Lurte på det samme sjøl en gang, bare med 1100, dette er det beste svaret jeg fant:
"Update - Purchased a 16 gauge barrel, and also the following parts which are indeed different between the 12 and 16 gauge standard 1100s: Interceptor latch Feed latch Action bar Bolt Assembly (includes extractor) All other parts I have found to be identical (trigger group, action spring, magazine tube, etc.). In fact, I'm not certain that the action bar sleeve itself was different, but the bolt carrier was certainly different between the 12 and the 16 gauge. Swapped the parts. Forearm has a slight play, but I have not tried to tighten the retaining cap with the monkey grip yet. Loaded up three shells, and to my pleasant surprise, by 12 gauge standard 1100 went Boom, Boom, Boom, slinging out 16 gauge shells."
https://www.refugeforums.com/threads/remington-1100-12-16-gauge.1021654/post-10779593
Regner med mye av det samme gjelder 870