Taper Bore Gun
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Basic Information

A type of ordnance, developed in the late 1930s as an anti-tank weapon1. Broadly an armour-piercing round similar to an APCR round was fired down a barrel which became steadily narrower as it neared the muzzle. Thus the soft alloy skirts of the shell were crushed down behind the penetrator as the round was forced through the shrinking bore.

This design enabled the weapon to use a much larger propellant charge in proportion to the calibre of the penetrator and, by using such aggressive obturation, more or less ensured that as much of the energy of the propellant as possible was transfered to the projectile. However, that same aggressive obturation meant massively increased barrel wear - even when the barrels were lined with tungsten - and an unusually high breech pressure for which the whole gun had to be re-inforced. It also made it very hard to fire anything but AP rounds from the gun as the ballistics of a non-deforming shell in a tapering barrel were dreadful: not only was the calibre of the round limited by the narrow muzzle, but there was a risk of the round clipping the muzzle on the way out…

The only taper bore weapon to see widespread production and use was the German sPzB412 which, despite being designated a heavy anti-tank rifle, was actually an anti-tank gun in all but name. Tapering from 28mm at the breech to 20mm at the muzzle it had an impressive penetrating ability for its size and weight but was discontinued in 1943 (although examples popped up in use until the end of the war), partly due to its impressive cost and partly because of Gemany's chronic shortage of tungsten.
Britain and the US also experimented with taper bore guns - or more accurately, taper-bore adaptors for normal guns such as the British "Littlejohn Adaptor" - but these were quickly made obsolete by the widespread deployment of APDS ammunition that did a better job at a lower price.


1. full source reference

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