As an alternate to that, (although that could work too...) Given his background (hated by the military?) It'd be easiest to assume that he is a mercenary, and is loyal to his employer...<br/><br/>We could assume that he's employed by our rogue trader, and (say, thanks to some reason injuries in close quarters combat, and a sudden realization of a greater need for an experienced gunner) is employed by the party's Rogue Trader himself, Cortez!<br/><br/>Being thrilled at getting the chance to prove himself in service to a powerful Rogue Trader (and seeing the opportunities it offers someone like himself), his loyalty will be very strong.<br/><br/>I'm a bit leary of shotgunning Tagan's plan to buff up the moral of the Ultramarine ship... and from the birthright description, I could see reason to value his current contract over his old Ultramarine background... at least to the point of doing what's best for the group...