Ah yes, the humvee. I like that line from the humvee commander in the movie Black Hawk Down. "These things are bullet magnets!"
I suppose a possible answer if they're determined to have heavier armour is compartmentalised and autonomous setups. Having the operator in a separate location to the cannon could reduce the armouring requirements of both and splitting them up could make delivery separately on those c130's possible. Of course the draw back is a platform far more susceptible to electronic warfare.