Picatinny adapter converts the 3/8" factory groove to accommodate red dot optics. Previous production or 1st gen Brass Big Boy guns were drilled and tapped on the barrel under the rear sight, they were NOT drilled/tapped on the receiver. You will need to use BB-CSM for 1st gen H006 Brass Big Boy series. In 2011 we began shipping 2nd generation Brass Big Boy rifles which feature receivers that are drilled and tapped for the scope mount.