As for barrel specs, well that is truly your call to make unless you are wanting to duplicate and existing barrel outline. If so, then getting specs is easy. Visit any of the major barrel companies and you will find their profile and dimensions on line. When I was searching for a company to do an 18" fluted bbl for a proposed build I found no shortage of profiles on many of the sites I visited.
Several of them have on line ordering and you can spec out your bbl and pick a profile.
Now for the question: How do you intend on rifling the bbl?
Last year I was working on a proposed build and in the AR world every component piece is available for purchase. Most are manuf by only a handful of companies. Barrels are dominated by Young Manuf as an example.
Sidebar: I just got back from the NRA show in Nashville. I stopped off at a booth at the urging of some friends who wanted my opinion on the new company which is getting a LOT of press in the AR world. Ok, looked good, saw quality from my eyes as a former machinist. Then I got one of the Sr employees aside and asked this: Do you manuf any components and or do you machine any components IN HOUSE...after a pause and a foot shuffle, they said no. In other words they assembling the gun from component parts, Yes. OK nothing inherently wrong, but you or I can do same same, in fact that is exactly what I was going to do last year (before I got so sidetracked on the whole house remodel).
Their AR line up runs about $4-$5k albeit they are heavily discounted...
The result being I was between a rock and a hard place with my friends who have bought into the magazine hype. I danced the old soft shoe around the reality its just another kit gun by a company that may or may not be in business 2 years from now. My friends bought 2 of them...
Take away: Like Clinton said "its the economy stupid" in the AR world its the barrel. No where in the build of an AR can one have more impact than the bbl. Sure I love an AR that I cannot slide even a cigarette thru where the upper and lower meet. But its the bbl that has a MAJOR role in the accuracy of the weapon.
So go dig thru some custom bbl maker sites and you will find bbl specs. Have fund and enjoy the experience and learning of how to make a bbl and keep us informed of your journey...