Slugging a barrel

[Bugscuffle]

I understand how to slug a barrel, but I'm not too sure about how to measure the slug that I get. Do you measure it from land to land, from land to groove or from groove to groove? I have been measuring from land to groove and selecting my bullet diameter from that measurement. Recently a Pard told me that the measurement should be done from groove to groove. My pistols have an even number of lands in the barrel, and the measurement from land to groove is a bit tricky. So, I get the eight year old girl from next door to come over and do the math for me to find the land to groove measurement. She explained it to me this way: Measure the land to land diameter and write down the number (important for us old farts). Then measure the groove to groove diameter (write this one down too for the same reason). . Subtract the land to land measure ment from the groove to groove measurement  and that is two times the groove depth. Divide that by two and you have the groove depth and  all the numbers that you need to figure all of the diameters. But, the quetion still remains, which measurement am I looking for?

[pony express]

For ordinary cast bullet shooting, all you need is the groove diameter, size your bullets .001-.002 larger. Then make sure that it will chamber freely, with that diameter loaded(don't know you caliber, I've read that some 44-40s are problematic in that regard.) Measuring from "land to groove" is going to get you a somewhat undersized bullet.

[Blackfoot]

For what we do, measure the largest diameter of the slug.  That will give you the groove diameter of the barrel then select a bullet that is .001 to .002 larger. 

Blackfoot

[Bugscuffle]

So, it's groove to groove +.001 to .002. Thanks.

[cal44walker]

And use an oversized soft lead round ball to measure groove diameter. Use a .457" for a Colt 45 for example and give it 1 good whack when you have just pushed/tapped it into the rifling. Then gently push it out the rest of the way. Gun oil helps it through easily.

cal44walker

[Bugscuffle]

And use an oversized soft lead round ball to measure groove diameter. Use a .457" for a Colt 45 for example and give it 1 good whack when you have just pushed/tapped it into the rifling. Then gently push it out the rest of the way. Gun oil helps it through easily.

cal44walker

That sounds like it will work O.K. for a relatively new gun, but an older gun will have some muzzle errosion that may give you a false reading on the bore size further down the barrel. Just my thoughts on that technique.

[HL Hunley]

I'm a total greenhorn and should keep my pie hole closed, but I do have some observations. I would appreciate wiser folks telling me where I'm wrong. Please understand I'm observing, and probably from a distorted point of view. But set me straight.

Here's what I see.

First, slugging a barrel sure seems like a lot of hard work that could hurt the gun. I've seen warnings about "crowning" (bowing?) the barrel. I would very much hate to bend my barrel trying to determine the bore. I have a brand new Smokewagon and would hate to turn it into a Chuckwagon.

Apparently, the argument is that calipers won't be accurate enough, so slug. Bash that lead all the way through the bore. But what do you measure the slug with? I'd use calipers. Mine are calibrated by know, guaranteed stainless shims.

Then there's the fact that most sizing dies come in only .001" increments, with many gaps and jumps. So, if theoretically a slug is more accurate than a direct caliper measure, and if you ignore the caliper measure of the slug, what difference does it make if the slug is slightly different to a direct measure? You can only get sizing dies in big increments, so why not just measure the barrel?

Then there's the whole leading concept. A small projectile will bounce as it exits, smashing lead into the barrel, causing leading. How is this different to an oversized projectile stripping skin and embedding it to the barrel as it exits?

I am in need of wisdom!

[pony express]

It's easier to get an accurate outside measurement than inside with calipers. Also, for more precision, you can use a micrometer to measure a slug, but not an internal measurement. Also, if there is any muzzle wear present, you won't get a true measurement of the bore diameter just using a caliper.

As far as damaging your barrel, I guess if you tried to drive a 50 cal ball into a 30cal barrel, maybe you would. You should use one that just shaves a small ring, similar to seating a ball in a C&B revolver. It also needs to be SOFT lead, like you would use in muzzle loaders, not hard cast bullets.

After all of this discussion, if you're using recently manufactured guns, in relatively standard calibers, with commercial cast bullets, you only have one or two options on size anyway,(ie: 357, 358, 427 or 429, 451 or 454) just buy some and try them out.

An undersize bullet will more likely lead than an oversize. This is because on an undersize, you have hot gas from powder combustion escaping around the bullet, melting some of it of, unless you're using soft lead and BP, that's a whole different story. A bullet even .002-003 oversize will simply squeeze down to fit.

