Originally Posted by Feriority
The covers of the Cain books are basically propaganda posters. They show CAIN using a bolt pistol, even though in the books, he turns down anything higher powered than his laspistol because his experience with the laspistol makes aiming it reflexive for him. They also show him standing triumphantly among the corpses of the enemies of the Imperium, which isn't particularly Cain-like.
And Colonels and Majors having bolt pistols is rather different from the rank and file getting the same; if I remember right, they don't even show any officers below that using bolt weapons, though I might be wrong. Heavy bolters are used as support weapons, and regular bolters don't come up much, as far as I remember.
Even Cain's suicide mission squad in For the Emperor, when getting armed with better armor and weapons, only gets hellguns, not bolters. Vail uses a bolt pistol, and Cain keeps his trusty laspistol.
I always got the impression that Bolt weapons were not unusual for Colonels, Majors, and Commissars. I don't think they were actually "standard issue", but at that rank you get to use your influence to get your hands on special gear, and you can get techpriests to maintain your gear for you. Cain's choice of gear seems to be the bare minimum for Commissars, I always got the impression that he's been using the same chainsword and laspistol he was issued with at the Schola. With the exception of some Carapace armor he hasn't really made an effort to get his hands on better gear. Which is a little odd, I would imagine that there must be at least one noble family on Perlia that tried to foist some family heirlooms onto him after the whole Korbul thing.