Most of the rifle barrels offered by the major makers that include, Rice, Colerain, Green Mountain and others will offer the production twist rates of around 1 in 66". That should be a good twist rate for round ball shooting for both hunting and target shooting.
The half stock barrel length is best to be from 32" to 36" with 32" being a bit easier to handle. One would want a tapered barrel from breech to muzzle (not swamped) to lessen the muzzle heaviness while improving the balance. If you can get a hollow under rib. This will lessen the muzzle heaviness.
I would suggest a hooked breech with a chambered flint lock breech plug. These are more difficult to properly fit but make cleaning so much easier. First the breech plug needs to be fit to the barrel. Both the shoulder of the breech plug and the threads should bottom out with the threads in the barrel. Second, the hook needs to be fit to the tang to perfectly match the hook to the tang. Once the fit is perfect, epoxy the breech plug to the tang. Inlet the tang and breech plug onto the stock. Continue to inlet the barrel with the stock tang and breech in place. Heat the breech plug to break the epoxy and clean up the surfaces.
Of course, you could use a fixed breech as I have on my 1803 Harper's Ferry Rifle and my Derringer Rifle. Use wedges to hold the barrel in the stock. The fixed breech is almost as easy to remove from the stock as a hooked breech. Remove the bolts holding the barrel in place, remove the wedge and the barrel can be lifted out. Note: I have lock bolts that go through the tang part of the breech plug, so I have a tang bolt and a lock bolt to remove on my Derringer rifle.
