My father-in-law and I went to Cabelas today and bought a full reloading setup, and during the 20 minutes the guy was helping us he had to tell at least a half dozen guys, "No AR mags at all!"
It is if you're doing competition shooting in a bolt action, but I don't think it's very practical for tactical rimfires. My buddy swears by it in his Savage, and that thing is scary accurate.