Any competent blacksmith at any military post or in any village would have had the capability of cutting down any musket barrel. They forged them by hand and had the capability of trimming them if needed. Hacksaws were part of the blacksmiths kit, along with proper files for finishing. Check with Colonial Williamsburg.
Hacksaws were just not in every tool box on every farm in those days.
Even the Native Americans in remote areas managed to cut down their fusils, or trim off the ragged stubs of barrel when they blew, keeping them usable for extended periods.
Build the smooth bore of your choice and shorten it to the length you desire. As I posted before. Rogers Rangers did the same, shortening their Bess Carbines to 30". It's not issue length, but is PC, as is shortening of any firearm when necessity required it.
I myself, have shortened modern firearms to meet my needs on many occasions, all legal I assure you (except for that shotgun I carried back in '72 when bad people were trying to do bad things to me. That was a bit stubby!). I once did a very professional trim on a .22 autopistol so it would fit in the clove box of an AMC Gremlin (showing my age here).
I thought they called them "CARbines" because they were short enough to use inside the car!!! Guess they would have been wagonbines beck then. :hmm:
The artifacts show that our ancestors did the same things we do. If you are fighting in the woods or close in and the barrel is too long you cut that sucker off! If you are running buffalo you cut it even shorter! I can see that 18th century frontiersman right now, rubbing his chin, trying to decide if cutting his $.50 musket barrel down is going to be a confusing factor to some 21st century history buff! :hmm: :crackup:
We can only offer opinions. You must make your own choices, just as you would have made while standing in front of the blacksmith as he reached for his hacksaw.
Unlike us today, they had the option of doing anything to any gun they owned with out repurcussions from the legal system.