I have a bushmaster lower. Can I buy a complete 308 upper and put it on my lower or are those two totally different things? I cannot find that info anywhere but maybe I am asking the wrong question.

.308 lowers and 5.56/.223 lowers have different magazine well dimensions. So presuming you are talking about a 5.56 lower than no.

Probably won't fit.  While similar in operation, AR-10s have a longer magazine well to fit the longer .308 cartridge, so a .308 upper would generally be too long to fit on a stock .223/5.56 lower.

You could easily make an AR-15 upper chambered for .308, but would have to single-feed it through the ejection port, since the AR-15 magwell is not long enough to allow the feeding of .308 cartridges in the normal fashion.

if you want 30 caliber in an AR15, your options are 300 blackout or 7.62x39 or almost 30 cal with the 6.8 rem spc
im waiting on the new 6x35 round that KAC cooked up.

Yes you can, but it will be single-shot.  Franklin Armory sells one.

http://www.franklinarmory.com/PRODUCTS_308-Upper.html

Best,
JBR

.308 Winchester?   No.

.308 Winchester? Yes...as others have said. Just only single shot. The cartridge will not fit in an ar15 magwell. However I wonder if anyone has considered an upper that would take .308 mags from the left side? Seems a cool idea where you wouldn't have to have a ar10 lower.

Did you watch that video contained in the linked page?  Is that what you'd buy, if you were out for a new AR-15?  A single shot?  Maybe I'm crazy, but I don't think that was the intent of the OPs question.

I did watch the video, and the guy says it can be run "as fast, or faster, than a bolt action."  Don't think so.  And, we don't buy ARs because we're looking for bolt action-type single shot operation.

Again, I'm not speaking for the OP, don't have any idea what he's really looking for - that answer from Franklin Armory is not why you'd typically want a .308 AR.  

Again, you want to run your AR like an AR, but in .308 Winchester - that upper listed above isn't going to do it.  No upper, onto an AR-15 lower, is.

.30 caliber through an AR-15 is there - a few choices.  .308 Win through the AR-15, running it like any other AR-15 - unpossible.

For this part, there's already a company that does something like this - ZEL Custom.  The calibers they make are .50 BMG, .416 Barrett, .338 Lapua Magnum, and the .408 CheyTac.  Those are bolt action, single shots, that attach directly to an AR-15 lower.  They make a .50 BMG model that has a 5rd left-side mag feed, and that mag also feeds the .416 Barrett round.  So, they have two models that have a detachable mag.

They don't offer anything in .308 Win.

what kind of lower do you have...

Or there's the 7.62x40 WT.  You can use everything from your ar-15 except the barrel.  Even 5.56 brass can be trimmed and re-sized to 7.62x40.