Gem-Tech. I have one for my G21 and it's an excellent barrel. I had no desire to purchase an over priced Glock factory threaded barrel in an unusual thread pitch. Not only that, but the chamber in a conventional barrel is much easier on brass... I can always tell when I'm resizing Glock brass in the Dillon.
The chamber isn't unusually tight. The only issue I've had is that 3 of my magazines that worked with the factory barrel failed to feed the last round on the new barrel. The cause was worn magazine springs.. and these mags were all 15 or more years old and have been well used and stored loaded. New springs and good to go with no issues
So far it's feed 230grn FMJ, 230gr Winchester SXT, and 230gr Hornady XTP with no failures after changing the magazine springs out. Only 3 mags needed it, but I changed them all out as my mags are all early LE Marked mags.