Sound advice so far...
All 1100s and 11-87s have O rings..It's part of the basic design of the gas system.. Sounds like yours needs a replacement.. I've had to replace several over the years...
Magnum 1100s were designed to function with 3" shells and 2 3/4" magnum shells...Some may cycle heavy 2 3/4 " loads like rifled slugs or certain higher pressure 2 3/4" loads...
If you want to shoot lighter 2 3/4" ammo, I'd suggest picking up a used 2 3/4" barrel..There are lots of them sitting around gunshops and online gun auctions and you should be able to find one at a reasonable price..Oh, and get yourself a couple of O rings for it....

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