From what I recall, that stupid-azz rubber gasket ring on the rear sprocket was only on the 2008 models.
Are you sure you have a 2010?????
Did you buy bike used? I guess it's possible a previous owner installed a 2008 sprocket????????
I think one guy machined his off. Another I think used a sharp chisel and sliced the thing off while the sprocket was still on the bike.
I'd recommend just buying a new sprocket - and get one with a few more teeth.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs