That is extremely impressive.
With the M60 I used to carry ("...when dinosaurs roamed the earth...") you would have melted down a barrel completely long before you got close to 850 rounds. Hell, I think 1/4th of that would have bent the barrel pretty badly if you fired it all without letting up on the trigger.
I'm sure everyone probably has a story about seeing an M60 get stuck firing wide open. It used to be pretty common, I guess. Pretty neat to watch, even if it was scary. Never failed some buck private would string a bunch of 100-round belts together, and one would get stuck, and the next thing you know the barrel would start getting orange, then yellow, then white, then the impact site of the rounds would start walking closer and closer back towards the firing line, until someone got smart enough to snap the belt.