Usually I have some idea or some direction in a project, however with this one it's new territory. I figured I go to shooters with some experience in this area instead of inventing the wheel. My son has an AR and he wants to build a .450 Bushmaster for black bear hunting in Maine. Usually the shots are 60 yards or less, the Bushmaster looks like a really good cartridge for what he intends to do. There are a few questions about what to get for an upper, what brand uppers would one recommend, and where/who do we get the parts from? Are there any additional parts needed besides the upper? He's preferably looking for at least an 18 inch barrel, or even a 20 inch barrel if possible. Also if there any suggestions besides what I have asked, please, they will be appreciated. Thanks for any help.