Weaver rail mount

Older Weaver systems used two pieces mounted a distance apart from one another, typically on the receiver of a rifle over the bolt opening, where the cartridge would be inserted and/or ejected.

The slots on a Weaver system are primarily used as a clearance of the locking screw that tightens the clamp to the rail.

[5] The lack of a locking bar on many Weaver-type accessories lends to another alignment issue: side-to-side canting.

Because scopes need to be mounted to a rifle in perfect parallel to the barrel, and to ensure the cross hairs indicate a point of aim (POA) exactly at a bullet's point of impact (POI) at a known distance, a small variation of even one quarter of one degree can cause massive problems at longer ranges.

When tightened down, stress exerted on the base can cause the scope's POA to be off from the POI by as much as several feet at 100–200 yards, and gets progressively worse with greater distance.

The locking bar system allows for even stress to be distributed and prevent canting of the scope mount.

[5] Many rail-grabber-mounted accessories can be used on either type of rail, and accessories designed for a Weaver system will always (SWP) fit Picatinny rails – although not vice versa because the Picatinny locking slot width is 0.206 in (5.232 mm) vs the .180 width of the Weaver, and the spacing of slot centers is 0.394 in (10.008 mm).