I bought this to install fiber cement lap trim. After struggling to remove the plastic cover, this unit fitted my Bostitch N66C. While overpriced for what it is, it really saves time nailing the siding. The dowel, which sets the height of the nail 1 inch from the top of the lap trim, is spring loaded, which is handy if you need to drive a nail. Two set screws hold the block securely to the gun. Overall, putting your house down is definitely worth the cost!