Measuring bore size

[Misfit-45]

Hello,
Other than "slugging" the barrel, can I measure the bore of my Lyman .50 GP Hunter using a dial caliper just inside the muzzle? If so, is it measured Land to Land, Land to Groove, Or Groove to Groove? Given the width of the lands and grooves, I could accomplish any of the three measurements and still be measuring the true diameter of the circle.

 

[Zonie]

The bore is measured land to land.

 

[M. De Land]

A caliper will never give a true reading on the inside of any bore.
If you slug it then you will be measuring the grooves(inverse) of the slug to get the bore diameter and that is not measured particularly accurate with a caliper either although better than an ID.
I use a slug to get groove diameter and a split ball gauge to get the bore diameter on even landed rifling. If you have odd numbered rifling you will have to use a tri-mic or make up a Powely gauge to measure the groove and a pin or plug gauge to get the bore. MD

 

[Ken Rummer]

My notes indicate the Lyman barrels have 5 grooves so when you take a micrometer reading you are measuring fron a land to the opposite groove.

I went to the Lyman Users Guide which gives the specified bore dimensions. The 50 GPH is supposed to be .502 land and .510 groove diameters. Actual direct measurement would require special equipment known only to the Quality Control tech's in the Lyman shop.

Slugging the barrel will help but you still have the same land-opposite-bore situation. You can get a good estimate of the groove depth off the slug and calculate the land and bore sizes.

I think the best course would be to shoot it and worry about bore dimensions if you can't get recommended loads to produce an acceptable group.

 

[Misfit-45]

Thanks for the replies. Yeah, generally If you have a .50 cal., you shoot .50 cal bullets.(I'm shooting conicals) I haven't shot it too much, just about a full can of Goex. The problem I had was the .503 bullets from my Lyman mold (#508656 395 grain) I was trying to ram down the barrel. After the first time, the barrel already had enough fouling to make it difficult. After two or three times it was impossible. I swabbed each time, but it seemed more difficult than it should be. (I've never seen it done before). I went to this forum and asked for information. The conclusion was that I was not using PURE lead. When I finally got around to casting pure lead and shooting, the situation improved, but not enough in my opinion. The whole point to this post was to find out the feasibility of buying a Lee Lube and Size Kit for $25 in .501. Naturally, the concern was making the conicals too small. In a Minie Ball, it might work, but my mold casts flat bottom bullets. I'm not sure it would expand enough to properly engage the rifling. I thought maybe my Lyman barrel would mic out to .501 or 2. More opinions are very welcome.
By the way, I wanted to try Triple Seven ff because I read it is much cleaner, less fouling. Haven't bought any yet.
Marv

 

[necchi]

If you already have over size projectiles you can slug the bore.
Find a peice of brass rod (or something softer than the steel bore) 3/8" about a foot long.
Drop it down the bore, pound in one of your over size projectiles,, then turn the barrel upside down and use the brass rod inside like a hammer,, bounce it a few times and it'll drive the OS projectile out for you to measure.

 

[majg1234]

yep you could slug the barrel to get the size of the bore(pretty darn close) and you could resize the projectiles place a felt wad over powder and go on from there.You could even use a patched RB which will help with your loading :v Good luck and enjoy the journey

 

[Billnpatti]

Slugging the barrel is the best way to get the bore diameter and can be done in at leat three ways. First, if you have the breach plug out, you can just drive an oversize plug of lead through the bore. I don't recommend removing the breach lug, though. Another way is to put a soft metal rod such as brass down your bore and then drive in an oversize piece of lead into your bore. Then shake the barrel up and down using the brass rod to drive out the lead plug. Lastly, you can buy a piece of a low temperature melting metal called "Cerosafe" from someplace like Brownell's. Then you just put a bore brush into the muzzle so that the threaded part sticks out. Cover the threads to keep them clean and then melt the Cerosafe and pour some into the bore so that it flows into the bristles of the brush. It will harden almost instantly and then you can simply attach a cleaning rod to the brush and pull it out. Don't worry, it will not stick to your bore. The brush, however, is a goner. Once you have the slug, use a micrometer to measure it.

 

[RedFeather]

I was also thinking to recommend Cerosafe. It is not supposed to shrink, so you will get a good reading. And I would measure all around. I once had a .43 Spanish rolling block which I slugged the barrel of and took to an older gun guru to measure for me. He said that one of the grooves was deeper than the others. Apparently not uncommon on some of the old guns that were rifled by hand or hand-set machinery. Don't know how they do it today but who knows?

 

[M. De Land]

I have and use cerosafe quite often and you have to wait for a period of time for it to reach full diameter before measuring. It shrinks initially(first 30 minutes) and slowly expands to full diameter at the end of one hour, when it should be measured. It should be removed when it reaches room temperature and not always will. It will expand about .0025 after 200 hours. I know most custom bullet mold makers will not except it as a slug from which to make you a new mold because of it's ability to shrink and expand. A lead slug is better over all for bore and groove measuring but for chambers it has no equal in my opinion.
It is made to order for chamber casts and some times will not come out when used for measuring bores and grooves. I've had it stick solid on more than one occasion when following directions precisely. I think this is because of bore diameter inconsistency and it molds so precisely. When that happens just remelt it inside the bore and pour it out. It melts at between 158-190 degrees. MD

 

[Misfit-45]

Hi, I used the "slide hammer" technique to run a bullet down the barrel about 12 inches and hammer it out again with a steel rod wrapped in duct tape.(put in the barrel first) It worked fine.
It appears that the bullet in engaging the rifling perfectly (to me). The first four to six inches of the barrel were a bit tight, but after that, the bullet slides quite easily. I'm starting to think that my "tight" bullets is strictly due to fouling. Comments?
Marv

 

[M. De Land]

It didn't expand into the groove. What you need to do is use a ball instead of a pullet that is bigger than the groove diameter in a muzzle loader so it can be swaged down by both groove and lands to get a measurement. You can use a bullet or undersize ball if you don't have a breech plug in by putting a steel rod under the base and swaging it between the upper and lower. If you have a breech plug than it needs to be over groove diameter to start with.
You may have some choke in your bore if it got easier as it went down. That is a good feature usually. MD