To be a bit creative about "run and gun" style of an ability:
Steady Assault (toggle-able "stance" or maybe gunnery specific ammo, making choice between ammo regen and mobility): Reduces your movement speed to '70%' of normal, but makes you completely immune to slows, roots and interrupts and makes you able to cast on the move.
I think I'd actually like something like this a bit more; it seems like a nice compromise between Run and Gun with something like Cover. Considering you would essentially moving at the speed of a sloth while it is active, I agree with the immunity to slows and roots. There is no need to be both immune to interruptions AND able to cast on the move. I'd also say this would need to be high in the Gunnery tree too, assuming it is not a base Commando ability. If it were the latter, it would make for some VERY interesting Combat Medic hybrids. Probably utterly overpowered ones at that (ie. a healer that is passively immune to interrupts at all times).