I recently had the opportunity to work on a Remington 870 that was having some feeding problems. As I began dissasembly of the gun the magazine cap flew off as I unscrewed it. Along with the cap came the green magazine plug for hunting. Thankfully I still have both of my eyes!
The customer was reporting cycling issues with the gun, and it became evident why. The magazine cap had been placed over top of the magazine plug, which was on top of the spring retainer. The plug should go under the retainer or else you will have the spring compressed by several inches making cycling problematic.
When I explained this to the customer, he said Remington had shipped the gun that way. This is the second time that a Remington 870 has come to the shop this way. I’m not sure how it ends up like this (customer or factory), but if you are experiencing cycling problems, look at the magazine spring setup.