I see that a lot of you here have high opinions of Green Mountain barrels. Are they a company that sells replacement barrels or are they a suppler for certain manufacturers?

Both and more. They make barrels for Knight and some of the T/C's and other companies, and they sell barrel blanks and complete barrels for muzzleloaders, cowboy guns, and barrels for Ruger 10/22.

http://www.gmriflebarrel.com/