I don't think leading will be a problem. I've never seen a leading problem even with cap and ball revolvers, which shoot balls of groove diameter.
However, shooting as described (powder, undersized ball, wad) will probably result in a rapid buildup of fouling. I did considerable experimentation a couple of years ago, trying to replicate native shooting methods with trade guns, based on what I had read. Trade guns nominally of 24 gauge appeared to run from around .570" to .600", based on actual measurement, as documented in the literature, while the standard "trade balls" were better standardized at about .550". I tried .550" and .562" balls in my .58 caliber/24 gauge Northwest gun, using tow, shredded cedar bark, and palmetto fiber wadding, over powder and ball, and just over the ball. Accuracy was poor, but that's beside the point and I'm a lousy shot anyway. What I found was that fouling accumulated heavily and rapidly. After as little as four or five shots, a .550" ball would not drop all the way to the powder, and had to be rammed. The .562" ball was about the same. If I remember correctly, fouling was worse with just the over-ball wad than with wadding over both the powder and ball.
I suspect, but can't prove, that the excessive windage of the undersized ball allowed a greater buildup of fouling. Ramming the over-powder wad had some minor "cleaning" effect. It is also worth noting that all of those wadding materials are flammable... In fact, shredded cedar bark and palmetto fiber make first-rate tinder for fire starting with flint and steel. Much better than flax tow. In any event, I thought (but could not prove) that the flammable wadding material might contribute to the fouling.
All of this, in my mind, helps explain the pictures of Indians carrying a spare wiping stick with the worm permanently mounted... They probably had to wipe the bore pretty frequently.
However, I saw no evidence of leading at all.
Best regards,
Notchy Bob
