d.6B > d.5B wont work midscreen for starters, but since that's not your problem, I'll move on.
When doing that combo, its usually best to instead do:
5A > 6A > 6C > 236A > 66C > j.D > d.6D > d.5C > d.6B > 236D
However as far as midscreen 5A combos go, you may want to practice:
5A > 6A > 6C > 2D > 623D > 66C > j.D > d.6D > d.5C > d.6B > 236D
In my experience, its not that reliable to do a 5D combo after an optic barrel link, not to say it wouldn't work, but its probably too much trouble for its reward.
Another problem is you're training dummy, Bang's hitbox isn't nice to 5D > d.6B combos, they still work on him, but they are made much harder.