11 mm dovetail rail (airgun and rimfire)

The 11 mm figure is nominal. Real dovetails on air rifles and rimfires measure anywhere from 11 to about 13 mm, so a ring or adapter that does not allow for that spread will either bind on the way on or sit loose.

A European 11 mm dovetail is about 11 mm at the top and roughly 9.3 mm at the base, while a US 3/8 inch dovetail is about 9.5 mm at the base and wider at the top, which is why the two are often cross-clamped by the same rings.

To run a Picatinny red dot or scope mount on an 11 mm gun, fit a dovetail-to-Picatinny adapter base first, then mount the accessory to the Picatinny surface it provides.