...For example, my personal favorite is the Heavy Hunting Weapon.
A group of intrepid adventurers arrives on a TL 5 backwater confidently superior in their cloth armor and gauss rifles and generally contemptuous of the backwoods militiamen. Until they see the heavy armor-piercing local rounds punch through the hide of the local fauna (and suddenly feel less safe in their vacc suit ‘armor’). And for up close and personal, these TL 5 ‘bumpkins’ have the use of any special shotgun shell from any Traveller book or TL that can be fired from the same ‘common’ hunting arm. ...
Actually, that comment makes me a little less interested in the book. A serious heavy TL5 armor-piercing hunting arm approximates the Terran elephant gun. Okay so far: the local militia feels the need for a large-bore high-power weapon to take on some problematic local beastie. The thing is big, heavy, the rounds likewise. One presumes they aren't fighting wars with each other, 'cause someone's going to get the bright idea to beat the other side by equipping some of the men with lighter rifles firing lighter rounds so they can carry more rounds into battle and resupply more easily. Someone might even come up with the idea of beating the cannoneers by using some variant of the TL5 Winchester. But then this man-portable cannon can fire shotgun shells - which means it is coincidentally the same diameter as a shotgun, and it is smoothbore, foregoing the advantages of rifling.
I'm probably missing something. My knowledge of firearms is somewhat limited.