Weapon:4, methinks... but I think the gun might have its own aspect of "Kicks like a mule!" that the enemy can compel for anything from making the wielder off-balanced after firing, to even simply inflicting physical stress (1 point, but that'll add up over time...) from having the weapon improperly braced.
Consequences from the compelled stress of having a gun like that repeatedly slam into you might go along of the lines of "Okay, now, anybody seen my shoulder?" "Dazed And Dislocated" "Recoilless, My Ass" and other ideas along those lines.
However, given what it'll do to your opposition, a little skeletal damage is a small price to pay.