Initial Problem: The gun would seize and lock back all the time. Extremely frustrating. Also, the tappet plate would skip on the sector gear, causing it to get stuck between the tappet actuating nob and the axle of the sector gear.
Fix: Marui gears and piston/ piston head, re-shim.
Nothing fancy but it works great now. Hopefully this will help someone else out there!
...and she's terribly partial to the periwinkle blue.