The primary reason the 31 functions so smoothly is the carrier.
It's as long and wide as the receiver itself. Pivoting on the side walls of the receiver on reletivly large trunions & at the extreme rear of the recv;r. Leverage and very small contact surfaces.
All that makes for a very easy job of it for the bolt/slide to operate it by simple cam motion.
The carrier on the 31 is a whole separate part from the trigger group unlike the 870 where they built the carrier into the trigger group.
The carrier in the 31 is under no spring tension at all. It is simply moved into position by the bolt slide going back and forth as you operate the slide.
When they do wear out and fail to feed, usually the cam surfaces on the carrier are starting to wear down and the carrier isn't being lifted quite high enough for the shell to be chambered cleanly.
Rebuilding the cam edge(s) a few .000" puts it back to life. The carriers are reletively soft compared to the bolt slide surfaces which are hard.
A shade tree fix for the above is to 'gently' squeeze the front area of the carrier arms together a bit. That makes the shell ride a little higher on the opposing carrier arms and can sometimes make them feed OK once again.,,sometimes.
But it really doesn't 'fix' the problem.
Don't squeeze too much!!
Lots of parts change configurations in the 31 as well as gauge specific parts,,check carefully before buying any replacements if you do need any.
