Have you looked at [ The Gun Works ]?

They have an English Sporting Rifle in several calibers.

"The English Sporting Rifle will feature the following:

Available in 45, 50, 54, 58, 62, and 69 caliber
Oregon Barrel Company tapered octagon 1-1/8" to 1" barrel
Barrel lengths: 28" to 36"
Twist and rifling depth are made to order. Forsythe Rifling available.
Davis lock and single trigger
Standard length of pull is 13-3/4"
Available in two grades of Maple or Walnut:
Select: with figure, but can fade out
Select Plus: with figure throughout"

They also have a 4 and 8 bore.

I know nothing of there Quality, but I do have one of there .69 cal barrels.

With .120" land and .160" groves, slow twist. looks good!

Have not shot it yet.