I threaded my ak47 barrel in 1/2x28 and wanted to know the best way to go about boring out an 5.56 AAC Brakeout to .30cal?

Boring it out on a lathe would be the best.

I don't know what muzzle brake you are trying to use but just boring out a muzzle brake may not be the best option.
Some of the muzzle brakes have a design that is for a specific caliber, if you take a smaller cal. brake & open it up it may not work as intended.
You can buy an adaptor to change your 1/2 x 28 to 5/8 x24 & then any .30 cal brake will work.

http://www.brownells.com/rifle-parts/barrel-parts/muzzle-parts/thread-adapters/muzzle-thread-adapter-1-2-28-to-5-8-24-sku100010536-54364-105317.aspx

they are pretty hard, but decent tooling on a lathe will do it.  Maybe a cobalt bit.

While it's been done, I think the wall thickness on a .30 in 5.56 is borderline too thin.