My 1100 20 ga. has 3 O rings; one rubber and 2 metal. A rubber replacement costs about 4 dollars so I'd suggest buying at least 1 and taking it into the field. Took the barrel off my 1100 in the field(don't ask) and the O rings slid off. Couldn't remember the exact order to replace them so my semi-auto became a hand cycle in the middle of a big dove hunt. When I got home I checked the manual and got the order right and it works fine. Made a copy of the diagram and taped it to the underside of the forearm. Don't know if the 1187 has multiple rings but here's my 2 cents worth.
I dont think its got 3 rings. its got a metal ring, or combo of rings that remington calls the gas collar and then one o ring near the top of the mag tube. I got a couple of the o rings from the local gun shop but they were kinda pricey so thats why i was wondering how often they need replacing. And i was trying to avoid what happened to you since if im shooting my 1187 at dove im already on my backup (bps is my go to) thanks for the thoughts man.