For the record, a .69 diameter bore is a 14 guage. A .729 diameter bore is a 12 guage.
A 16 guage bore is .662 in diameter so I think that would be the size Treestalker would want.
My old Dixie Gun Works catalog says the Zouave made by Armi Sport/Italy had a barrel that tapered from 1 1/8" (1.125") at the breech to 15/16" (.938) at the muzzle. It also shows a Zouave made by Euro Arms/Italy that has a barrel that tapers from 1 1/8" at the breech to 7/8" (.875") at the muzzle.
The Armi Sport's Zouave bored out to .662 diameter would have a barrel wall thickness at the muzzle of .138".
The Euro Arms Zouave bored out to .662 would have a barrel wall thickness at the muzzle of .107".
My CVA 12 guage double barrel shotgun has a barrel wall thickness of .065 at the muzzle so, either of these Zouaves would be thicker than that so, I think boring a Zouave out to a 16 guage size (.662") would be quite safe. Of course it will still handle like the Rifled Musket that it is so it would be a very poor choice for shooting birds on the wing.
Now, the real problem is finding a good barrel maker that would be willing to bore the guns barrel out.