I currently own a SBEII and an M4, and I'm considering adding an M2 to the mix. Are all M2 receivers drilled and tapped for a picatinny rail? I'd like to get the 21" field version, and add a 26" barrel for bird shooting. In the off season I'd like to run the shorter barrel and add some sort of sight to it. Another option would be to pick up a tactical version and run the 26" on it to bird hunt. How easily is the rear ghost ring sight removed? Any opinions on these options?