Mike,
I think your findings re. flint and percussion has more to it than just muzzle wear.
One thing that Might contribute to this, is if a percussion gun is converted from flint, the shooting (apparently according to Hawker ) very often suffered as a result.
So, some of the old percussions you tried, if bored for flint, might have been spoiled in conversion.
What you say I too have found. Have a double 18 bore flint, and it would shoot a killing pattern at 50 yards or more, regular as clockwork. (Once shot a right and left at British Jays, at over 70 yards with it)
The boring is important, and we nowadays are at a disadvantage in only having cylinder bored (or jugged) barrels whereas the old ones varied along their length most of the time. (Some tighter at breech, others slacker at breech and V often relieved at muzzle to some degree.)