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Weaver rail

The older commercial base: narrower 0.180 inch slots with no fixed spacing, and the one-way compatibility that trips people up.

Rail cross-section, not to scale
Designation4.57 mm slot, non-uniform spacing
Rail standard
Slot width
4.57 mm (0.180 in)
Slot spacing
not standardised, cut where needed
Slot floor
rounded bottom
Top width
close to Picatinny, so most rings clamp
Seats on it
Weaver-cut ringsmany low-profile Picatinny rings that clamp on width
Does not seat
Picatinny mounts with a 5.23 mm recoil lug

A Weaver slot is 4.57 mm wide against the Picatinny 5.23 mm, and Weaver slots are not on a fixed pitch, so a mount that relies on indexing into a specific slot spacing may not line up.

A Weaver-pattern mount usually fits a Picatinny rail because its thinner cross-bar drops into the wider Picatinny slot. The reverse fails: a Picatinny lug is too wide for the Weaver slot.

If a ring only clamps the rail on width rather than locking into a slot, it may hold on either pattern, but recoil-locking mounts should match the pattern they were cut for.

Rings for a Weaver base
Search Amazon for Weaver scope rings 1 inchaffiliate searchFind Weaver scope rings 1 inch on eBayaffiliateSearch Amazon for Weaver scope rings 30 mmaffiliate searchFind Weaver scope rings 30 mm on eBayaffiliate
Related rails and fitments

Sources: Picatinny vs Weaver rails, Weaver Mounts · Scope mounting: Picatinny vs Weaver, NRA SSUSA · MIL-STD-1913 Picatinny rail dimensions, Wikipedia