I'm not sure from your picture, but that could either be the piston/piston seal assembly, or thebarrel seal activator.

If you have a 11-87 Super Mag (2 3/4" - 3 1/2"), it's probably the barrel seal activator. You need to remove it when shooting 3" and 3 1/2", and leave it in while shooting 2 3/4" shells. It'sRemington's version of agas regulator.

If you don't have the Super Mag, then I'm not sure what it is. But I wouldn't worry about it, if this is your case.

Good luck