Lundy,

The earlier foresight tunnels (from the classic era of 10 m 'springers') were made to fit ca. 9 - 9.5 mm barrel grooves.

Most of the 'modern' tunnels fit 11 mm grooves.

The HWs use ca. 9 - 9.5 mm grooves.

You can make shims from 1 mm sheet steel, and these allow the fitting of 11 mm tunnels to the HWs.

The advantage of the 'modern' tunnels is that the elements are much more available, and standardised.

Below is a picture of my HW99S with a Centra Score tunnel (11 mm)

Have fun & a good weekend

Best regards

Russ