A post on a model railway Boche buster model train pack highlighted an event that would be know as 'The King's Shot'. 

It is an event that is recorded in the The Royal Artillery War Commemoration Book (The  King's  Shot (pages 295 - 296)

The Royal Artillery War Commemoration Book

THE KINGS SHOT

During May, 1918, there arrived in France two 14-inch "railway" guns - the heaviest used by the British Army during the war.

Each gun weighed 270 tons with its mounting, 90 feet in length, moved on four "bogies" having a total of 20 wheels, and fired a shell three-quarters off a ton to a range of 20 miles.
 

These guns were taken over by the 471st Siege Battery R.G.A., and no battery could have been prouder of its equipment - the finest guns yet manned by the Royal Regiment. One known as his Majesty's Gun "Boche Buster'" was sent to the neighbourhood of Arras under Major S.M. Cleeve R.G.A. ; the other, His Majesty's Gun "Scene Shifter", under Captain M.B. Elderton, to the vicinity of Bethune.

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At 2 o'clock on the afternoon of Thursday, August 8th, 1918, H.M.G. "Boche Buster" was waiting at the firing position at Maroeuil with the gun detachment , a fine burly lot of men, specially selected for their physique and experience, formed up alongside under Lieutenant R. W. Tickner, R.G.A.

Suddenly a ripple of excitement spread over the waiting parade. Congratulatory murmurs could be heard, and broad smiles of felicitation appeared on the faces  of all present. It was the colonel of the "group", resplendent in a brand new and glistening pair of French gaiters, donned for the occasion.

Soon after this a number of motor-cars came around the corner, and from the leading one  out stepped H.M. the King, who went straight up to the gun and climbed on to the loading platform, followed by the battery commander, who accompanied him around the top of the mounting. The King frequently stopped for explanations. Arriving at the loading platform, General Sir Henry Horne, who was with the party, asked his Majesty whether he wished to see the gun fire. The King answered "Yes", and asked what target it was proposed to engage, and was told the railway station of Douai, some 18 miles distant.

The gun detachment sprang to their positions in action, and the gun was loaded and elevated. The section commander, who was on the gun sight, did not know the King was looking on just behind him, and turning suddenly round from the sight,  was surprised by finding himself confronted by someone on the very narrow gangway. His look of extreme indignation, followed by a barely suppressed and somewhat vigorous expletive, tickled the King immensely, who, suggesting they were apparently in the way, led the file back on the loading platform and down the latter to a position some little way off.

The gun then fired, and thus began the opening phase of the bombardment of Douai railway station that was not without its effect on the big push by the 4th Army that started that day east of Amiens.

The King suggested, as Douai station was a very important railway centre, that if a certain number of rounds were fired from the gun at uncertain intervals during the night it would probably hamper the enemy in sending re-enforcements from the north  to the big battle of the 4th Army down south. The night saw twelve more three-quarter-ton shells put into Douai station.

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After the armistice, when the civilian population returned to the town, evidence was obtained  from them showing that this first shell to land in Douai was an absolute "O.K.", bursting right in the centre of the railway line and causing enormous damage both to the permanent way and to German rolling stock concentrated in the station. It came as a complete surprise to the Germans, who from that day onwards never used the railway station again (though they repaired the actual line each time it subsequently became damaged), excepting for clearing the forward area preparatory to retreating.

This, when it came to light, was a source of much satisfaction to the battery, as the round had always been known among the men as "The King's Shot," and was looked upon them as having marked the turning point of the war, bringing luck to the battery and victory to the British Armies.

THE KINGS SHOT [Source: Imperial War Museum]

THE KINGS SHOT

Two 14  inch (356mm) Railway Guns were constructed at the Elswick Ordinance Work, Newcastle Upon Tyne. The barrels came from an uncompleted order on a Japanese battleship and were offered to the War office in August 1916. The offer was accepted and the barrels were mounted as railway guns. [1]

Once completed the guns were shipped to France via the Military Port of Richborough arriving in France 26th May 1918. [2]