A .40 Cal with a swamped 13/16" barrel even 44" long can provide a well balanced rifle. The butt stock comb and most important Cast off and LOP depending if you are right or left handed makes or breaks the deal for off hand rifles. If you can find an orginal early flint lock with a straight wall barrel it will be most rare. The orginal flint locks pre 1820 before the use of mass produced hammer forged barrels were swamped or tapered.
The modern semi-custom rifles like TVM etc cut cost with straight wall [Perc. Barrels] and no measured cast off. These are muzzle heavy and all but impossible to shoot 20 strings of 5s in an off hand match. I have one Don Getz 43" 15/16" .54 straight wall 7 flat groove but it is a dedicated cross stick rifle.
We love our "Rock Rifles".