Make sure you have all the parts,I bought a used 1187 that would not fire more than 1 shot,I tried all brand of ammo light load heavy load nothing would work,replaced recoil spring ,seal, piston,gas cylinder spring ,all to get nowhere.Than one day i pulled up a schematic and figured out one part was not in my gun,is the gas cylinder collar it goes over the magazine after the barrel as been inserted in to the receiver,then you have the forehand and finally the magazine cap.I see how easy would be to forget to replace it after a cleaning .If you decide to look at a schematic be sure you have the one for the 1187,not 1100 or 870,these gun do not use that part.
Remingtoncountrystore.com has a clear photo of the part i am referring to again be sure you are looking at the 1187 page.
Sorry for being so long winded.
Danny.