The Firing Line Forums - View Single Post - Slugging Barrels for Lead Boolits Really Necessary?

FrankenMauser · August 24, 2012, 04:19 PM

You can always use the "trial and error" method to find out what a firearm wants. The reason cast bullet shooters (and milsurp shooters) suggest slugging the bore, is because it can eliminate a lot of time and money wasted on reloads that don't work, and/or time wasted on hours spent scrubbing lead out of the barrel.

As dunerjeff discussed, most barrels are not at the 'nominal' dimensions, and most are over by 0.001-0.002". Some, like .444 Marlin, are frequently over (from the factory) by as much as 0.005".

It is not uncommon to find 9mm barrels with a groove diameter of .357-.358". If you went with a .357" projectile (for 0.002" over nominal), you'd likely encounter a lot of leading.

I have a .380 with grooves that measure .358" x .359" at the muzzle, but are significantly larger than that mid-way through the bore (estimated to be in excess of .363"). I bought 500 lead bullets for it, quite a while ago, and couldn't figure out why I couldn't get it to stop leading. Once I slugged it, I knew why. Those .356" bullets were just rattling down the oblong, over-sized barrel. Had I slugged it, to begin with, I never would have wasted my time or money on the wrong bullets.
(Since it's rather difficult to get .359-.360" bullets appropriate for .380, it has the loose section, and I don't want to size down Makarov bullets... it now gets hollow base .356-.357" bullets that can expand to the proper size and 'ride the grooves'.)

It is not uncommon to find 30 caliber barrels with narrow bores and deep grooves. Rather than the nominal .304" bore / .308" grooves, you may encounter a .302" bore and .309" or even .310" grooves.

.45 Auto and .45 Colt share the same problem. One .45 Auto might have a groove diameter of .451", while the next is clear up at .454". And, its sibling .45 Colts might have a groove diameter of .452", while the next is at .456". Going with a slight over-size of the nominal bore dimensions won't work with those deep grooves.

And, then.... you can get into arguments about revolvers and rifles.
Many people believe it is more important to size the bullet to a rifle's throat, than to the groove diameter. Rifle chambers are quite variable, and the throat changes along with them. Since the throat is also the most erosion-prone part of the barrel, it's also the part that gets enlarged faster than anything else. Without slugging (or taking a chamber cast)... you'll never know what size the throat actually is.

And, revolvers have even more areas to worry about:
Do you size it for the cylinder throats? -Are they smaller than the bore? Smaller than the forcing cone? Are the mis-matched?
Do you size it for the forcing cone? -What if the bore is smaller?
Do you size it for the groove diameter? -What if there's a constriction where the barrel threads into the frame (very common)?

Revolvers can be quite a headache, if you have mis-matched dimensions. But, slugging before aimlessly tossing lead bullets through them can provide some insight into what might be the best starting point.

If you don't want to slug your barrels or take chamber casts, don't. Just throw a bunch of lead through it, until you find something that works.
But, when we suggest slugging it... we're just trying to help, by saving you some potential frustration (and bore scrubbing).

August 24, 2012, 04:19 PM	#4
FrankenMauser Senior Member Join Date: August 25, 2008 Location: In the valley above the plain Posts: 13,424	You can always use the "trial and error" method to find out what a firearm wants. The reason cast bullet shooters (and milsurp shooters) suggest slugging the bore, is because it can eliminate a lot of time and money wasted on reloads that don't work, and/or time wasted on hours spent scrubbing lead out of the barrel. As dunerjeff discussed, most barrels are not at the 'nominal' dimensions, and most are over by 0.001-0.002". Some, like .444 Marlin, are frequently over (from the factory) by as much as 0.005". It is not uncommon to find 9mm barrels with a groove diameter of .357-.358". If you went with a .357" projectile (for 0.002" over nominal), you'd likely encounter a lot of leading. I have a .380 with grooves that measure .358" x .359" at the muzzle, but are significantly larger than that mid-way through the bore (estimated to be in excess of .363"). I bought 500 lead bullets for it, quite a while ago, and couldn't figure out why I couldn't get it to stop leading. Once I slugged it, I knew why. Those .356" bullets were just rattling down the oblong, over-sized barrel. Had I slugged it, to begin with, I never would have wasted my time or money on the wrong bullets. (Since it's rather difficult to get .359-.360" bullets appropriate for .380, it has the loose section, and I don't want to size down Makarov bullets... it now gets hollow base .356-.357" bullets that can expand to the proper size and 'ride the grooves'.) It is not uncommon to find 30 caliber barrels with narrow bores and deep grooves. Rather than the nominal .304" bore / .308" grooves, you may encounter a .302" bore and .309" or even .310" grooves. .45 Auto and .45 Colt share the same problem. One .45 Auto might have a groove diameter of .451", while the next is clear up at .454". And, its sibling .45 Colts might have a groove diameter of .452", while the next is at .456". Going with a slight over-size of the nominal bore dimensions won't work with those deep grooves. And, then.... you can get into arguments about revolvers and rifles. Many people believe it is more important to size the bullet to a rifle's throat, than to the groove diameter. Rifle chambers are quite variable, and the throat changes along with them. Since the throat is also the most erosion-prone part of the barrel, it's also the part that gets enlarged faster than anything else. Without slugging (or taking a chamber cast)... you'll never know what size the throat actually is. And, revolvers have even more areas to worry about: Do you size it for the cylinder throats? -Are they smaller than the bore? Smaller than the forcing cone? Are the mis-matched? Do you size it for the forcing cone? -What if the bore is smaller? Do you size it for the groove diameter? -What if there's a constriction where the barrel threads into the frame (very common)? Revolvers can be quite a headache, if you have mis-matched dimensions. But, slugging before aimlessly tossing lead bullets through them can provide some insight into what might be the best starting point. If you don't want to slug your barrels or take chamber casts, don't. Just throw a bunch of lead through it, until you find something that works. But, when we suggest slugging it... we're just trying to help, by saving you some potential frustration (and bore scrubbing). __________________ Don't even try it. It's even worse than the internet would lead you to believe.