Wheelweights is about as hard as you want to go with a cap and ball revolver.
The problem lies in seating the ball. It requires force to seat the ball in the chamber, and if you use hard lead a considerable amount of force may have to be applied. This is hard on the rammer parts.
Wheelweights typically measure 9.5 Brinnel Hardness Number (BHN). Pure lead measures about 4 BHN. Lead alloy of 1 part tin to 20 parts lead (1:20) measures about 8.5 BHN. Linotype measures about 22 BHN; it's much too hard to use.
There is another problem when using hard bullets and black powder: leading. For reasons unknown, hard bullets tend to cause leading when propelled with black powder.
Conversely, soft bullets propelled by black powder cause no or very little leading, all things being equal (diameter, design, bullet lubricant, etc.).
I think you'll find that hard bullets, when used with black powder, will give you leading problems --- especially if you use a petroleum-based lubricant such as Alox.
To get the best accuracy from black powder loads, and to avoid leading, you must use a soft lead bullet, a lubricant made for black powder use such as SPG or Lyman Black Powder Gold and a bullet sized to no more than .001 over the diameter of the chamber mouth.
As far as using hard bullets in that Single Action Army with smokeless powder, no lead alloy bullet is too hard --- as long as you're not using maximum or near-maximum loads.
The Colt Single Action Army is not nearly as strong as the Ruger so pay particular attention to which loads you use when you peruse your reloading manual.
NEVER accept a load on the internet at face value until you've checked it against a recent reloading book or a reputable source. The only exception to this warning about the internet is the actual websites of the component makers themselves, such as Speer, Hornady, Alliant, Winchester, etc.
Don't use loads in books older than about 10 years. The reloading books of the 1970s or older are particularly suspect. Components have changed over the years, especially the powders.
The Unique powder made by DuPont in 1973 isn't the same critter as today's Unique made by Alliant.
Some years ago I read of an experiment to wear out a Smith & Wesson .38 Special using nothing but lead bullets. After firing 100,000 regular strength (not Plus-P) cartridges the testers gave up. There was very little wear on the revolver and it was still quite serviceable.
Conversely, jacketed bullets can cause measurable wear to a bore in 10,000 rounds or less.
The lesson is clear: if you wish your revolver to lastfor generations, you won't hot-rod it and you'll shoot lead bullets.
