As someone mentioned on this site once you get the black gun fever, you can not have just one. I have a M4 flattop that I purchased about a year ago. My current plan is to build a heavy barrel AR15. Researching the possiblities is what led me to this site. One guestion that has come up in my mind if one purchases a "stripped lower" and adds a trigger setup, how does one go about registering the weapon?