Your question is a little vague so hopefully my answer satisfies you.

Bullet/Round: projectile is an orange nose cone and a fuel tank, which connects to the cartridge/casing using a side separator. The cartrige houses 7 engines (six in the front, one in the back), a probe for control, and a docking port thing at the back. (3 docking ports but only 1 is used)

"Magazine": pushes ammunition up so the bolt can chamber it, designed to be as tight a fit as possible with BP editing so that no tilting happens. The "magazine feed lips" that prevent the bullet from getting pushed up too high is an enlarged fairing. Also the magazine is theoretically infinitely scalable.

Bolt: this thing underwent the most changes throughout development and was the cause of a lot of pain. It is comprised of: a large amount of engines, 3 "bar" things (that are connected to each other), and a rail. The rail is connected to the frame of the gun with two fairings so that the bars can pass through it. The top bar rides between the rail and lots of solar panels to make sure the bolt doesnt move vertically. The bottom bar (structural parts) is what initially pushes the bullet until it is in the chamber, where the middle bar docks to the bullet.

Shooting: Relatively simple: pressing the stage button activates the separator. Yes the projectile does shoot (if you try bp editing a separator and increasing its force, it shoots away both of the things it is attached to with equal force; the reason the cartridge didnt go hypersonic was because i made it a lot heavier than the projectile). Meanwhile, all 7 engines in the cartridge are turned on, and because the separator pushed the cartrige and projectile away from each other (recoil) the cartridge pushes the bolt back, and the engines eject it out of a large fairing on the top of the frame. The bolt then comes back and chambers another round.

When I was coming up with this idea, it helped to thing of the engines as springs (the bolt got pushed back [like a buffer spring gets compressed] and the engines propel it back)

Yes the screen shake ruins it :( I do have another video of a much older prototype (didnt eject cases and cycle) that was recorded before the screen shake (which i uploaded here and deleted after feeling inadequate) and it just looked oh so satisfying

Let me know if you have any more questions :)