There are 4 buildable "master grade" weapons.
Super Shotgun: Powerful for a shotgun at 36x7 as well as extra accurate and relatively lightweight. However single ammo type, and shotguns are already very accurate as is. Not a good choice.
Commando Rifle: High damage, accurate, fast to fire, single shot rifle. Scales with reactions. Downside no gunbutt strike, and not a sniper given H2s LOS penalty. Okay pick
Bossar: A hybrid Rifle/Autogun. 60 damage is nothing to scoff at and it's accurate even in x5 auto mode. However rather weighty at 43 loaded and has neither sufficient rate of fire to truly benefit it's armor shred or really stellar per shot damage. Meh pick
Kustom Blunderbuss: Okay now here's something to get behind. Powerful, accurate, fast, multi ammo shotgun. Gold ammo scales off TU which while oddball makes for alot of punch in the right hands. Also has chem and concussive ammo. Can use blunderbuss ammo too. Smidgen heavy at 33 loaded.
My list best to worst:
Kustom Blunderbuss
Commando Rifle
Bossar
Super Shotgun
Given that you can't disassemble them to get parts back the blunderbuss remains relevant for longer, and in more situations. Also lower in the tech tree.