I saw pipe rifle sold at Shady Sands trader.
EDIT: Killian can sell sawoff shotgun. My first visit there doesnt find it, but on 2nd return, 80 days later, he spawned that. No doubleshot mode for that one or pure shotguns though.
Khans got two sharpened spears, which is the only group got this kind of weapon so far.
The Raiders at Irwin Farm got .44 revolver, which is the reliable place so far for it.
Grease gun in the hand of Boneyard's combatants, and sold in Gunrunners. I dont remember the Hub's merchant like Jacob stock it. This is the upgrade after SMG, though you can find or buy combat shotgun at The Hub. The somewhat rare encounter with gangs around Boneyard can spot one or two guys wielding grease gun. A rare chance (require very high luck) can have Adytum merchant stock this. But the bottleneck is .45 caliber bullets. It's rare as hell and you are going to stock this for quite a while just to burst it all off in a few battle.
Jacob and Gunrunners quite rarely stock jackhammer. And with my single digit barter, I have to sell almost all of my inventory to get one~ They never stock Bozaar in my two games of trading with them. But turn out it's a matter of high luck, as Luck10 build get this twice in a short period.
Saw Laser Rifle ext cap at Military Base, no earlier. Its extended range does provide alternative to where the Turbo Plasma Rifle cant reach, like, to shoot at Rocket/Minigun Super Mutant from extended range. So there's a chance the rockets/minigun would miss.
The pulse pistol, also at Mariposa, is of lesser utility to my Jinxed Luck2 game even without the nerf: we quite likely meet destroyed weapon events. Katja lose 2 so far. So without spares, this usage of pulse pistol is numbered in days~ Also ONE laser pistol with magneto lens.
A small thing of note is assault rifle with expanded mag. i dont remember it's from a locker, or from a robobrain. but I got out of Mariposa with a couple of them. Also highest luck can have gunrunners spawn this.
Zip Gun, which is basic energy gun, is in the hands of random encounter gangs around Boneyard, which I dont meet anywhere else. But at that time we probabbly already had plasma pistol from Necropolis, or Gunrunner. Zip gun will only be useful if player ever do a straight run from the Hub to boneyard AND get a Zip gun gang right then. So it's of use from there until necropolis.