I sure can't think of any off the shelf. I think some of the barrelmakers would do it as a custom job for more dollas tho. A .56 caliber sounds nice to me but the only one i've seen besides on antique rifles is a smoothbore. Maybe someone else can elaborate.
I checked both Colerain an Green Mountain websites and it appears that barrels jump from .36 to .40 calibur . No .38 cal. barrels were listed. You may have to go the custom route. hope this info helps.
If you want a .38 I would jump the little bit and use a .40 caliber. Ball is easier to find, they do not foul as much, you do not need a custom barrel, and in some states that rifled caliber is even legal for deer hunting if you wanted. All though that would be a small deer caliber IMO.
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