This is the pump action carbine I did earlier, except I added comparisons of the magazine and ammunition and made a perspective view from the top, bottom and left.

I also added a ruler in inches, the barrel is about 22 inches total, and after much thought caseless ammunition is retarded.

I have no idea what to call the caliber, the bullets are 4 inches long and 7/8 of inch wide. A serious business bullet for sure. The single stack magazine holds 5 of the suckers, standard capacity for a shotgun so it is pretty good.

Might even make up some schematics for this in the future, or at least a cut away, it is pretty simple really, I figured the pump would push back the bolt and simultaneously strip a round out of the mag. It then pushes it into the bolt face which would be similar to an M-16 bolt face in that it grasps the rounds. Then as it comes forward it is held by the bolt face and slid into the chamber. One fired it acts like a M-16 bolt face and pops the casing out the ejection port. Might take a bit to get it all copacetic but in theory it may work.