@Zaffy-Laffy, You've never seen anybody shield cancel with guns? Go talk to some double alch switching gunners some time. Or better yet, just watch this, this, and that. These aren't very fast shooters, but you can get the idea.
It works the same way with all weapons; you can shield cancel after the attack is done, and it's just a tiny bit faster than letting the animation finish on its own (I'd assume).
I'd be curious who shield cancels with guns.
Blitzing? You mean charge attack or just the normal 6 shotter.
I don't think you can though... it is just how the attack animation works
Shield cancelling normally works for swords though