So I am in the process of building my first AR gonna go with a BCM. I also want something with a little more punch than a 5.56 for hunting whitetail. So the question is can I buy a larger caliber upper and it be compatible with my 5.56 lower? I just dont want to throw out any more cash than what I have to. Thanks for the input I am clueless when it comes to AR's other than 5.56.