According to a recent post a 45 colt rifle, loaded with a 410 shot shell has been determined by the PGC to be legal for small game despite the regulation saying no rifle over 23 caliber.
A 50 caliber smooth bore would be a 32 ga.
The answer is that it probably depends on the WCO's mood that day.
I have a 58 caliber centerfire rifle for which factory shot cartridges were available. ( likewise, factory 44-40 shot cartridges were available for 44-40 rifles) My feelings are that it would still have rifling and exceed 23 caliber even though loaded with shot.
However, the regulations say only single barrel muzzle loaders during the regular firearms season, yet the digest says ANY muzzle loader. So anybody's guess.
(1) Permitted devices. It is lawful to hunt turkey during the spring turkey season with any of the following devices:
(i) A manually operated or semiautomatic, centerfire shotgun or muzzleloading shotgun using shot ammunition no larger than #4 lead, #2 steel or #4 of any other composition or alloy of nontoxic shot approved by the Director of the Unites States Fish and Wildlife Service or an authorized representative under 50 CFR 20.134 (relating to nontoxic shot).
(ii) A bow and arrow as permitted under subsection (a)(1)(iii).
(iii) A crossbow and bolt as permitted under subsection (a)(1)(iv).
(2) Prohibitions. While hunting turkey during the spring turkey season, it is unlawful to:
(i) Use a centerfire, rimfire or muzzleloading firearm using single projectile ammunition.
(ii) Use or possess single projectile ammunition, except arrows or bolts.
(iii) Use drives or any method other than hand or mouth calling.
(iv) Use or possess an electronic caller or a live turkey as a decoy.
(v) Use a device not provided for in the act or in this subsection.
Not real clear. The inline is not a shotgun under (b) (1) (i), but if loaded with shot, it is not prohibited under (2)(i) or (ii). It seems to fall into an in between area.
The Eley Brothers, (Eley Ammunition makers in England) started by manufacturing a wire cage for shot to be muzzle loaded into shotguns that would concentrate shot groups at very long distances. The wire cages wouldn't tear apart until down range several yards. It was a flop in the 1800's because of price. Anyway, I am not sure if it would have a benefit for turkey hunting or not.
In addition, most in-line barrels are rather short for shooting shot anyway. Some folks claim that rolled newspaper shot cups make a big difference in shooting shot from muzzle loaders. I recall reading that early reloaders, would drip wax into the shot in the shell on the theory that range was extended. But it reportedly had negligible effect.
I would figure that if there was a trick to getting tight long distance groups with a muzzleloader, it would have been discovered a century ago.
This was 2 ,50gr powder pelets, a sabot type holder a ton of #4 shot and a small piece of cotton ball, it put a ton of BBs in the turkey target head at near 70 yards. Nothing like I ever seen. The middle of the pattern was on the neck and it took the neck off the target. My 12 gauge with #4 buck and the best choke I have found does not compare at 50 yards to what the inline does at 70.
I would say illegal because they are probably talking smooth bore when they say shotgun.
I always heard that rifling in the bore would cause pattern issues but honestly ive always wanted to try something like that out of one of my MLs just out of curiosity . It won't hurt the gun and you never know till u try. I dont feel like buying a black powder smooth bore just yet, but a friend of mine who shoots one laughed at me when I bragged about getting 230 hits in a ten inch circle at 40with my 870. He said his will do that at fifty on a bad day. I never saw the proof but i know the guy enough that i dont doubt him. If you do some range work, post your results with pics. Im curious.