I was shooting full auto with the stock folded, and a jagged piece of metal from the mechbox flew back into my face (full face masks are awesome). I took these pics when I got back.
http://www.pbase.com/shadowmatter/inbox
I am now sold on reinforced gearboxes. The spring guide was cracked, and shredded 4 piston teeth on the Systema gears. The gears were fine, the stock TM piston was a write off needless to say.
Cheers.