What is the size of the o-ring and materal for a Rem. 1100 12ga.?

there graphite coated, run of the mill hardware o-rings don't last very long, Brownell has them on sale

http://www.brownells.com/shotgun-pa...on-1100-11-87-gas-o-rings-prod54774.aspx

Yeah get the proper ones, hardware shop ones break real fast.

Buna N will fail very quickly, but can save a hunt if that is all that is available. McMaster Carr has the good ones cheap, but they come in a pack of more than I would use in the rest of my shooting days.

Exactly right. The 4-H club was using O-rings from a local hardware store tray assortment in Rem. 1100's when I got there. The shotguns worked with new O-rings but the O-rings crapped out pretty quickly. Switched to the proper high temp rings (Brownell's). Between that and keeping where the rings seat free of carbon buildup the rings last a long time. It's trap shooting and I'll replace an O-ring every year or two depending on use. Eventually the heat will rob them of their elasticity. The rings are of a high temperature elastomer of some type, there are several, with Teflon.

Check McMaster Carr for Viton o-rings. I believe that #19 is correct, but I am not 100% sure. There are no longer very many Remington 1100s in my family since small game hunting has dwindled, and I can't remember the exact number of the o-ring. I do know that a pack of o-rings from McMaster Carr would last a lifetime in my family.