Keep in mind a standard bolt is .75 caliber. That is just shy of 20mm, and a bit bigger than a 12 gauge shotgun shell. I can't remember where I read it (i'll have to go through my fluff) but a standard bolter fires in three modes, single, 4 round burst, and full auto. They never do give the size of a heavy bolt shell though. I have some free time coming soon. I'm going to model a Stalker Pattern Bolter, and the Vindicare Assassin weapons.