Unless you have a very old weapon that has been used extremely poorly you wouldn't need to have the exact bore size. Nearly every black powder shooter's guide will tell you the proper bullet to use, whether it's a patched round ball or unpatched slug. Black powder barrels don't usually get "shot out" like a modern rifle barrel. I doubt you're going to be shooting MOA accuracy anyway.

 

[Loyalist Dave]

I have noticed several threads about measuring bore size of muzzle loaders that often seem to involve pounding lead into the bore. I remembered an old gunsmithing book has a section about measuring chambers, a technique which should work with bore sizing.

Just Curious or are you planning on target shooting in excess of 200 yards? Why isn't the use of the caliper a few inches into the barrel at the muzzle sufficient to give you a bore measurement?

LD

 

[paul harm]

Do inline muzzleloaders have a chamber ? You remove the breech plug in any muzzleloader and you should have the bore. Isn't it pretty easy to just pound or push a oversize RB or slug down the bore ? Or do as Papa said if you're fooling around with a real muzzleloader. Good question, Dave.

 

[Snidely70431]

This is a picture of two CVA Quick release breech plugs. The unthreaded portion towards the muzzle generally measures about .560 or .570 on the breech plugs I've checked. The threaded part is about .625 or 5/8 -18 threads per inch, so, no, there is no "chamber" per se. The barrel begins just ahead of the plug.

As to the size being marked on the barrel, that is the nominal barrel size, not the precise internal diameter. I understand that the actual size of a 50 caliber barrel may vary from .495 to .503.

Do inline muzzleloaders have a chamber ? You remove the breech plug in any muzzleloader and you should have the bore. Isn't it pretty easy to just pound or push a oversize RB or slug down the bore ? Or do as Papa said if you're fooling around with a real muzzleloader. Good question, Dave.

 

[Snidely70431]

Since I haven't used sulphur to make a chamber casting (in this case, a bore casting) in many years, I thought I would go ahead and make a couple just to be able to communicate a usable method.

I made a casting pot out of a tuna can and a coat hanger. I used a propane torch to heat the can. I measured the mixture with the little electronic scale I use for measuring out my loads.

The sulphur I bought at a feed store, a couple of pounds for about $6. Lamp black being unavailable, I used some graphite sold at hardware stores for lubricating the internals of locks.

I made castings of two CVA Optimas I use for my test beds for breech blocks and other stuff, a .45 and a .50. I swabbed the bores with a patch with WD 40 on it; both castings came out with a light tap with a rubber mallet on the ram rod that came with the guns.

Not having access to a cork, I improvised a barrel block by wrapping a cartridge case in aluminum foil. I forced it into the bore from the muzzle towards the breech with a bullet starter, then compacted it in the bore using the ram rod from the block end. I secured the barrel in a vise, muzzle up.

I then heated the sulphur mixture and poured it into the barrel. It solidified almost immediately in the cold barrel. After a couple f minutes I removed it from the barrel. It was slightly warm to the touch.

The .45 barrel miked at .458 and the .50 .508 at the widest, the grooves. For the picture I removed the barrel block from the .45 casting, left it attached to the .50