I had a 223 in a 14inch contender barrel. I did get a small fire ball but nothing bad for daylight only shot it once at night without too much flash.
You will almost alwyas get some unburned powder and muzzle flash unless you do a lot of working on loads. Once the flash and unburned powder is gone you will loose speed because, once the powder is used up all that is pushing the bullet is it's inital momentom and it will staart to slow down due to friction in the barrel.
You will be better off finding a bullet/load combo. that the gun likes then worring about flash and unburned powder.