Bsharp - Ed Rayle, I have heard, makes a 16 bore slightly oversize so a .662" Lyman mould (standard) can be used.
Varsity - my whole reason for choosing the .69, was for the "best" all-round calibre for large and/or dangerous game, as per Forsyth's book.
My rifle has a 66" twist, .012" rifling depth and has shot a number of 1" to 1 1/2" 5-shot groups at 100 meters, from a bench rest. I also got one
2 1/2" group at 200yards with it, again, bench rested. These all "took" 165gr. 2F GOEX & .030" denim (12ounce) patches. It will shoot decently to
100yards with a lowly 85gr. 2F GOEX & .021" denim (10 ounce) as well, using the same .682" ball. With the .675" ball, I used the same .030: denim patch.
So, maybe the 66" twist is best afterall?
GRRW bl. 1 1/8" flat to flat. .690" bore .714" groove diameter 66" RoT.
The groove at 11 o'clock shows some rod wear, but doesn't seem to hurt accuracy. About 5,000 shots through it.