Does anyone know where I might find a 20" 300 blackout barrel?  I want to build a 300 blackout and would love a 20" heavy barrel fluted.  Thanks

At best you are looking at a custom job. You really do not gain a whole lot after 16 inches.  There may be someone making a non custom 300 blk  heavy fluted, but I have not seen it.

Last night on Brownells I saw where you could order 24" AR barrels in a variety of calibers. It's not fluted or anything, more likely closer to a standard barrel. If 24" is too much for your fancy then so be it, but I know I will be ordering one.

Here's the link:

http://www.brownells.com/rifle-parts/barrel-parts/rifle-barrels/24-223-rem-med-rifle-gas-barrel-sku080001018-56797-112677.aspx

There's almost zero advantage to a 20" barrel.  A few FPS faster isn't worth the extra weight, IMHO.  Also, 20" isn't going to give you any better accuracy than 16".  The fast-burning pwoders used in the 300 BLK are done well before the 16" mark.  I love my 300s, and I'm always tempted to try to make them something they're not.  As Nancy Reagan used to say, "Just say, no."

I don't see a 300 BLK barrel longer than 16" there.

No advantage will be seen out of a 20" barrel...powder burns in the first 9".

Exactly what they said, powder burns up in 9" and after 16" you get drag.  I've got 8.5 and a brake and can hit anything you can with a 20" heavy 5.56 (practice required). Like the short, it'll like you back long time.

If you want a 20" barrel I'd look at the 30hrt if you are thinking a custom handload only thing is up your alley.  There was one company that was making 20" 300blk barrels....I thought it was Houston Armory, but I didn't see them on their website.  The company that makes them used to chime in on these 20" barrel threads to counter all the 300 blk 20" naysayers.  IIRC the numbers they were posting seemed like it would be a worth while thing to do for the few who wanted to go that route.  I don't recall any posts in the recent past, though.  If I figure out who it was I'll post it.  YMMV

advntrjnky