There are a number of factors that can lead to leading in a cap and ball revolver:
1. Projectiles of relatively hard lead. Pure lead is BHN 4, wheelweights are about 9. I would not use bullets that are harder than 6 BHN.
2. Inappropriate lubricant, such as the hard lubricants used with hard-cast bullets intended for smokeless powder cartridges. Proper black powder lubricants should be moist, and made of natural (animal or vegetable) oils or greases. Petroleum-based oils and greases should be avoided; an exception is canning paraffin, a pure paraffin that lacks petroleum's annoying propensity to create hard, tarry fouling when used with black powder or its substitutes.
3. Mismatched chamber throat dimensions, as compared to the bore dimensions. An overly large or small projectile can create leading. Chambers that are only a little larger or smaller than the bore don't have this problem.
4. A poorly polished bore. I have a 1970s Colt 1849 pocket pistol with machine marks across the lands. Lil' devil picks up leading terribly, and is woefully inaccurate for this reason. I use it for a decorator.
5. A pitted bore. Pitting scrapes lead off the passing bullet and increases with each shot. Badly pitted bores can pick up so much lead that they get lumpy with it.
6. The use of hollow or concave-based bullets. Years ago, in my Colt 2nd generation 1851 Navy, I tried the relatively new Buffalo Bore bullet with its concave base. These left a ring of lead where the forcing cone angles down to meet the rifling. Accuracy was terrible, even with the supplied felt wads under the wadcutter-shaped bullet.
Then I recalled that, during the development of the M1 Carbine, the first bullets made to test it had a deep, concave base. Testers were getting a ring of copper in the rifling ahead of the chamber.
The reason: at the high pressures of the M1 Carbine cartridge, that concave base was blown out to resemble a badminton bird. Each shot, some copper was sheared from the bullet from this flared base.
I realized that the same thing must be happening with the soft lead Buffalo Bore wadcutters and their concaved base.
I've tried the Buffalo Bore wadcutters in a variety of .36 caliber revolvers since, but the Colt 2nd generation 1851 is the worst offender. Accuracy in the other revolvers is no better than the Lee conical, and not nearly as good as a ball of .380 inch, so I stopped buying them. Besides, at nearly $20 for 50, they were expensive.
7. The gods playing pranks. For reasons unknown, because each firearm is an individual, some guns are just naturally prone to get leaded bores, despite all efforts to cure the problem. The gods must have their laughter.
Cures for leading include the use of:
a. Soft lead projectiles.
b. Moist, non-petroleum greases and oils for lubrication of the felt wad, on the grooved conical bullet, or over the seated projectile.
c. Lubricated, hard felt wad between ball and powder. A slightly oversized wad that fits snugly in the chamber when seated, made of hard felt, has a scraping effect upon fouling in the bore. The wad must be made of 100% wool; much of the wool felt sold today is polyester (plastic) and may leave plastic deposits in the bore.
A hard, lubricated felt wad will also seal off the projectile from the eroding effects of the hot gases.
d. A cylinder with chambers very near the dimensions of the groove diameter.
e. A projectile with a relatively narrow area for the rifling to grip.
This is not so much a factor today, with constant rate rifling, but the old original Remingtons and Colts had gain twist rifling.
Gain twist rifling has a pitch rate that increases as it goes down the barrel, from breech to muzzle.
A long bullet in gain twist rifling tends to lead because the front portion of the bullet is turning faster than the rear half. This is why the old conical bullets were so stubby -- to present a relatively narrow area for the rifling to grip.
Balls don't have this problem because their driving band area is naturally narrow.
Conical bullet designs of long ago were also stubby because they were used with paper cartridges, and these cartridges couldn't be so long that juggling them into an empty chamber was difficult, especially when you were dodging enemy bullets.
f. A bore that is leaded will pick up more lead. Sometimes, removing every speck of lead in the bore will cure the problem.
For this task, I use Iosso Bore Cleaner or J-B Bore Cleaning Compound on a tight-fitting patch. Both are creams with very light abrasive that picks up leading and other fouling.
I prefer to use a short length of wooden dowel, with its blunt end, to tap a tight patch through. The patch should have either bore cleaner worked well into its threads.
Often, the first run through of a tight patch will result in long ribbons of lead, scraped from the grooves of the rifling.
For the .36 caliber, I use 5/16" dowel; for the .44/45 I use 3/8" dowel.
I prefer the Iosso Bore Cleaner as it seems to do a little better job than the J-B.
g. Polish the bore with back and forth motions of a tight patch containing Iosso or J-B bore cleaner. This is a paste, a consistency harder than toothpaste, and will smooth the bore. It's particularly helpful in new barrels, to remove traces of protectants applied at the factory.
H. Use a conical bullet with a flat base, or if it is hollowbased then it should have a thick skirt. Actually, I've almost always found balls to be the most accurate projectile available, and easier to load if not cheaper.
An exception is the Lee 450-200 conical, cast of soft lead and lubricated with Gatofeo No. 1 Lubricant (search the net). This has been a very accurate combination in my Uberti-made 1858 Remington. At 20 yards, from a benchrest, it will consistently group six into 1-1/2 or 2 inch groups -- and no leading!
Leaded bores are often curable, but every once in a while one comes along that defies all logic. This is particularly true in smokeless powder firearms.
As Robert Service observed, "The gods must have their laughter."
