The chaps at 858Airsoft have posted a review of the Marushin M500 8mm Shotgun. I'll leave you to read the review, but my reason for posting it here is that they've highlighted the issue that I also came across with one of the pivot points breaking after only light use.

This happened to me on two of my own 8mm M500s, and on a further two that I know of myself. Sadly it looks to be a manufacturing flaw and affects pretty much every 8mm M500 at some stage in their lives. Initially I thought it might have been down to overstress from excessive pumping, or use of propane gas, but it doesn't look that way.

I chatted to the guys at RedWolf about the issue a while back, and they find it impossible to get spares for any Marushin gear.