If I may,
I belive it was in Inquisitor or Necromunda that it was made clear that Bolt Pistols are 50 cal, Boltguns are 75 cal, and heavy bolters 100 cal. It was also stated that these were all nominal values, with a great deal of variation between patterns (how the hell does the Ordo Munitorum do it?).
I have to do a tonne of digging to find the source material, but Adeptus bolters fire self-propelled shells out of a standard cartridge. Simply put, their is a boom in the gun, and then the rocket lights off after the round clears the barrel. Thinking about a 75 caliber shell, you are talking about something akin to firing a solid slug out of a 10 gage shotgun, with a one handed pistol grip.
Non-astartes bolters have similarly been described as caseless, so the round is a pure gyro-jet with no casing to eject. This makes the individual round lighter, and cuts recoil to almost 0. The down side of this is the same is real gyro-jets, they take a certain amount of time after clearing the barrell to reach lethal velocity. Depending on the design 5'-30' for the real thing.
The best illustration of the difference would be putting your hand over the end of the barrell. For a non-adeptus, the round has aceclerated from stop for about 8", and would hit your hand. The rocket would have enough power to push your hand away, and would eventually foul the barrell, and possibly detonate the rest of the rounds sympathetically. Doing the same thing to an adeptus bolter would result in you hoping you have enough requisition for a cyber hand, because your's would be gone, and probably not even trigger the mass reactive detonator.
Also of interest, way back in 2e, bolt rounds were listed as having a fail safe that limited their effective range. It would disable the mass reactive charge so missed shots didn't turn into a mine field the marines would have to advance through.
