A pure lead sphere of .526" diameter weighs exactly half an ounce, or 32 to the pound.
I have a Hawken halfstock built around a nominally .54 caliber GRRW barrel. I haven't gotten around to getting an accurate measure of the bore size, but these are notorious for being more or less undersized. The first time I took it to the range, I tried a .528" round ball from Missouri Bullet Co. and had to use a mallet to seat it. I had some .520" balls with me, though, and these loaded as they should.
If you like fifty-fours, I think it's a good idea to have a variety of balls on hand in sizes from .520" up to .535". It's nice to have a few .526" balls incase the need arises.
In the book, Small Arms and Ammunition in the United States Service, author Berkeley R. Lewis cited several government documents and showed a number of tables indicating the .54 caliber US rifles and single-shot pistols of the first half of the 19th century used a .525" ball in the standard service load. Interestingly, he also noted that "...musket [smoothbore] balls were always considerably smaller than bore diameter. Prior to 1845 the standard difference in diameter of ball and bore (known as windage) was 0.050 inch. Thus, a smooth-bore arm to use the caliber 0.525 rifle ball, had a bore of 0.525 plus 0.050, or 0.575 inch" (page 49). These smoothbores, including the various "cadet" muskets as well as some treaty muskets for issue to the Indian tribes, were known as "rifle caliber" muskets (p. 48).
In the literature of the fur trade, you see occasional references to a "half-ounce trade ball," and we know the most common bore size for Northwest guns was 24 gauge. A true 24 gauge would be .579", but by actual measure, a great many of these old trade guns are closer to .60 caliber. Oddly enough, moulds and balls of .550" are frequently associated with fur trade sites (see Hamilton's Colonial Frontier Guns and Gooding's Trade Guns of the Hudson's Bay Company). An actual .550" ball in a .60 caliber gun would still give .050" of windage.
In any event the .526" round ball is historically significant, and this ball size does still have practical applications for .54 caliber shooters today, whether they simply want a slightly easier-loading ball, or they need the smaller size for an undersized bore.
I'm already in the queue for one of the .526" moulds from Track of the Wolf.
Best regards,
Notchy Bob