Hi Enduroman.
I have two Steyr Pro Hunters from Sportsman : .243 + .30-06. Both were supplied factory cut with 1/2 x 20 UNF threads. Some people think this is not a good thread, especially for a .30 Cal round, but I have never had any problems. Both barrels are 20", and I use muzzle mounted mods, not reflex type ( personal choice ).
I use a Roedale Delta on the .243, which is superb, but went for a heavier ASE Jet-Z on the .30-06 - which is also excellent. Can`t help personally with the A-Tec, but a friend has one, and swears by it.
All the best.
Neil.