[lurkingknight]

60 bucks sounds rather high for just opening the gun, but that's all about who's your preferred/trusted guy to work on it is.

Spring binding on the spring guide/gearbox DOES happen, I've seen it, so it's not out of the question that this is what happened.

If the piston bound at the back while the gears tried to turn, there wouldn't be metal shavings... there'd be pieces of the piston rack all over the inside of the gearbox if the rack was plastic. IF it was metal, (and more often than not if it was plastic) the motor would just jam and pin the piston back, unable to turn.

The shavings could be just from wear and tear, or could be from the piston slowing on the rails and causing premature engagement, gears grinding on the rack as the piston comes forward cause the cycle is too slow by the time the gears start another cycle.

Shavings are also more likely to come from bevel/pinion gear meshing or just gears wearing. If they're not directly from the piston, that can be attributed to poor shimming/wear and tear.

There's definitely some investigation work to be done to pin the cause of the jam and fix it so it doesn't happen again. Since it's one of those more bizarre issues, it can take some time to figure it out and apply the fix, especially if the technician takes is time to do it right. The price is not outside the realm of reasonable.


Several things need to be considered for the loosening of the buffer/release of the spring to happen:

If the piston were jammed and released by loosening the buffer tube screw:

the sector gear would have to be outside of the piston rack, which means the piston is jammed FULL back beyond contact with the release tooth. Springs and spring guides can jam the inside of the piston when it comes back at the top of the stroke.

The gun would sound like it was turning with a stripped piston though. He described no function.


If it was jammed MID cycle with the sector engaged in the pistol, the buffer tube screw might dislodge it a tiny bit, but the ARL would engage and catch it until you flipped it open, then the piston would release.

Metal shavings in the top of the gearbox near the piston would indicate the release tooth was being chopped at. If the shavings were at the bottom of the gearbox, that's from the pinion/bevel.

Gears free spinning would not cause scratching unless there was material caught between gears and gearbox shell. If there's a circular pattern ground into the shell, that would be from a bearing failing and the gear falling into the side, out of alignment. But the invoice stated no new parts needed.

I recently serviced a gearbox with a spring guide that unthreaded itself and the bearings came lose and out of alignment, opening enough gap for the spring to slip into and come out of alignment. This caused the piston to jam mid cycle. Pulling the trigger would make a click and motor whine as the motor tried to pull through the jam, but it wasn't strong enough.