I am planning on buying a lower (maybe two) in preparation for building an AR. Not sure what caliber I want yet. Was considering doing one in 300 blackout and one in 308. But I want to get the lower now, while I have the ca$h, meaning before the wife spends it!!! Are AR's like old school bolt rifles where it is either long action or short action, and that is pretty much what I have to decide before I purchase the lower? I know there is way more to the lower, but not knowing what caliber I want, what other questions do I have to know the answer to before pulling the trigger on the lower? I know, rookie AR questions... But bear with me, I am an avid shooter, but have only ever shot the standard hunting rifles.