Re: Steel or Aluminum Rings?
Aluminum is quite a bit easier to machine, and is much easier on tooling than steel. All things being equal, manufacturers will be able to hold better tolerances, and produce rings faster if they make them out of aluminum. (I'm talking about milling / turning from solid. Extruded rings are generally of quite a bit lower quality / softer aluminum.) What this means from a practical point of view, is that they can either offer aluminum rings cheaper, or hold higher quality for the same price as steel.
At Murphy Precision, we've gone to the other extreme -- We make rings out of stainless steel or titanium. We Wire EDM the ring bodies out of 3/4" plate. This takes maybe ten or fifteen times longer than it would take to mill them out of aluminum, but it offers unmatched accuracy. We are able to hold .0002" tolerances on the ring diameter, and better than .0001" parallelism between the bore and the Picatinny mounting surfaces, since they are cut at the same time, in the same setup. What this means is that as long as your Picatinny rail is straight, and un-stressed, there is no need to lap your rings for a near perfect fit.
- Cameron