[Kos-Mos]

Try a manual test.

Remove everything but the piston from the mechbox (trigger parts are ok).
Close the shell with the screw as if it was complete.
Push the piston all the way back with your finger using the cylinder opening.

If it jams, it means the mechbox shell is poorly made and the rails contact the piston too much... You can try to find out where and sand it down, but it is a long and touchy job to do... remove too much or from the wrong spot and your piston will jam and strip.

If it is ok, add the spring guide only and try again. Some bearing spring guides have bearings that are a tad bit too large and hold the piston in place.