Slugging barrels: size does matter

Guns Magazine, Feb, 2008 by Mike Venturino

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How do you actually slug a gun's barrel? It seems so elementary I've never written about the mechanics of the process. Then the other day a reader wrote and said he'd inserted a tight fitting wooden dowel into his barrel and then poured molten lead down on it, figuring he could then push the resulting "slug" back out. He said instead the lead "locked" the dowel in and getting both lead and dowel out of the barrel was extremely difficult.

Slugging a barrel is so easy it can be done in a couple of minutes if the proper items are on hand. Conversely, slugging can ruin a good barrel if improper tools are used or if someone goes at it like they're driving a golf ball.

What It Means

First here's a brief explanation of just what "slugging a barrel" is, and why do it at all. Slugging a barrel simply means pushing a lead "slug" into a barrel and then removing it in order to measure it with a micrometer or caliper so the barrel's interior dimensions are known. Why do this at all? Mostly it is necessary for the cast bullet shooter, so he can match his bullets as-cast size, or sizing diameter, to his gun's barrel.

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Why is this important? With smokeless powder loads, I would rather have a cast bullet be .003" over barrel groove diameter than even .0005" under it. In my experience, undersize bullets either shoot poorly or just tumble altogether. If black powder is the propellant, then a soft cast bullet even several thousandths undersize will be "slugged up" by the powder's explosive force and fit the barrel. That might net the shooter acceptable accuracy, but usually not of the high level a proper fitting bullet would give. And in regards to jacketed bullet shooting, I say if you are shooting them in a firearm of reasonably modern manufacture then don't waste your brain cells on the topic of barrel slugging. It's a moot point.

Don't Bother

Another area where you shouldn't waste your time (unless your understanding of mathematics and especially trigonometry is exceptional) is with barrels having an odd number of grooves. That's because none of the grooves' imprints on the slug will be exactly opposite of one another, making it virtually impossible to measure accurately with a micrometer or caliper. For instance, Smith & Wesson revolvers have always had 5-groove barrels and early American military rifles such as Trapdoor Springfields of any model always had 3-groove barrels.

I've heard of guys well versed in mathematics putting slugs in fixtures with a right angle and then measuring across the opening to get bullet diameter. That's where the trig knowledge is handy. I'm not nearly as smart as that. Luckily for us cast bullet shooters, most firearms have barrels with an even number of grooves--two, four, six, etc.

The mechanics of slugging a barrel are simple: Drive a very soft lead slug into the barrel and then push it back out. Alternately, the slug can be pushed all the way through the barrel. This latter method also will give the "slugger" some idea as to whether or not the barrel has tight and loose spots and is the one I prefer. This is the way I discovered the barrels on Pedersoli replicas of Sharps Model 1874s are tapered. Once the slug was pushed a short distance into the barrel from the muzzle end, gravity took over and it simply fell the rest of the way. Tapered barrels aren't a bad thing, those Pedersoli Sharps shoot very nicely.

The Roundball

Since I shoot cast bullets more often than jacketed and usually do so in guns whose barrel dimensions can be all over the map, I have refined my barrel slugging techniques to make the process quick and easy. First off I keep a supply of pure lead roundballs on hand of the type sold by Speer and Hornady for muzzleloading rifles and revolvers. Such pure lead projectiles are very soft and slide through a barrel easily. Don't ever try to push hard lead through a barrel! The effort required to do so will just increase the chances you will damage the barrel. Here's another don't. Don't ever try to drive any slug through a barrel with a metal cleaning rod. That also will increase the chance of damaging the barrel.

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What I do is place a pure lead round ball on a gun's muzzle, chosen because it is significantly larger than the barrel to be slugged. For instance, to slug most .38 caliber revolver barrels, knowing they will range somewhere between .354" to .360", I use the .375" round ball. To slug a 45-caliber rifle or handgun barrel, knowing they will be between .451" and .460", I use the .490" roundball. To slug a 50-caliber barrel those .530" roundballs work fine.

After giving the round ball a squirt of spray oil to help "grease the skids" it is gently tapped flush into the barrel's muzzle using a wooden or rubber mallet. The "ball" is now a "slug" with parallel sides. Next it is tapped several inches deeper into the barrel with the aid of a small piece of hardwood dowel say 3" to 6" long and of the proper diameter to fit inside the barrel. I've also used small brass punches but you must be very careful with them as even brass can damage a barrel if it is banged forcefully against it. For the rest of the trip through the barrel a hardwood dowel slightly smaller than the gun's caliber and of course of the proper length can be used to tap the bullet onwards.