Funny name, serious design. This kit is designed to allow the shooter to customize the length of the stock to the shooter. The average stock length for AR-15 style rifles is approximately 15 inches (from trigger to end of stock). However the length of stock for the RFB is approximately 14 inches (from trigger to end of stock). Each spacer will add approximately half an inch (0.55 inches) to the length of the stock, allowing the shooter to customize their level of shooting comfort. Kit includes three (3) spacers. **** During assembly, do not over-tighten screws; Doing so could cause damage to the rubber pad and / or brass threads. ****