The base and scope tube mating surfaces of many low-cost aluminum rings are formed by extrusion, a process which is sometimes followed by machining operations. Extruded surfaces are neither uniformly flat (base surface) nor cylindrical (ring surface). These surfaces typically have linear striations that result from machining marks or wear on the extrusion die. All extruded aluminum rings will require lapping to eliminate excessive stress in the scope tube. Beware of low cost rings that include thick protective tape or fabric liners (see the paragraph on Leapers AccuShot rings below).
Figure 1. Left: Non-uniform contact between ring and scope tube due to extrusion marks. Right: uniform surface contact between ring and scope after after lapping the rings.
Cast steel surfaces are not uniformly flat or cylindrical either. These uneven mating surfaces impart stress to the scope tube when clamped to flat bases and cylindrical scope tubes. All cast steel rings will require lapping to eliminate excessive stress in the scope tube.
Stamped steel is sometimes used to fabricate ring caps and provides an inexpensive and lightweight, yet strong ring design. Stamped caps are not uniformly cylindrical and will require lapping to eliminate excessive stress in the scope tube.
Higher cost aluminum and steel rings are often machined to make the mating surfaces uniformly flat and/or cylindrical. Starting with an extruded or cast form reduces material waste and machining time, and can produce a precision mount after the mating surfaces are machined. Even with this higher precision, problems with bases can cause the rings to be mis-aligned. We highly recommend that you lap these rings as well.
Machined rings have tighter tolerances and more uniform mating surfaces than extruded, cast or stamped rings. Scope tube diameters still vary a lot, however. Less expensive (<$400) scopes can vary by as much as 0.010” in tube diameter. Higher end scopes tend to have tighter tolerances, and tube diameters usually vary by less than 0.003”. On traditional rings the ring cross section is thin enough for the ring to conform to the scope tube it is tightened to the proper torque value. On tactical style rings, however, the cross section may be too thick for the ring to flex. In this case the ring diameter must be matched to the scope tube diameter.