I've got 2 Hart barrels that are amazing shooters. I know Defensive Edge uses them in their rifles. Hart Rifle Barrels in near Syracuse, NY.
Hart Rifles in a different company, R.W. Hart and Sons in north east PA. Can't tell you much about them aside from getting a rant about someone else stealing their idea about a wildcat. I wasn't impressed by that experience, but I've heard good things about their rifles.