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 this is way too clever by half. I'd much rather have the "run and gun" proc attached to stock strike that's been proposed. Casting on the move is pointless when you have 70% movement speed, especially since you'd probably be unable to stance dance properly. Even snipers have to displace a decent amount. You still have to run, and when you do 70% movement speed reduction is not gonna be something you want.