If you have a turned down barrel (Norinco for instance) your external diameter should be JUST under 1/2 inch, usually about .498+/-.

If this is true, your best bet is to thread it for 1/2-28 and use an AR style brake thats reamed out to accommodate 7.62. Bear in mind that if your AK is a ban model, in order to add a flash hider or other muzzle device, you'll need to be 922(r) compliant.