I have always thought the same as Old Slacky. Perhaps mounts claiming to be Weaver weren't actually 'in spec' or as below, the slot spacing is off?
AI says...
Weaver mounts are a type of firearm accessory mounting system, characterized by a specific slot width and spacing, primarily used for attaching scopes and other accessories. The slots on a Weaver rail are 0.180 inches (4.57 mm) wide, and the spacing between slot centers is not always consistent. While Weaver mounts are designed to be compatible with both Weaver and Picatinny rails, Picatinny mounts are not universally compatible with Weaver rails due to differences in slot dimensions and consistency.
Key Features of Weaver Mounts:
- Slot Width: 0.180 inches (4.57 mm).
- Slot Spacing: Not always consistent.
- Compatibility: Generally compatible with Weaver and Picatinny rails.
- Typical Use: Attaching telescopic sights and other accessories to firearms.
Weaver vs. Picatinny:
- Slot Width: Picatinny slots are wider (0.206 inches or 5.23 mm) than Weaver slots.
- Slot Spacing: Picatinny slot spacing is precisely 0.394 inches (10 mm) center-to-center, unlike Weaver.
- Compatibility: Picatinny mounts might not fit Weaver rails due to the wider slot.
- Durability: Picatinny rails are often considered more durable and rugged due to tighter manufacturing tolerances.