the reason is that the laser has to be a certain length - unless you go with the spud in the end of the barrel thing- in its bare form the laser module is longer than 39 mm,unless you can figure a way to cut off the battery stack and still power the laser-it just barely fits in the 223, which is 45 mm- personally, i think you're better off with an OPTICAL boresighter- the bushnell has been around for years and is kind of the standard