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In July of 1918, the British military formally adopted a Parker-Hale system of adapting .303-caliber arms to .22 rimfire for short range training. The system involved lining standard barrels with .22 caliber blanks that were machined with full size .303 chambers. Special cartridge inserts were used which had the external dimensions of .303 cartridges, but held .22LR cartridges inside. This allowed the weapon to use the same feeding system as it normally would, and to duplicate the handling and trigger mechanism of a standard .303 caliber gun wile only firing a small rimfire cartridge.
Parker-Hale built kits of this type for the Lee Enfield rifle, the Lewis LMG, and the Vickers HMG, and all were used by the British military. In the Lewis and Vickers, there was no accommodation made to actually cycle the guns; this sort of .22 practice drill was done by manually cycling the actions after each shot. Still, it was a good way to practice basic drill (loading, unloading, etc) as well as basic marksmanship without the noise of full power cartridges and without the need for a full-power-rated backstop.
The system for the Lee Enfield was replaced in 1920 (after only about 2 years of use) with the No2 MkIV* rifle, a dedicated .22 conversion of an SMLE. Parker-Hale kept the system in their commercial catalog into the 1930s, however.
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