Yep, guncrank, I was pretty sure we were talking about the same part. The first sign of failure is when you start to get double feeds. Later is when parts get spit out, and pn55 is the primary part getting spit. You usually lose the circlip at that time.
The 1100 is a good shooting shotgun, but it has a defined use life measured in rounds fired. As suggested by the gunsmith, I had it stuck back together one more time and then sold it. Then, after having studied shotguns a bit and talked to owners, I went toward the Browning humpback design and bought a Remington Model 11. Took it to Briley in Houston and had their screw in choke system installed. No problems after that.
I used to entertain hunters on a corporate lease. Not my primary job, which was trading oil, but to entertain the folks I did business with. One day we shot doves in south Texas and we ran a lot of ammo through our shotguns. Most guys had 1100's, and there was a cleaning station back at the ranch just for them to clean up nasty gunked up gas guns. I was busy dunking my metal parts in cleaning liquids (Hoppes, I think), trying to get it clean enough for the next day, in the hope that it wouldn't jam from carbon fouling, when one of my customers walked over to the gun rack and put his uncleaned Browning A5 in the rack. I asked if he wanted to clean it. He said it didn't need cleaning. I said "BS", and he took the gun apart quickly to show me that it was indeed clean. He said that all of us young dumba____ didn't know to buy recoil operated shotguns. No gas system to clean. So when my 1100 gas gun died, I went to a recoil operated shotgun. I will never again own a gas operated shotgun.