Here's a couple of WW2 Anti-Aircraft Gunnery certificates for an Amble AB who spent many years on the Broomhill Colliery Steamers (my grandfather)
During the war I think the Board of Trade took more control of how men were deployed and you could end up working on any line, not staying with your original employer.
I'm not sure what the Broomhill Colliers had as armaments during the war, perhaps just a lewis gun on the bridge? who knows.
We can see he qualified for the .303 Lewis Gun, Oerlikon .30 Marlin, .303 hotchkiss, and the dreaded (for the operator) PAC or Parachute and Cable.
Some of the IWM photos of these guns:
Oerlikon, on a Merchant Ship:
THE MERCHANT NAVY DURING THE SECOND WORLD WAR. © IWM (A 11269)IWM Non Commercial Licence
Lewis Gun:
Gun, Lewis, .303 inch [2nd issue] (IWM). © IWM (FIR 9220)IWM Non Commercial Licence
Merchant Navy training with the .30 Marlin, 1941
MEMBERS OF THE ROYAL MERCHANT NAVY AT FIRING PRACTICE DURING THEIR ANTI-AIRCRAFT GUNNERY COURSE. 1941. © IWM (A 5395)IWM Non Commercial Licence
Twin Hotchkiss on a Merchant Ship, 1942:
TWIN HOTCHKISS GUN ARMAMENT ON BRITISH MERCHANT SHIPS. 7 MAY 1942, ON BOARD THE TROOPSHIP SS SCYTHIA, AT LIVERPOOL.. © IWM (A 8519)IWM Non Commercial Licence