First of all, AR/Chaos isn't really viable. Chaos/AR might be, but IMHO AR doesn't really bring anything to the table and takes up a precious slot if you want to use its grenades. That being off the table, you are picking between Hammer/shotgun and Shotgun/chaos. I'd personally go for hammer/shotgun, mainly to avoid having to reload too often which triggers an annoying GCD which leaves you unable to interrupt anything. With shotgun as a secondary you can do thirty second rotations with just one reload, or using the passive combat reload you can tank without reloads! Also tanking with the hammer PBAOE will be easier than with shotgun's blasts for your basic skill. And burning wrath (5e power ability) combines well in terms of rotations with rocket pod, and rocket pod has a nice passive which can net you a nice damage reduction (26% without diminishing returns accounted for). And with that said, it also is a starting class which makes it very easy to roll. You can roll shotgun/hammer until you got the AP to flip to hammer/shotgun, which is doable by the end of KM.