“How to slug a barrel” - Looking for the best resources
- Thread starter Michael
- Start date
Measuring the diameter of a barrel is simply a shortcut to finding the proper sized projectile. You get a measurement to narrow it down. We all know the ideal projectile size will be different from whatever the exact size of the barrel is.
For some purposes like headspace the exact dimensions are critical. For getting a baseline to determine the right size projectile it is not. Its simply a starting point for testing. The differences in the size of a casting or a pushed slug is relatively unimportant. It's just a number to start your experimentation.
A poured lead cast will be closer to the actual dimension of your ideal projectile diameter than a casting of cerrosafe. A cerrosafe casting will be the exact size of the barrel. You can measure the cerrosafe casting and reduce that size a bit for a starting point. Or you can use linotype and start there. Either way it's just a reference point to arrive at the dimension to begin testing. The ideal projectile diameter can't be determined by either method. And either method will provide a suitable place to start.
That being said I'm not recommending anyone pour molten lead in the barrel. I simply stated that barrels and chambers are measured with cerrosafe and that linotype could also be used. I shouldn't have even mentioned it and Firewalker wouldn't have had to tell me how wrong I was.
Neither cerrosafe nor linotype would be practical for an airgun or a powderburner to measure your bore to size a projectile. You can just push a right sized short slug in there and measure that.
For some purposes like headspace the exact dimensions are critical. For getting a baseline to determine the right size projectile it is not. Its simply a starting point for testing. The differences in the size of a casting or a pushed slug is relatively unimportant. It's just a number to start your experimentation.
A poured lead cast will be closer to the actual dimension of your ideal projectile diameter than a casting of cerrosafe. A cerrosafe casting will be the exact size of the barrel. You can measure the cerrosafe casting and reduce that size a bit for a starting point. Or you can use linotype and start there. Either way it's just a reference point to arrive at the dimension to begin testing. The ideal projectile diameter can't be determined by either method. And either method will provide a suitable place to start.
That being said I'm not recommending anyone pour molten lead in the barrel. I simply stated that barrels and chambers are measured with cerrosafe and that linotype could also be used. I shouldn't have even mentioned it and Firewalker wouldn't have had to tell me how wrong I was.
Neither cerrosafe nor linotype would be practical for an airgun or a powderburner to measure your bore to size a projectile. You can just push a right sized short slug in there and measure that.
Last edited:
Upvote 0
Upvote 0
I have cerrosafe and have used it to determine what unknown barrels (pb) are chambered in. For specific dimensions of groove diameter, I will continue with the lead slug method. Cerrosafe is kinda finicky knowing how to obtain the correct dimension (as evidenced above by needing to purchase software to calculate properly).
I suppose cerrosafe might be useful in airguns if you suspect you have a bore that is grossly outside the dimensions reasonable for caliber.
Dave
I suppose cerrosafe might be useful in airguns if you suspect you have a bore that is grossly outside the dimensions reasonable for caliber.
Dave
Upvote 0
A lead bullet cut in half is the way I've always done it. About as long as the diameter of the bore. Tap it in with a wooden dowel past the leade and then back out. If there are no witness marks at the bottom of the grooves I fatten one a little with a hammer and try until the slug is against the groove surface.
If you use a whole slug on a rifle you have a lot of friction. You may change the dimensions trying to drive it out. Or get it stuck.
I like Firewalkers idea using a round ball. That would eliminate sticking as well as deformation driving it out. Great idea!! I've never done it like that for lack of appropriate sized round balls I can fit to the bore. After sticking a couple full length bullets I started cutting them down to reduce bearing surface. It works. A round ball would probably work better.
Your rifling design is going to be the big variable in how much lead is displaced by the engraving of the lands. So the ideal bullet dimension isn't a set number based on barrel diameter. A barrel with an odd number of lands will be more forgiving than a barrel with an even number of lands. A barrel with the "micro groove" style rifling displaces very little lead in engraving. With the funky rifling designs on some barrels today it's probably a craps shoot. So the right sized projectile can be a process even with a perfect barrel measurement.
