With use, gun barrels wear away inside so that the bores get larger. With larger bores, there is more room for the gases to expand and the muzzle velocity is reduced. Wear varies for each gun and is dependent on the number of rounds fired, the type of round and the charge used. In late 1952, 16 Fd Regt carried out a regimental calibration in order to correct for the wear induced by extensive use of their 25-pounders.
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The muzzle velocity of one gun (the regimental standard gun) was measured electronically and the remainder of the guns of the regiment were fired and measured against the standard gun over varying charges and ranges (comparative calibration). The muzzle velocity slide on the range cone was adjusted to reflect the true velocity so that for any given range on varying charges all the guns would match the standard gun and the rounds would land in the same place. It was a very precise exercise and hence it was important to have, among other things, shell batches of a consistent weight, to standardise performance. Each gun was laid by clinometer.
Each round was washed in kerosene and weighed individually. Batches of equal weight were made up. As you will see by enlarging the image below, the batch in front weighs 25 pd 2 oz. Our battery surveyors interpolated the range table values and arrived at accurate corrections for each batch of ammo.
For the true 25pdr buffs the ammo is: 25pdr HE Mk 1. RDX/TNT. Lot no C7511 945 Fuzed 119BMk. 10. Lot no. 1078 Wt. 25lb 2 oz.
As a matter of interest the scales used were the Regimental postal scales. The postal clerk, Bdr Woon, was not fit to talk to for days when he found out what his precious scales were being used for.
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Washing each round in kerosene prior to weighing.
(Click to enlarge)
Weighing the ammuntion.
(Click to enlarge)
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