Problems:
-The rubber sealer pushes out the front part of the turret, and the gears won't be able to align
-It's annoying as fuck to re-assemble the turret once the barrels are glued in
Solutions:
-Make the gear extrude farther from the turret base
-Essentially elongate the center bar
First we'll focus on the gear problem. Your going to need to cut off the gears on the bottom half of the turret. Use a thin saw blade and always keep it parallel to the turret base. We need a perfectly straight and level cut with as little material removed as possible. Ignore the washer in the picture, I fucked up and glued it on too soon.

Now buy two washers that are a good size for the task at hand that are 1/16" thick.

Place the cut of gear into where it would rest in the front half of the turret. Cut up one of the washers and glue bits of it to the back of the gear. Make a mark on both halves of the turret so that they line up when a barrel is centered around the air-output.

Put glue on the pieces of rubber and hold the turret base above the front turret half. Put the center bar through both halves, and then press the turret base down.

Now remove the pin and turret base. Now glue the other washer onto the base as a sealer, and put more cut-up bits from the other washer onto the turret base so that the front half remains parallel to the back half.

Lube every thing up with silicone GREASE. Glue some tubing that fits snugly over the spacer on the center bar.

Put the turret halves together, and insert the center bar from the front. Drill a hole in the tubing.

Put some coat hanger wire through the hole. Put on the spring and endcap. Put the turret right-side-up on the ground, and push down on the ends of the coathanger wire until you get the endcap all the way down. Then drip some superglue into the exposed end of the end cap.

And there you go, your rear-loading turret is ready.