Rifleshooter.com repeated their previous .300 Winmag test with a rifle chambered in .223/556.
He borrows his friend’s Remington 700 chambered in .223 Remington. They start at 26″ and cut the barrel down 1″ at a time until they get to 16.5″. They used a chopbox and a generator at the range to cut the barrel down. They did not bother re-crowning each cut. They use four different factory loaded cartridges; Remington UMC 223 Remington 55-grain U223R3, Federal XM193 5.56 M193 55-grain ball (the box is labeled both XM193 and M193 and has an 02 head stamp), Winchester M855 (08 head stamp) and Black Hills 223 68-grain Heavy Match ammunition.
There are tons of data that they collected. Go check it out.