Just a bit on the locking piece...
by this I mean the piece that cams up into the barrel extension from the bolt.Also the slots and camming surfaces inside the bolt that the locking piece moves in. A little goes a long way, but I like something heavier that oil on the locking piece/barrel extension. The slide bars can get by with a slight coating of oil. Also, a dab applied to the sear. If you are going to be in wet, more lube, sand, less or lube that dries so as not to attract dirt. The gun will work very dirty(I have cleaned sticks and leaves out of them when I worked at a gun shop years ago-the guns still worked) the thing is that the dirtier it is the quicker it will wear out. I have seen a few that were so worn they would close on a no-go headspace gauge, but they were rough, I cant remember if we could just put in a locking piece but I believe that we would replace bolt assy and barrel if need be, but that was uncommon in my 6 years there(was a full service gun shop and did many repairs) . Now a Remington model 11- avoid like the plague. It is a copy of browning A-5 made under license. There are friction rings that go round the magazine tube and they can be set for light loads or heavy by turning them around. Most people left them for light loads and shot anything and everything , or the rings do wear out. I think I only saw one Browning break, but one day took apart 6 Rem. mod 11s all the way down to the bolt, pretty much a detail strip, every bolt was broken where there are semi circular cuts that the locking piece rides in on both sides inside the bolt. My plan was to buy one, boss told me they would all be broken and he was right. Remington made guns for Browning for a year or two when Belgium was overrun by the nazis. These suffer the same fate, but are pretty rare, as are military model 11s, but do not shoot one unless you have it checked out. They are a bear-tons of tiny parts. If you ever shoot an A-5 that is kicking the snot out of you the friction rings are probably set wrong, and it is easy to set, just take the fore end off and the rings are around the mag tube. I have blathered enough, hard to hurt an 870, Always keep extra trigger mech pins though, most common malady with this gun, and though rare, it has happened.