I'd buy whatever sporter I liked and then pay a smith $50-75 to lop it off at 20-21" and give it a fresh crown. That's assuming the same rifle wasn't already available in the shorter-barreled version. When you open the door to bobbing a couple inches off a sporter, you have more options than just factory carbines. It's not like 2-4" of sporter tube really weighs that much out front anyway.
