Just spoke with Benelli USA. Not overly helpful, I have to send the busted piston into them so they can look it over. No big deal.
Here is a picture of the busted piston.
The piston is fairly easy to remove from the firearm. Actually there are two of them. From the picture above, the piece that broke off is the portion that extends thru the receiver and impacts on the bolt during the impingment.
I'm pretty sure it broke within the first 20 or 30 rounds. The weapon became sort of slugish, I could feel the action working. When I looked into the weapon, I could see nothing wrong because the piston was still free floating in place. Only when it made its way into the receiver did it cause a major failure.
Basically, the weapon was running off one piston for 20 rounds or so. It would probably even work still, not recommended though.
I did not remove the pistons from the barrel when I first received it. I simply lubed it lightly along with the rest of the shotgun before taking it out.
The gas pistons only move about a quarter inch during their travel. There is a shelf the shoulder hits on the barrel to keep it from protruding too far. Where that shoulder is, and the piece that impacts on the bolt face meet is where it sheered.
I rechecked how many rounds went down range. I shot about 50 or so. Most were 2 & 3/4 bird shot loads at 3 dram. Definately not some shoulder busters.
Looks like my brand new shotgun is out of commission for at least a few weeks. Not pleased at all.
Those of you who have the M4, what the hell does the 273j, 274j & 275j parts listed in the gas system on page 16 of the spare parts manual? I can't disassemble that part, I hope all those pieces are in there.
-Steve
Edited to add:
The piece of the piston was above the feeding round when it caused the complete failure.