Some clarification, since it's come up before: Be sure your upper and lower are for the same caliber weapon.

You're certainly not asking that, but in the past there've been questions raised along the line of "can I convert my current .223 AR to .308 simply by changing the upper receiver"?

Very simply, "no"

No offense intended, in all likelihood you already understand that. However,there may be someone out there who'll read your question and wonder....