20x110 questions

1 & 2 in the pic 20mm Oerlikon, #3,4 & 5 20 US Navy. The rest are? I think the last one is Oerlikon also.

Last FOUR are Oerlikon 20x110…the first Three are Fired (expanded neck) and the last Unfired.
The expansion is due to the “anticipated Firing” cycle of the Blowback-Recoil Oerlikon mechanism ( Firing pin falls as the cartridge is still chambering, but before reaching battery ( Bolt head is already within chamber);This allows for a light breechblock, and no “locking” mechanism. Hench the rebated rim-case, which can sit in a case-body -diameter Bolt, which enters the entire chamber.

Hispano-Suiza Guns use a normal rimless case, as the cases headspace on the shoulder, and the bolt has a positive “locking wedge” when in battery…Recoil aids extraction, and a gas-bleed unlocks the Wedge.
Doc AV
AV Ballistics Technical Services


Nos. 3,4,5 and 6 all look like 20x110 Hispano to me. Although they were used by the USN more than the USAAF/USAF (especially in the decade after WW2), I wouldn’t describe them as 20mm USN as that might cause confusion with the 20x110 MK100 series of the 1950s and 1960s, which were USN/export only and used a wider case (a slightly lengthened 20x102 Vulcan case).

Thanks Tony!