little round plastic piece

shooting in the backyard and when i closed the breach i noticed a little round plastic piece fall out. maybee 200 rounds thru her now. piece was 1/4 inch in diameter by 3/32 deep by 1 millimeter thick. anyone know what this is. seems to shoot alright still. just wonder if this is something i should be concerened about. thanx in advance and happy shooting. ken