Preventing barrel leading
A reader asked me to comment on successfully shooting lead bullets in revolvers. It seems that he's been getting indifferent accuracy coupled with severe leading, and would like to know the "secret" to using lead in his gun.
I thought I'd covered this topic once before, but a thorough search of the archives failed to turn up the expected article. Guess I'll have to do this from scratch!
Please note that I'm not a "hardcore" cast bullet shooter. I don't cast my own, which means that I'm dependent on commercial sources for my projectiles. As a result, it's taken me longer to learn this stuff than it would have otherwise. Thus I'm no expert; but Ed Harris, who sometimes checks in here at the RLA, is - hopefully he'll see fit to comment. (Ed, if I get anything wrong please drop me a note - I'll make your response into it's own post.)
The first thing to understand is that your lead bullets need to fit the chamber throats of your gun. If, for example, your throats measure .358", your bullets should be no smaller than .358, and no bigger than .001" over that measurement. Smaller bullets won't be as accurate, and will let the erosive combustion gases blow past the bullet causing severe leading around the forcing cone.
(Many bullet makers will size their products to your preference; if they don't make that service obvious, just ask. A surprising number are happy to oblige, usually at no extra cost.)
The forcing cone of your gun must also be in good condition; roughness in that area will result in leading at that point.
Assuming that the gun part of the equation is in good shape, and the bullets are of correct size, the hardness of the bullet becomes the critical issue. Most bullet makers advertise really hard bullets as being the "cure" for leading. It sort of stands to reason, doesn't it? A harder lead won't smear as much as it goes down the barrel, and will leave less residue - right?
Guess what - it isn't true. In fact, it's completely off base!
Think about this: you probably have a .22 rifle hanging around. Most .22 LR bullets are plain lubricated lead - very soft lead, no less. Compared to your average hard cast bullet, a .22 slug is almost like butter - soft as can be. Yet I'll bet that if you looked at the bore of your rifle, you probably won't see much leading - if any at all. My .22 rifles will fire a thousand or so rounds between cleanings, and I've never seen lead in my bores despite the bullet traveling at 1,200 fps.
What's the reason? Obturation.
A bullet, under great pressure from the expanding gases behind it, grows in size to fit whatever hole (chamber throat, barrel bore) it is being shoved into. This phenomenon is called obturation. As the bullet obturates it seals the hole, and keeps the gases where they belong until the bullet actually exits the barrel.
If the bullet doesn't obturate, the very hot gases will rush past while it is in the bore. The lead where the gases pass is melted and deposited on the barrel's walls - producing leading. This kind of leading is the most difficult to remove, as it really "sticks" to the bore - as if it's been soldered there. In fact, it has!
It follows that we need to make sure that they bullet obturates in our bore. In order for a bullet to obturate, the metal used needs to be soft enough to deform easily under the amount of pressure being applied to it. If the bullet is too hard, it won't obturate and there will be no sealing.
So, the bullet has to be soft enough to obturate. Why not just make all bullets out of super soft pure lead - won't that cure the problem? No, it won't; a bullet that's too soft will also cause leading, as it won't be strong enough to maintain the necessary seal in the bore. It also won't be resistant to the heat generated by the friction of travel down the bore. Both result in lead left in the barrel.
The bullet has to be hard, but not too hard; soft, but not too soft! The variable is the amount of pressure generated by the firing cartridge.
The higher the pressure, the harder the bullet needs to be to resist excess deformation - but remember that it has to be soft enough to obturate properly. A mild .38 Special target load needs a softer bullet than a fire-breathing .357 Magnum in order to obturate; putting a too-hard bullet in a mild cartridge is as much a problem as a too-soft slug in a hot one.
Bullet hardness is rated on the Brinell (BHN) scale. Pure lead is 5 BHN; "hard cast" bullets can be close to 30 BHN. Somewhere in that range is the ideal bullet for any given cartridge; how do we find it?
As it happens, there is a way to determine the optimum bullet hardness. First, you need to know the amount of pressure your load develops. That's easy - your loading manual will have that information. (Pressure is listed in either CUP or PSI; they are slightly different, but for this particular question either will be close enough to get the answer we need.)
There are two formula: one for the ideal hardness, one for the maximum hardness.
Ideal hardness in BHN = Pressure / 1,920
Maximum BHN = Pressure / 1,422
Let's say it's a .38 Special using 4.5 grains of Hodgdon Universal Clays and a 158 grain SWC bullet. The pressure for this load is 16,700. Our formulae look like this:
16,700 '/ 1920 = 8.69 BHN ideal hardness
16,700 / 1422 = 11.74 BHN maximum hardness
You can (and should) round those to the nearest whole number. Thus, for this load I want a bullet of around 9 BHN, but no more than 12 BHN for best results.
For a heavy .357 Magnum load, using the same bullet, the numbers are dramatically different:
33,600 / 1920 = 18 (rounded) ideal
33,600 / 1422 = 24 (rounded) maximum
Big difference! If I buy bullets of 21 BHN for my Magnum, and use them in the light Special loads, they won't obturate properly and I'm likely to get leading.
Guess what? That's exactly what happened! It wasn't until I bought some bullets of a nice 10 BHN for my Special loads that my leading problem was solved. As I said at the beginning, it doesn't seem logical that softer bullets leave less residue behind - that is, until you understand the physics behind the problem.
With this information you can now go bullet shopping with confidence. You'll probably find that purveyors of "cowboy" bullets are your best choice to get the alloy hardness that you need to keep the lead where it belongs: on the target, not in your barrel!
-=[ Grant ]=-