Inner velocity governor for autocockers. These include Nelson IVGS in the plug and the screw style, and classic Sheridan IVGS. If you have a plug IVG that uses nelson springs with a nelson spring hammer you need a SHORTER NELSON SPRING. If you have a threaded wgp autococker body that uses a screw type IVG in nelson spring size with a nelson spring hammer then you need a LONG STANDARD LENGTH NELSON SPRING. All Sheridan spring setups use plug style IVGs. Sheridan are smaller OD and Nelson springs are larger OD.