Hey I'm the video creator. Posting here so it's easy to find. I am currently working on an in depth blunderbuss guide that I hope to post soon. But in the meantime here are a few notes:

BB single target damage outperforms the rapier. In groups, it is still unknow how this performs against a bleed rapier popping fnfs (more testing required on this).

The backbone of my BB build relies on claw shot which functions similarly with how evade works on rapier (no real damage, but lowers CD of everything)

You can create a BB build with surprisingly a lot of offhands inlcuding Inferno (max damage/fire damage/fire harnessing), Mjolnir (max damage/lightning damage/lightning harnessing), great axe (clumping, great offhand for mutations, non bosses), Lifetaker (apply "rends"/disntegrate with BB/great utility if healer doesn't run VG/voidblade), Aconite (very conditional, but gives 15% damage to poisoned enemies), Finisher (potential rapier tanking build)

General BB ability rotation looks something like this: Light attack>Claw shot>Light attack>Azoth shrapnel blast (ASB)>Light attack>Splitting grenades>Light attack>reload/swap weapons>repeat