I don't know about the new"microgroove rifling" but the original would loose accuracy if you didn't keep it clean on the centerfire ctgs. Never heard anything good/bad about the rimfire. The differance between guilding metal(most centerfire) vs. lead/coated lead, the rimfire probably didn't have had a leading problem unless you rapid fired alot. The only centerfire calibers that I remember with micro groove rifling were .44 & .30-30, there may have been more.