My impression is that 'five nines' moniker was mainly used for the shells, rather than the guns – which themselves were rarely seen by Tommy unless such artillery was overrun.
Some versions of Wilfred Owen's 'Dulce Et Decorum Est' end the first stanza with:
…deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.
…instead of 'gas shells dropping softly behind'. Apart from scanning better, the 'five-nines' can only be referring to shells.
As for 'Jack Johnsons', another common name was 'coalboxes'.
Edited By Mick B1 on 12/10/2018 17:50:38