This very unusual design was patented jointly by Purdey's and Ernest Lawrence, their factory manager, on May 13th. 1929. The barrel insert is essentially a dummy cartridge which is inserted into the breech of the gun. When the gun is fired, the striker hits the dummy percussion cap in the centre of the dummy cartridge which completes a circuit between the batteries and bulb within the insert. A beam of light is then thrown from the muzzle of the gun to show point of aim. The object of this ingenious device is explained as follows by The Hon. Richard Beaumont in his book Purdey's The Guns And The Family:-

'In 1929, King George V became very ill, but during his convalescence he felt he must exercise his arms otherwise he might lose his swing and sense of timing; however, he was extremely safety minded and would not allow a gun in any room in the house apart from the gunroom. Athol and Harry Lawrence overcame his problem by building an exact replica of the King's guns in wood - the same stock measurements, two dummy hammers and hollow wooden barrels. The weight and balance were the same, but inside the barrels of his imitation guns were placed bulbs and batteries with small fitted lenses so that when the triggers were pulled a beam of light was projected onto the object aimed at. This idea worked perfectly and His Majesty was able to practise in his rooms until he was well enough to resume normal shooting.
As a result, the try-gun in the Long Room was fitted with the same electrical appliances, and customers have been fitted for their guns with this system ever since. A white card bearing the silhouette of a flying pheasant has been fitted to the wall of the Long Room and, after his measurements have been placed on the try-gun, the customer can see by the projected light exactly where he is aiming. The added advantage is that older customers who have been shooting badly and cannot understand why their shooting skills have deteriorated so much, can be shown where they are aiming'