I can find several web sources for new 1100 o-rings, but it seems to be a guarded secret as to what the actual size is supposed to be. I guess that is the only way they can get a buck or two for them. I just happen to work where quality o-rings are plentiful and cheap. They won't cost me anything if I know the correct size to use. I took the new gas system oring off my Remington 1100 shotgun to work today. I used our o-ring measuring devices and came up with this... The O-ring was measured out to be a size 0-22 with a durometer of 70-75. I am guessing the material to be Buna or Viton. Does anyone have any information to verify or disprove this information?