Although most firearms are banned, apparently, by the Coast Guard on civilian vessels (or so I’m told) and at many foreign ports, they usually do allow flare pistols.
For a while someone was making steel inserts that would fit into the chamber of a flare gun and accept a pistol or .410 shotgun shell; sort of an “adapter”. Although only a single shot, it would be better than nothing and the chamber insert and a few rounds would be easier to hide aboard a boat than a shotgun or rifle, I would suppose.
I made an 11-inch long 1” OD .58 caliber muzzle loading barrel out of schedule 80 seamless steel pipe that slides into the breech of my German 26.5mm flare pistol and extends out beyond the muzzle. I use if for firing salutes on the 4th of July, but I could have just as easily bored it out to take a 12-ga. shotgun shell - although the recoil would be brutal and I would go to jail if the BATFE ever caught me with it.
Some Gunsmiths have made similar inserts that chamber .44 magnum, .30-30 rifle rounds etc.and are rifled..
the tricky part is getting the extractor to work. Otherwise you have to poke the empties out with a stick from the muzzle.
I recall quite a few years ago reading a story somewhere of some poor bastard who had to spend a lot more time in St. Kitts than he had planned for violating this.
Also very good+ surplus Egyptian Ljungmann rifles in 7.92mm were available cheap. 10 shots. They wouldn't get closer to you than 500 yards without hurting.
Don't think you're right. You can take along any gun you want in US waters or on the high seas. The only problems come when you make port elsewhere.
My understanding is that most decent places allow guns aboard ship to be left in the custody of the harbor master or customs office, and not treated as if they have been imported into the country. There are probably significant amounts of paperwork involved.
At the other extreme, Jamaica supposedly will give you life in prison or hang you if they catch you with a gun anywhere in their 200-mile exclusion zone.
Experienced yachtsmen are invited to elucidate further.
-ccm
-ccm