There's a WHOLE lot more to be said about cast bullet diameters, you can check out a Lyman cast bullet manual to get all the science of it.

[cal44walker]

Forget all about trying to measure the inside diameter of a barrel with calipers. Absoloutly non indicative of the true barrel diameter as you will never measure it accuratly enough and you are also only measuring the muzzle. A round lead ball slightly over sized with oil and an aluminium/wooden rod just uner bore size are all thats needed. If possible go from the breech end as theoreticaly there should be more erosion here, and once seated in the rifling, give the ball a solid thump. Then push it the rest of the way out feeling along the way for any changes in resistance that could indicate changes in bore diameter. Its very easy and very quick if you have all the materials you need. Measure the maximum diameter of the ball with a micrometer screw type tool and use a bullet around .001" over that diameter.

cal44walker

[HL Hunley]

Pony express - thanks for that on the "gas melting lead" with an undersized, makes sense.

Pony & Cal44 walker - I went ahead and slugged my barrel today! It was very scary, but actually incredibly easy and smooth!

I'd seen videos of folks pounding away, but maybe they were using hardened alloy balls. My .454 (.44 cal ball) pounded into my .44-40 with just moderate resistance and cut off a ring a lead. Did it twice in fact, it was so easy.

Thanks guys

[cal44walker]

Yea, those guys you see pounding away are doing it all wrong. It scares alot of people away from doing an easy job that can provide alot of benefits to accuracy. Let us know how the shooting goes. Try several diameter of projectile at, and over bore size. Something like +.000/.001/.002/.003. You would be surprised at the difference it makes to some guns, and amazed that some others will shoot anything Let the experimanting begin!

cal44walker

[HL Hunley]

Thanks, Cal. I think I've got room in my chambers for going up, I only worry about the brass (starline take .427 easy, don't know if it would start balking at .429). But I shall begin the fun!

[cal44walker]

I use Starline brass in 44/40 in a Pedersoli Lightning which has a pretty tight chamber. I size to .429 and have no problems at all chambering the round. I'm moving up to .430 and .431 next.

cal44walker

[HL Hunley]

That's VERY helpful, Cal. I didn't want to buy 3 sizing dies to experiment. I was thinking of machining dummies on my mini lathe to test fit in the brass, but it seemed like a lot of work for a bad machinist! (I'm more of a chip/swarfmaker.)

Measuring my slug about 20 times in 3 positions, I get the same three figures every time: .427, .4275, and .428. Obviously a but different depth on one of those grooves. So I'm thinking going .429. Now I can start there with confidence. Much appreciated.

[pony express]

Here's another thing, HL. I don't remember if you mentioned what kind of gun you are loading for. If it's  revolver, you might try also slugging the chambers. From what I've read, (I never got that involved with chasing the tinyest groups) it can be more important to match the bullet to the diameter of the chamber mouth, than the barrel. So if the chambers are 429, and bore is 428, then go with at least 429 or 430. If, on the other hand, the chamber mouths are smaller then the groove diameter, then it's probably not going to shoot great, no matter what you do.

[HL Hunley]

Howdy, Pony. Yup, a revolver. An experienced mate told me to check the chambers like you said. The projectile has to fit both he said, and I hadn't even thought of the chambers! They're bigger, but I'll slug to find out how much. Thanks for that!

[Fox Creek Kid]

http://www.grantcunningham.com/blog_files/Measuring_chamber_throats.html

[Coffinmaker]

Here is a fairly simple way to check the throats.
Clean the barrel bore and run a light oil patch thru it.
Drive a lead bullet thru it.  Measure the bullet at the raised portion left by the barrel grooves.  Do it again so you have two bullets.
Drop one bullet into the cylinder chamber.  You should be able to push the bullet thru the throats with a light tap.  If the bullet sticks tight,
The throats need to be reamed a half to a full thou over bore size.  For guns shooting lead bullets, I prefer a full thou over bore size.  Different story for guns intended for jacketed bullets.
Your reserved bullet that hasn't been stuck in a throat will provide your measurement for the reamer.

Coffinmaker

[Blackfoot]

After determining the size of bullet that you need, get some of the proper size and make certain that they will pass through the cylinder throats without "resizing".  If you need a .429 and it is .428 after going through the throat you need to ream the throats.  Keep in mind that all of the throats in a cylinder may not be the same size.

Blackfoot