1) Titan Mauler lets you weild lower penalties from size.
2 handed Firearms can be used in one hand with -4 hit (but just thought that was interesting).
2) The size of as weapon doesn't affect how many hands can be used.
Thus, Titan mauler can wield large Shotguns (or blunderbuss) with -2 size to hit, but he lowers this with Massive Weapons ability (assuming 6 levels Barb) to -0.
You could get a huge one, but you'd need 12 levels Barb to negate the penalty.
The penalty to hit from size should be fine either way since hits touch AC.
Either way, I thought that was cool that you can technically get a massive gun.
You might wonder where you get the gun? Gunslinger doesn't say the gun must be appropriately sized.
So a Barb multiclassing into Gunslinger now has a free Huge Blunderbuss. Awesome damage (2d6->3d6? or is it 2d6->4d6? I forget weapon increasing size chart says).
<I'm probably going to do this every time a new thread comes up.>