Law of the Sea treaty applies outside UN approved borders... and it also has some considerations... such as requiring non-lethal measures be used and ignored by the belligerent before lethal force can be used. Likewise, it's entirely the Skipper's choice on whether to authorize use of force, but if the admiralty or the UN don't agree, the skipper's personally liable. The crewmember using the force may also be liable.
Then there's the issue of flagging state. the flagging state can deny a captain the right of self-defense of the crew and/or vessel. US vessels have to observe US federal law in US territorial waters; they're expected to operate under US Law or international law while in non-territorial waters or foreign territorial waters, and foreign law when more restrictive in their territorial waters... as limited by the Law of the Sea treaties still in force.
There's no where on earth one isn't subject to some bit of law or another ...
Still not seeing why any of that is an issue with snub pistols in Traveller.
The classic ones in the book are revolvers. Put 2 rounds of tranq, 2 rounds of solids and 2 rounds of HEAP and you've pretty much covered the "non lethal first" issue.
US Federal law has issues with explosive bullets in civilian hands. Some states have additional laws.
If you're on a US flagged ship, yeah, you're right, you're still under US law.
On the other hand, at TL7-8, there's not much reason for 10mm HEAP. At TL10-12, when Diplo armor is effective enough that hijackers might be wearing it, or you're dealing with pirates in armored vacc suits... yeah, I can see the logic there.
I suspect that if the US hangs in there until we get to TL11, the law will change. Heck, with some of the court cases and bills to repeal the NFA, it might be before that.... which would be really interesting.
For our purposes, though, the Third Imperium doesn't have issues with people having ... well, anything
on thier ships . There's LL0-ish when you're outside the planetary law. Nuclear ship-to-ship missiles are a grey area where a non-military ship is going to either not have them or have to jump through legal hoops. If you stop at an LL0 world and buy FGMP-15's for you whole crew for "shipboard security" the IISS Customs Boat might shake their head at a poor choice of indoor weapon, but not be able to confiscate them or even write you a ticket until you walk outside the ship planet side.