Copyright International Ammunition Association, 2010. All rights reserved.
Cartridge of the Month April 2010

20 x 110 mm types- Oerlikon, Hispano and U.S. Navy
Specimens and photos courtesy of Keith Pagel

These three cartridges are all 20mm with cases 110mm long, and their identification can be confusing unless further information is provided.

20 x 110mm RB Oerlikon

The first, and oldest type is the 20mm Oerlikon properly designated 20 x 110mm RB (for rebated base). This was widely used in a 20 mm automatic weapon originally designed by the Swiss firm of Oerlikon in the 1930s, and was probably produced in higher numbers than any other AA weapon of World War II. The United States produced nearly 125,000 guns during World War 2, the British another 55,000 and they were also used the the Germans as the Flak 28 or Flak 29, and by the Italians and Russians. These were popular as anti-aircraft guns, and served as the most common U.S. Navy anti-aircraft gun during WW2, replacing the .50 caliber Browning Machine Guns, but were eventually replaced by the 40mm Bofors with their greater range and larger projectiles. This ammunition was also used in the SSG-36 AT rifle, Polsten. [no further information available.]

This cartridge is frequently found as an inert dummy or drill type having a brown painted projectile brazed to the case which has three holes to identify it as a dummy or drill round. The example shown here is a variation with a regular fired case and primer and inert projectile.

Note that the 21.9mm rim is not as wide as the rear of the case body.

20 mm Oerlikon guns on USS Iowa BB-61 about 1943
The crewman on the left of the elevated gun mount is adjusting the trunnion height
Note the Mark 14 Gunsight and that the censor has obscured the radar antennas on the director and mast
U.S. Naval Historical Center Photograph # K-16469
(For more info on the 20mm Oerlikon guns in naval use, check out

20 x 110 Hispano Suiza

The second type is the 20 x 110 Hispano Suiza round developed in the mid-1930s for the French designed Hispano-Suiza HS 404 cannon, and variants built during World War 2 by the British (Hispano Mark I through Mark V) and the United States M1, M2, M3 and M24. These typically fired a 130 gram projectile at around 850 m/s (2,800 fps) at a rate of about 600 round per minute.

It is similar to the 20x110RB described above, except that the rim is the same diameter as the body and the shoulder is sharper. The gun was adopted by the RAF as the 20mm Hispano, remaining in service until replaced by the 30mm Aden in the mid-1950s, and was also used by US aircraft in and after WW2. The last version of the USAF's gun, the M24, had electric rather than percussion priming. This gun and the postwar HS 804 have been widely used as aircraft and AA guns, and the ammunition remains in production. The cartridge was also used in the postwar Bofors m/45 and m/49 aircraft guns. Besides the U.S. and the U. K. the guns and ammunition were also used by Argentina, Sweden and Yugoslavia, and perhaps other countries.

This example is a "board dummy" as indicated by the remnants of solder where screws were used to attach it to a board.

Note that the rim and base diameter are both the same, about 24.8mm.


20 x 110mm U.S. Navy

The third member of the 20 x 100mm family is the 20 x 110mm U.S. Navy. This was used in 1950s era aircraft guns Mark 11 and Mark 12 used in most U.S. Navy aircraft through the 1970s when the 20 x 102mm M61 "Vulcan" gun replaced it. The USN did not initially share the USAAF/USAF's interest in new revolver and rotary guns. Instead, they improved the performance of the old Hispano design by speeding up its rate of fire and rechambering it for a powerful new round, the 20x110 USN. This was developed by taking the base dimensions of the 20x102, but stretching the case length to the maximum which the Hispano could handle. It hardly seemed worth the effort as the performance is only marginally better than the 20x102. Like its smaller brother, it uses electric priming. The modified Hispano was designated the Mk 12 and was the USN's standard aircraft cannon from the mid 1950s until the 1970s when it was finally replaced by the M61. As well as the Mk 12, the new round was chambered in a remarkable gun, the Mk 11. This is a twin-barrel eight-chamber revolver with a mixture of recoil and gas operation and with partly smoothbored barrels. It was used only in the Mk 4 gunpod. This round remains in production but is steadily going out of use as the planes which carry the guns are retired, and it is likely to become obsolete soon.

The rim and base diameter are both the same, but at 29.5mm the are much larger than previous cartridge type.

Note that the fuze and explosive filler material are not shown in this round.

Anthony Williams, Introduction to Collecting 20mm,



Copyright 2010 by the International Ammunition Association, Inc. All rights reserved.

[Close this page to return to the IAA Cartridge of the Month Index]

Return to IAA main page

  Revised 9 August 2010