Not to doubt your findings, I'm sure they're going much faster than 3k. .308 uses roughly 20 more grains of powder and you're getting 3k out of an of the shelf AR with .223 alreadyMy reloading is fun and the expense depends on what I am trying to build in a 308. There are spendy match components that take time and money to assemble or a man might build hunting rounds with specialized bullets that ain't cheap.
I get a wild hair to have fun and build 308 ammo that fires 223 bullets. The build is really fun and the shooting a lot cheaper and it tickles the hell out of you to drive a 223 bullet out of your 308 at 3000 fps.