Mk I of 80 cwt (4 tons) as originally built was an Elswick Ordnance design which was weakly made and fired only an 80-pound (36 kg) projectile.

Mk I guns remaining in British service were rebuilt with "chase hoops" added around the barrel for strengthening, and the barrel was shortened by 3 inches to keep its centre of gravity at the trunnions.

[4] The breech was rotated to the left to lock it, unlike standard British service BL guns made by the Royal Gun Factory which all rotated to the right to lock.

Australian colonies and New Zealand purchased various 6-inch guns direct from the manufacturers, usually Elswick Ordnance Company, and these versions do not correspond directly with the official "Marks" as adopted by the British government.

[5] The gun equipped some gunboats of the Australian colonial navies : Also on armed harbour vessels in Victoria :[6]