Thanks for those suggestions, everyone.
Daryl,
I have worked over the muzzle, as you have often suggested, so maybe the .360 ball will load easy enough. My only concern is that this is a hunting rifle, so relative ease of loading is a must.
I have ordered a bag of 360 cast balls and will order some .358 balls for comparison. I found some 350 balls that were used for buck loads in modern shot shells, so I will have some of all three diameters for reference.
The lands appear to be about the same width, or slightly more narrow than the grooves. The rate of twist is supposed to be 1:45" but it appears to be somewhat faster, just from looking into the bore of the 39" barrel. The grooves are a semi-round bottom style, but I can't tell any appreciable difference between this one and the grooves on any other barrel.
I gotta say, that the barrel arrived before I anticipated, so I am not prepared to test fire it. I don't even have a 36 cal jag, so cleaning would be a bit of a problem too.
Oh well, an order to track will take care of those details.
Thanks again, and God bless