You can get a 26.5mm flare gun like this which will fire solas flares and then get an adapter insert that lets you fire 12G shotgun shells from it.
This may let you get around the firearms regulations in MOST countries you might travel to if the adapter/shells are kept separate from the flares and flare gun.
I have no probem carrying a firearm or using it in the appropriate circumstances BUT there is a HUGE downside to carrying guns on board when cruising.
1. Virtually all countries make you declare firearms aboard when checking in.
2. If you say you have one...most confiscate them and hold them in bond till you check out....defeating the purpose of having one aboard.
3. If you don't declare it and they find it...you risk losing your boat/big fines/jail depending on the country and sometimes all three.
4. The biggest drawback for me is that you have to return to your port of entry to reclaim your firearm which is REALLY inconvenient if you want to visit a few places then move on. Imagine checking in to the Abagos and working your way down to Gerogetown and needing to go back to the Abacos to get your shotgun.
For me...the inconvenience has led me to the M&M defense....Mace and Machete's! Won't cover every possible situation but the incidence of fire-arm bearing boarders in most places is extremely small and I stay away from places that have any reputation for violent crime.