I believe the Warne fixed rings are sized for either Weaver style or Picatinny bases. Weaver slot widths = 0.18" (4.572mm) and may or may not be evenly spaced. Picatinny slot width = 0.206" (5.232 mm) and all slots are spaced at 0.394" (10.008 mm). Picatinny slots often are numbered to make replacement (at the same position) easier.