I look at the base of a sized and fired bullet to see how much lead is being pulled off. If it's dragging lead off the base it's probably sized too big and deforming the projectile. If it's not deforming enough to show witness marks in the grooves it's probably too small. In my experience they are most accurate when engraving displaces just enough lead to fill the grooves without too much lead being pushed over the base. But in some designs of rifling this may not be the sweet spot.
Only testing with different sizes will know for sure. In my experience with my rifling patterns it is a bit shy of the actual diameter. With a 5 groove barrel it's mighty difficult to even measure accurately. So I've given up trying to find the exact size of the barrel. I could care less. I'm looking for the right sized projectile. Getting in the ballpark by slugging the barrel just gives me a place to start.
That is all relative to my very limited experience shooting cast lead bullets in a powder gun. How this relates to airgun pellets or slugs I have no idea. I suspect the principles are the same but that is just wild conjecture.
If you use a whole slug on a rifle you have a lot of friction. You may change the dimensions trying to drive it out. Or get it stuck.
I like Firewalkers idea using a round ball. That would eliminate sticking as well as deformation driving it out. Great idea!! I've never done it like that for lack of appropriate sized round balls I can fit to the bore. After sticking a couple full length bullets I started cutting them down to reduce bearing surface. It works. A round ball would probably work better.
Your rifling design is going to be the big variable in how much lead is displaced by the engraving of the lands. So the ideal bullet dimension isn't a set number based on barrel diameter. A barrel with an odd number of lands will be more forgiving than a barrel with an even number of lands. A barrel with the "micro groove" style rifling displaces very little lead in engraving. With the funky rifling designs on some barrels today it's probably a craps shoot. So the right sized projectile can be a process even with a perfect barrel measurement.
I look at the base of a sized and fired bullet to see how much lead is being pulled off. If it's dragging lead off the base it's probably sized too big and deforming the projectile. If it's not deforming enough to show witness marks in the grooves it's probably too small. In my experience they are most accurate when engraving displaces just enough lead to fill the grooves without too much lead being pushed over the base. But in some designs of rifling this may not be the sweet spot.
Only testing with different sizes will know for sure. In my experience with my rifling patterns it is a bit shy of the actual diameter. With a 5 groove barrel it's mighty difficult to even measure accurately. So I've given up trying to find the exact size of the barrel. I could care less. I'm looking for the right sized projectile. Getting in the ballpark by slugging the barrel just gives me a place to start.
That is all relative to my very limited experience shooting cast lead bullets in a powder gun. How this relates to airgun pellets or slugs I have no idea. I suspect the principles are the same but that is just wild conjecture.
Last edited:
Upvote 0
Upvote 0
From an engineering point my main focus is on friction between the projectile and the bore and how consistent is that friction along the length.
Many people mentioned 0.0005 inch oversize ... in my eyes that is very aggressive overlap = 0.0127 microns.
Yes, the projectile is from a lead, it is relatively soft, but
I would not go 0.005 microns to 0=zero microns overlap on smallest groove OD somewhere there inside ... considering the bore is consistent enough along the length.
And btw, air will not escape at zero gap.
Many people mentioned 0.0005 inch oversize ... in my eyes that is very aggressive overlap = 0.0127 microns.
Yes, the projectile is from a lead, it is relatively soft, but
I would not go 0.005 microns to 0=zero microns overlap on smallest groove OD somewhere there inside ... considering the bore is consistent enough along the length.
And btw, air will not escape at zero gap.
Upvote 0
Lead is displaced when rifling engraves the projectile. Different designs rifling will displace more or less lead. Some is pulled rearward but the projectile also deforms diametrically. The ideal sized projectile will depend on your specific rifling design, bullet design and alloy.
I'n my limited experience with my PB guns a cast lead projectile is the proper size a tiny bit UNDER the outside groove diameter. Your results may (and probably will) vary based on these variables.
I'n my limited experience with my PB guns a cast lead projectile is the proper size a tiny bit UNDER the outside groove diameter. Your results may (and probably will) vary based on these variables.
Last edited:
Upvote 0
Upvote 0