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The actual standard is the US Survey Foot. Its the
- Mendenhal Order of 1893. 1 yard = 3600⁄3937 meter or 1 meter = 39.37 inches.
- The International inch standard 1 inch = 25.4 mm (exact).
- Order in Council 411 (1898) 1 imperial yard = 36/39.370113 meter or 1 meter = 39.370113 inches
- Industrial Inch (1927) 1 inch = 25.4 mm (exact) (UK 1930) (USA 1933)
To return to the beginning, there are reasons to be cautious about quoted calibres: are they referring to bore or projectile diameter?TZoli wrote: Friedman's both RN book (British Cruisers, and British Destroyers and Frigates) mentions this 0,661inch - 16,7894 or 17mm machine gun a new light AA for ships under development in 1935 or after 1935.
My friend provided me a low quality sketch drawing of a side view showing a sextuple mount (two rows, 3 gun each) in a mount very similar to the 40mm/39 Pom-pom quad. But due to copyright reasons I cannot share it.
He provided these info:
Also some data . 6 barrels in two rows of three, belt fed, Weight of mount around 3 tons (Vickers, no ammo) or 4tons 15 with 1200 rounds weighing 7.5 cwt and 10,000 rounds on the mount (L class Cover)
Generally based on 2pdr pompom (quad?) firing 85gm bullets at 300rpm. MV 3300 ft/sec. Official calibre 0.661in
(= 16.7894mm to be exact. I wish modern authors wouldn't convert exact inches to approximate but anachronistic metric. 17mm looks like a designed size, and 0.661 its conversion. The bullet weight would determine the calibre.)