A stock sets length of pull and cheek weld height -- the two dimensions that make the mount consistent rather than approximate. On a PCC used in IPSC competition, where the gun is mounted frequently across stage elements with multiple targets, a stock that reaches your shoulder correctly and positions your eye at the optic immediately saves a small amount of time on every mount that accumulates across a stage. The PMX stock is specific to the PMX receiver. Confirm compatibility on the individual product page.