These Rollers are the ones we use for our spearguns. They are very strong, made from Delrin. The standard has no bearings, and are made in two sizes; one for 9/16th-5/8th (14mm-16mm) rubber and one for 3/4in (18mm) rubber. The ball bearing roller easily accepts 9/16 (14mm) and 5/8 (16mm) powerbands. They are sold individually.