I don't think it should be possible to actually carry around an Autocannon and fire it from the hip...from my understanding, Autocannons are man portable, in a sense that they can be moved by one or two men into position. but to actually be fired they need to be set up, and shouldn't be possible even for an extremely strong man to fire it without it being deployed on a wall or tri-pod thing. As far as damage goes, I think that is about right, since they are anti light vehicle and aircraft, a space marine doesn't really stand a chance if he's being targeted by one.