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Discussion Starter · #1 ·
There is a legend that while watching an aircraft carrier performing launch operations for a simulated strike during an exercise, Secretary of Defense Robert McNamara asked, “Why are they so many different type of aircraft?”

And thus, demonstrating his total lack of understanding of military aircraft design.

Whether or not this was based on fact, there were some points of duplication within the Department of Defense that did defy logic, and cause some confusion to this very day. One of these was U.S. 20mm ammunition.

Many people even today think that U.S. 20mm ammunition only came on one or two flavors, but that is hardly the case, by the 1960’s the US had no less than six (6) different types of 20mm ammunition that were all mutually non-interchangeable, and one of them was further divided by the linking exclusive to a gun type.

Starting with the first 20mm introduced in to the inventory: Just prior to WW2, the US adopted the 20mm Automatic Gun, M1, a US produced copy of the British produced, Swiss designed Hispano-Suiza HS404. This used the 20mm X 110 Hispano cartridge, and was found in the likes of the P-38.

Just after the war started the US Navy when looking for a better light AA gun than the Caliber .50 and settled on the Oerlikon 20mm, Mk 1. Coincidentally, the HS404 was a development of the Oerlikon, so it should not be surprising that the ammunition would be similar, but different. The Oerlikon, being a straight blow-back, with advanced ignition, required a rebated rim, thus used the 20mm X 110RB. When Hispano redesigned the case for the HS404, they just increased the rim diameter to the body diameter and left the length, shoulder angle and other geometry the same.

Both of these two cartridges have similar ballistics, a little over a 1/4 pound shell with a muzzle velocity of 2,750 fps.

While the Oerlikon, or as the US navy referred to it, 20mm Automatic Gun, Mk 2, (or Mk 3, fewer cooling ribs, or Mk4, one-piece spring, most common) was regarded as a reliable weapon, the M1 was not. The M1 was redesigned and designated the M2, and was still found lacking, so another redesign later it became the 20mm Automatic Gun, M3. The M3 is found on many late war, or immediate post war cannon armed aircraft such as the F4U Corsair, the P-61 Black Widow, the AD-1 Skyraider, F8F Panther.

Around the end of the war a new safety feature was introduced – the electric primer. Now, a gun could be rendered safe simply by pulling a breaker, even if somebody did accidentally drop the bolt on a live round. This was added to the M3 gun (along with a few more changes to improve the still less that desired reliability) to become the 20mm Automatic Gun, M24, and later M24A1, because reliability still wasn’t where they wanted it.

And, here is where it gets really complicated: When they went to electric priming, nothing else changed in the cartridge, to include the designation nomenclature. Here are the ammunition types available:

M/M2/M3 . . . . . . . . . . . . . . . M24/M24A1
M95 AP-T . . . . . . . . . . . . . . . M95 (Electric Primed) AP-T
M58 HEI . . . . . . . . . . . . . . . . M58 (Electric Primed) HEI
M97 HEI . . . . . . . . . . . . . . . . . M97 (Electric Primed) HEI
M97A1 HEI . . . . . . . . . . . . . . . M97A1 (Electric Primed) HEI
M96 Incendiary . . . . . . . . . . . M96 (Electric Primed) Incendiary

The Navy also wanted to be safe, so they also went with an electric primed, but they decided to go with a larger case body diameter, about 0.19” bigger but kept the same case length. This became the Mark 100 series 20mm ammunition and was used in yet another improved version of the Hispano HS404, the Navy’s 20mm Automatic Gun, Mark 12.

Now, you see we have four 20mm cartridges, all nominally 20mm X 110, but all
non-interchangeable:

20mm X 110RB for the Oerlikon
20mm X 110 Percussion for the M1, M2 and M3
20mm X110 Electric for the M24
20mm X110 U.S.N. for the Mk 12

But wait, there’s more.

The USAAF, and later USAF, never happy with the Hispano’s reliability contracted Springfield Armory to develop a new 20mm based on some of the ideas imported from Germany. This would result in the M39 revolver cannon, but in addition to making a better gun, they wanted to make a better 20mm cartridge, which they did in the 20mm X 102. This was a big improvement over the old Hispano, and could toss that 1/4 pound shell out at 3,400 fps. The M39 and the new ammunition was fitted to later models of the F-86 and worked quite well. Concurrently, Springfield was having GE develop a 20mm based on the old Gatling gun principle, this would also use the new 20mm X 102 ammunition. Unfortunately, the links for the M39 were not compatible with the M61, so the two weapons did not completely share ammunition.

And last, the Army wanted something bigger and more ‘anti-armor” capable than the Caliber .50 to put on top of their new scout vehicle, the M114, so they got Hispano’s latest design the HS820, that used the 20mm X 139 cartridge. This gun was slated to be put on the MICV, but its reliability wasn’t stellar and the Army found a way to get rid of it and replace it with the 25mm Bushmaster (the development of which is and interesting story in itself).

