I am looking for a good upper for my colt ar-15. I would, like a larger caliber, maybe a 6.8. Is there a .308 upper? Please suggest who to buy from, and what price to pay!
.308 won't work on a 5.56 lower.
You can find alot of info on 6.8 SPC here.
.338 Spectre from Teppo Jutsu sounds like a good choice, especially if you live in a county where the sherriff will let you buy a supressor.
If you want a larger caliber then you should look at .50 Beowulf, .458 SOCOM or .499 LWR.