I actually remember some artwork with Magnus carrying an enormous boltpistol. The things was so fucking huge that he'd hit way more than your head if he shot you. Think using a .44 magnum to shoot a mouse. Though, with his psyker senses (pregoc, heightened mental awareness) I actually think it's quite safe to assume that Magnus was in fact a good shot. I mean, even if he's orginally off target he could probably tweak the bullet's trajectory in mid-air. Though some might consider that cheating.
The human appendix. Proof of a higher power. A divine kill switch so to speak.
No one really likes a smartass, but people tend to like a dumbass even less.