MK111 wrote:What changes have to made to a AR upper to install a 450 Bushmaster barrel build?
Welcome aboard MKIII.
The only thing you have to do to a functioning AR is make sure the ejection port opening is wide enough to pass the fatter 450b case through during ejection. Some come that way already and some do not. There are multiple references to this including dimensions to measure beforehand, in this sub-forum. For the best luck finding them go to Google and enter the search term site:450bushmaster.net "ejection port"
just as its written. The first search hit should be just what you need.