I gave a look on a lot of BM videos and information and here my opinion…
The Quarrel Shooter build is a trap, it looks cool but I think that a skill based build might be stronger, specially with Full Leather Jacket, Mozambique Drill having low cooldown, lothe of OH and high % damage. Smash bullet looks great too, so putting only one skill point in the dual stance might be optimal in a BM 2 build.
The best skills don’t require the Dual gun instance so a SR 3 - BM might work great even as a AA build with Limacon having 20 SP per shot (use Arcane Energy people, we already buy Enchanter Fire scroll anyway), I would focus on Silver Bullet, Full Leather Jacket and Napalm Bullet.
Blood OverDrive is trash, the hits are just like the animation, random and have a max of 17 hits.