Full Build Spec found here - http://bl2skills.com/commando.html#55050145015500515010005055004000
Derp 12 Pounder - Togue Grip, Torgue Exhaust, Tediore Sight. Derp prefix gives highest damage per shot, increased damage from the Torgue Grip and Torgue Exhaust, and increased reload speed from the Tediore sight.
The Sham - 94% chance to absorb enemy bullets and the secondary projectiles from guns like the Ker-blaster and Logan’s Gun as additional rocket ammo.
Low level Logan’s Gun - Does minimal self-inflicted damage while still being able to replenish rocket ammo via the Sham when fired at feet. I use a level 17 Logan’s Gun obtained as an assigned drop from Wilhelm during NVHM.
Magic Missiles - Mainly used as a slagging instrument. Could have also used a slag singularity, slag transfusion / O Negative, or even rotate in a Slagga and free up the grenade slot for an explosive Fastball.
Legendary Engineer class mod - Use this if you want to get the most damage out of the 12 Pounder. The constant turret usage will also soften up your enemies, thus making them easier to blow up.
Explosive Elemental relic - Get more damage from your 12 Pounder. Boom baby.