Brno/CZ have been playing around with 12 groove rifling since the late 60's, I have a model 2 from 68 and a model 5 from 74, both with 12 groove. I don't think they are noticeably less accurate, but at the same time, they aren't my most accurate Brnos either. They do shoot quite well, but like any 22 rimfire, you need to find the ammo they prefer.