Thinking about buying my first alternate complete upper in a different caliber. However, I'm not really certain what caliber I should go with. I'm thinking 7.62x.39 since it's still relatively cheap to shoot and I like that caliber as an option for whitetail hunting also. But there are just so many different calibers for the AR platform that I really don't know what I should be considering. Any recommendations on calibers and good places to shop for them?