Tapering one is a pain without a hydraulic follow rest or similar. That said, an AR typically has straight, non- tapered steps, which is much, much easier to do. I've done a bunch of them with no issues, both cut and buttoned varieties of barrels. I've never felt that any of them left anything on the table as a result of turning them down. IMO, if it hurts anything, it's little enough that it's lost in the noise of the platform on which they are going on. Some may claim better, but IME, an AR left in semi-auto form is only good for about .3 aggs, at best, regardless of whether it's barrel was lapped before of after contouring.