20mm Automatic Gun, Mk4 (Oerlikon)
rebated rim, percussion primed
Magazine fed (supplied with a light coat of grease on case)

20mm Automatic Gun, M3 (used on F7F, F4U, A3D, P-61, F9F, and some PBRs)
M21A1 (brass) or M21A1B1 (steel) case
M36 percussion primer
M7, M8 or M10 links (cases must be lightly oiled prior to loading)

20mm Automatic Gun,M24 and M24A1 (B-36, B-47, B-66, F89, and some PBRs)
M21A1 (brass) or M21A1B1 (steel) case
M52A1 electric primer
M10 links (supplied with a hard wax on case)

20mm Automatic Gun, M39 (late F-86, B-57, F-100, F-101, and F-5)
M103 (brass) or M103B1 (steel) case
M52A1 electric primer
T61E3 links (loaded dry)

20mm Automatic Gun, M61 (F-104, F-105, F-4E, A-7D and all later fighters)
M103 (brass) or M103B1 (steel) case
M52A1 electric primer
T76 links (loaded dry)

20mm Automatic Gun, Mark 12, Mod 0 (A4D Skyhawk, A-7A and B, F-3H, F8U, F11F)
Mark 5 case (brass only)
M52A1 electric primer
Mark 2, Mark 4 or Mark 6 links (loaded dry)

In Vietnam, many riverine patrol craft were armed with 20mm, and what was used was a mixed bag, mostly driven by weapon and repair parts availability. Some got the old WW2 Oerlikons, some got the M3s, and others the M24 or M24A1.

So, when you look at it, 20mm ammunition was a logistics nightmare in the 1960s, and especially in Vietnam.
 

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FWIW, this summer I met a local, retired mechanical engineer who is also a long time gun collector. His first job after finishing college was actually Springfield Armory up in MA, where he worked from June 1964 until June (or Sept) 1966. As a junior engineer he told me that he was introduced to many interesting weapon systems, including the infamous SPIR that Springfield was working on at the time.

Anecdotally, he said his main project at SA was working on an aircraft mounted 20mm cannon, but that the project was ultimately canceled. He told me the name of it, but I don't recall any details as we mostly talked about the M14 rifles. Next time I see him I'll ask him more questions about that 20mm cannon given all the info in Lysanders thread...
 

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Discussion Starter · #3 ·
I have much over-simplified the story of the 20mm X 102 cartridge, this cartridge actually dates all the way back to WW2 and the is interwoven with the Caliber .60 cartridge that was supposed to replace the Caliber .50.

As to 20mm cannon of the 1960s . . . One of the bigger projects was to improve terminal effects of the 20mm HEI on ground targets. In Vietnam, it was found that most 20mm ammunition was optimized inflicting damage on light, relatively fragile, aircraft components, and when used against fixed target or even heavy vehicles they did not create enough damage. So, there was a lot of work on good AP rounds that did not require discarding sabots, as DS are definitely not for aircraft use.

Other projects involving 20mm guns were: the gas operated Vulcan, that was used in the external gun pod, mounting the M139 on Helicopters (which saw limited use in combat), the Navy's Mk 22, the 20mm Vehicle Rapid Fire Weapon System (VRFWS), which in the 1970s yields the 25mm Bushmaster, the M621 and M693, French designs, the Rheinmetall Rh 202, the Hispano-Suiza HS827, and the General Electric GE120.
 

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o_O

Um, OK.
 

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I have a TM for a 20mm aircraft cannon that was made by Oldsmobile but I don't remember which plane it was in. I will try to find it and post a picture of it.

eQ
 

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Discussion Starter · #7 ·
The biggest problem with aircraft 20mm was dodging empty cartridge cases falling from a plane doing a strafing run. Those things HURT!!
And, if you're under any of the M197 armed Cobras, you get three live rounds dumped on you after each burst . . .
 

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As I recall, McNamara was the reason the Army and Air Force had to wear the same fatigues, combat boots, caps, dress shoes, socks, underwear... 🤪
 

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DSA (Defense Supply Agency) really took off under McNamara. McNamara came to the Kennedy administration after working many years at Ford Motor Company and he was a firm believer in standardizing everything and using centralized procurement. A great many items were standardized under McNamara. The Marine Corps was required to cease procurement of the unique Marine Corps utilities (equivalent of army fatigues) and use the same uniform as the army and air force. This resulted in a crisis regarding the Marine Corps emblem and USMC lettering on the left breast pocket of the utilities. This had previously been screen printed by the manufacturer but was omitted on the standardized DSA utilities. The first attempt to address this was the use of a brass plate stencil by supply units that involved black marking ink and a stubby brush but this proved to be a poor solution that produced equally poor results. Into this fiasco stepped a retired Marine by the name of Ken Norton who devised an effective iron-on marking and that turned into a good size company and the iron-ons were sold in the Exchanges throughout the Corps for many years.

Virtually everything ended up being procured by DSA (later DLA) so when you see an item of equipment with the DSA contract number, that's where it started.
 

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Didn't he also have something to do with the first M16's not having a chrome plated bore that caused a bunch of problems?

eQ
 

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As I recall, McNamara was the reason the Army and Air Force had to wear the same fatigues, combat boots, caps, dress shoes, socks, underwear... 🤪
Years after I always heard the question: "Was he a brown shoe?"
Still seeing military movies with Navy guys in Black shoes . . . during WWII
Even Marines!!!
 

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No, the M42 "Duster" used them in Vietnam. The M42 was designed for anti-aircraft but was used in a ground support role in Vietnam.
 

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Discussion Starter · #20 ·
Years after I always heard the question: "Was he a brown shoe?"
Still seeing military movies with Navy guys in Black shoes . . . during WWII
Even Marines!!!
Brown shoes versus black shoes is mainly a Navy thing.

The Army used brown leather for all uniform items up until they adopted the greens in 1954. The Marines used brown leather for all service uniform items and black for the dress uniforms until WW2, in WW2, as a cost savings measure, they dropped the black and used brown exclusively for all items until 1957 or 58 when they switch to all black.

The Navy used black leather for all dress uniforms items even the tan and grey working uniforms, except for the Aviator uniforms which was for winter a forest green uniform that is often confused with the USMC service dress, but are actually tailored like the Navy's tan summer working dress uniform. Even after the Aviator Working Greens were abolished in 2011, the brown shoes have remained for aviation personnel.
 
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