In addition to the Stokes the British produced a 6-inch mortar during World War 1. It was a pretty crude affair by modern standards, mounted upon a wooden base held together by an angle iron frame. It had no bipod but was supported by three adjustable guys, one for elevation, the other two for traversing. In addition to being dug in as shown in Figure 13, the bed was often weighted with sandbags and/or secured with pickets to ensure its steadiness in action.

The British SBML 6-inch mortar of World War 1 fired bombs weighing 60 lbs filled with high explosive and fin-stabilised, both primer and propellant charge being accommodated in the fins. Firing was by fixed striker in the breech.

Laying the mortar in the original line (then called the 'zero line'), was accomplished by plumb line and centre line inscribed on the barrel as with the 19th century mortars described above.

A combination clinometer designed to cater for both elevation and line was clamped around the barrel, round which was engraved a direction scale in degrees marked LEFT and RIGHT of a zero point. To lay for line the clinometer was turned on the barrel until the index coincided with the direction required, i.e. right or left of the zero line. The transverse bubble of the clinometer was then levelled by manipulating the traversing guys.

To lay for elevation the clinometer was set to the amount required, and the bubble levelled by adjusting the elevation guy.

Maximum elevation was 77°, minimum 45°; ranges achieved being 100 yards and 1420 yards respectively.

Figure 13 SBML 6-inch mortar

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