14K Yellow Gold Straight Barbell, 5.5mm Inner Length, 18 Gauge, 2.5mm Ball Screw. These are your go-to all purpose straight barbells. The 2.5mm ball on one end screws on and off, while the other end is fixed. The inner length is 5.5mm, but it measures 10.3mm end to end with the ball screwed on. The post thickness is 1.0mm, which works with 18 Gauge piercings.