Right, so we all know the -4 to Hit penalty of using one of these guns is a joke. The reason it's a joke is because you are a gunslinger and you target touch AC, so a -4 to hit in exchange for +1d12 for a single target or 1d8 for multiple targets is pretty good.
Besides, it's kind of fun lugging a giant gun around.
The way it works I mentioned once in N. Jolly's guide thread about the Gunslinger. You cannot be knocked prone, while prone. Dropping Prone is a free action, and there is a single level dip with full BAB that will get you what you need. Namely, Unarmed Fighter and Monkey Style.
Monkey Style provides you with NO penalty to AC for being prone, and allows you to stand up without provoking AoOs. It also allows you to stand back up as a swift action. Together this negates ANY penalty to being prone, and actually grants you an advantage to being prone: the +4 to defense against ranged opponents.
With this one level dip, you can make use of these powerful firearms. From there you need to make use of Musket Master and alchemic cartridges combined with Rapid Reload for free-action reloads and you are all set. You are a level behind on getting Dex to Damage but the extra d8/12 is too good. Especially with Rapid Shot.
At level 6 if you hit with all 3 shots you have an extra 3d8 (per target) or 3d12 damage which comes out to an average of 12.5 damage for the Culverin and 19.5 damage for the Double Hackbut.
If you want to use a Culverin, consider the Siege Gunner archetype, which grants you your gunslinger level in precision damage for a grit point. This archetype stacks with musket master for a wonderful burst or death aimed wherever you please.
Build:
Unarmed Fighter 1/Musket Master 5 (Siege Gunner 5 Optional)
BAB: +6
Saves; F6, R4, W1
HP: 10+5d10 HP
22 Skills
Feats:
1: Improved Unarmed Strike
1: Monkey Style
1: Point Blank Shot
3: Rapid Reload (Firearm of Choice)
5: Rapid Shot
5: Deadly Aim (Siege Gunner)