Unlike Germany, which developed and deployed successful high-velocity 12-inch guns, Britain in this case switched to guns firing larger and heavier shells at lower velocity which could achieve similar performance in range and armour penetration but could deliver a heavier explosive charge on impact.

[1] In 1939 three WW1 BL 14 inch Railway Guns (named Gladiator, Piece Maker and Scene Shifter) were removed from storage at Royal Army Ordnance Corps Chilwell and recommissioned.

[2] In 1940 these guns were issued to the Royal Marine Siege Regiment at Dover in Kent to bombard German batteries and shipping in the Calais area.

A 13.5/8-inch hypervelocity gun (a type of very large-calibre artillery) for stratospheric experiments was developed and deployed near St Margaret's in Kent.

The intended burst zone for the smoke shells was 30 miles (48 km) horizontally from the gun and at 95,000 feet (29,000 m) altitude.