__________________ Assumption is the mother of all ****ups. Know what you know, Know what you don't know. -Paul Pedzolt "Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul
You can switch between a 5.56 mid and carbine upper by just swapping uppers.
308 will not fit on your lower unless you have a Colt 901. The Colt 901 is the only lower I know of that can interchange between a standard ar15 and ar10 upper, with the help of a magwell adapter for 5.56 size magazines. Another excpetion is a custome bolt action upper for larger rounds. I seem to remember reading a few years ago about seeing .308, .50 bmg, .300 win mag and .338 single shot bolt action uppers that will bolt to an ar15 lower. Not sure tho. But all these rounds are waaaaay to long to fit in a standard ar15 magazine well.
Here's a short list of ar15 compatible uppers (NOT COMPLETE) 6.8 spc, 5.7, 6.5 grendel, 7.62 x39, .300 blk, much much more.
For simplicity sake, don't mix and match Ar10 and Ar15 receivers and parts.
There are several pistol calibers that will work on an AR as well.
"Democracy is two wolves and one sheep voting on what to have for dinner.
Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin