Remove the barrel. About 3/4" from the fill nipple there is a setscrew that will bleed the air out of the bottle, then unscrew the bottle, pull the cocking handle back, drill down through the rear barrel setscrew opening, you can see the air valve through it, then drill the barrel. Be very careful and square everything up. Here is a drawing of the gunhttps://www.crosman.com/fileuploader/download/download/?d=0&file=custom%2Fupload%2FBPBD3SRT_EVP.PDFRegards, Tom