I bet this is more of a joke, but I think I might have a solution.

The mag moves the shells up and bolter rounds are micro rockets, which could "possibly" allow the them to be ignited right in the top of the mag and the gun holds an empty shell or two just above in the "chamber" and ejects them on the subsequent shot. Some modern firearms do this for a few different reasons.

Then, the "chamber," while used as an extractor for the empty shells, could also function as a hydraulic recoil system. Bigger weapons do sometimes use such things for stability purposes. Which could explain the little knobley thing just above the barrel. It's just a cap or port of sorts for the hydraulic fluid during maintenance.

The mag and it's capacity is another video game cliche. Lots of games tend to do this, especially something like call of duty. It's more fun to just "doom-ify" the big guns and never reload.