It has a heavy 18" barrel.I wonder how much velocity I will lose?

I forget the link and don't have it here, but there's a website (rifleshooter.com maybe?) that posted a series of articles about velocity vs barrel length tests; one of those was on the 6.5 Creedmoor. I think they went down to 16" with that one. It's only a sample of one, but interesting reading.

Just my opinion, but I'd rather have a sporter weight 22" barrel than a heavy 18" with a Creedmoor. Of course, ARs are easy to change barrels, it just comes down to $$ and what you're willing to pay for the barrel you want.

You lose 25 fps per inch

http://rifleshooter.com/2016/02/6-5...ngth-on-velocity-cutting-up-a-creedmoor/

Yup, that's the one I was thinking of. Good article.