From SAAMI from their official glossary:
CHOKE, JUG: A type of shotgun barrel choke, wherein a slight recess is formed in the bore approximately one inch behind the muzzle. The recess causes the shot to gather before leaving the muzzle resulting in a denser pattern.
BACK BORE: A shotgun, chambered for a specified gauge, whose barrel bore diameter is greater than the nominal specified for that gauge, but does not exceed SAAMI maximum.
That being said, I would think Jug Choking would be your best option as the metal is releaved just behind the muzzle of the gun where pressures are at their lowest. The 'smith would use a special reamer, lengths vary but I.I.R.C. the norm is around 2.5" to 4" total. A small amount of metal is removed with the remaining "choke" tapered on both ends. There is very little metal removed and it has been used in very thin round barrels.
There are different definitions out there for backboring. The one listed by SAAMI above is touted by some as simply "overboring" and not true back boring which some say is releaving only the area imediately in front of the
chamber...in the case of a muzzle-loader you would have to relieve the whole breach area as they don't have "chambers". Relieving this area may not be practical in a ML barrel as the thinnest portion would be in the highest pressure area of the barrel and you may have to rebreach and go with a bigger plug. Either way your breach/bore is bigger so in essence you just have .53 or .54 caliber at the breach or full length....what's the point? You are, in essence, overboring already by having the barrel reamed out.
Advice on thinnest barrel wall thickness is best solicitated from the 'smith doing the work but I know Getz and probably others have .54's in 7/8" octagon and they have tested safe with full power loads.
Hope that helps. Enjoy, J.D.
P.S. Thanks, but don't bother defending me with Roundball. He stuffed me in the box the first day I was on board I.I.R.C. I can't help him and I have no time for childishness....which is the only way I can explain his "ingnoring" so many people. I do however support school yard humor and good old fashioned teasing. :blah: