[mcguyver]

The mechbox itself is what holds the upper receiver so solid on G&P guns. The front pin keeps it from sliding off, and really is the only thing that holds the upper on the gun.

In a few post-2005 G&P guns that I've owned or worked on, that front receiver pin was very loose, and needed to be physically altered (with a hammed and chisel) to deform and enlarge it to fit the holes in the receivers. This was the only gripe I could find with the guns really.

But mechboxes where users would use semi-auto, the switch stop block would break in the same fashion that Systema complete mechboxes would.