Popular worldwide All-2-Mate Barb-Style Insemination Gun requires no O-ring to secure the sheath to the gun. Sheath slips on quickly and is held in place with the barb. Use the B6-4500 Unslit Sheath with blue insert (available separately.) The All-2-Mate™ Gun works equally well for 1/2 cc and 1/4 cc straws.