MILITARY CARTRIDGE RELATIONSHIPS

© A G Williams

Amended July 2010

This is based on an article which first appeared in "Guns Review" magazine in April 1994, but has been much modified and extended.

It is taken for granted that whenever a new commercial rifle or pistol cartridge is introduced, it will probably be a derivative of an existing round. The 7x57 Mauser, .30-06 and .375 Holland & Holland Belted Rimless Magnum Nitro Express (to give it its formal title) have all parented huge families of variations in calibre, length or shape and are still doing so today. If a cartridge is genuinely new, such as the 10mm Auto, it is likely to find itself rapidly shortened (.40 S&W), lengthened (10mm Magnum) or otherwise molested in the interests of achieving some marginal advantage.

In contrast, military cartridges are rarely interfered with in this way. They are developed with little regard to cost for a specific purpose and once adopted remain unaltered for decades. However, there are well-known exceptions; the 7.92x57 was shortened to make the 7.92x33 Kurz cartridge for the WW2 German MP 43/44/StG 44 assault rifles, and the American .30-06 (7.62x63) was shortened to make the 7.62x51 NATO. Perhaps more surprising is that the 5.56mm NATO was developed from a commercial small-game cartridge - it's usually the other way round.

There have also been some heavy machine gun and cannon cartridges which were derivatives of existing service rounds and these are the subject of this article. Some of these are well known, others less so and some are quite surprising.

Based on the .50" Browning.

The .50" Browning (12.7x99) was among the first of the heavy machine gun rounds, being introduced shortly after the First World War. There is a persistent myth that it was a copy of the earlier German 13mm round (13x92SR) but a glance at the two rounds in this photo will show that they are quite different.

During the interwar period a number of competitors emerged, of which the only survivor is the Soviet 12.7x108 round which uses a unique and slightly larger case.

One which achieved considerable success in the 1930s, being adopted by the French, Italian and Japanese (among others) was the 13.2mm Hotchkiss (13.2x99; 13.2x96 and 13.2x93 are also found). The gas-operated gun was completely different from the recoil-operated Browning but the cartridge was remarkably similar in all dimensions except for calibre; in fact only an expert can distinguish them without having them side by side (the 13.2x96 was made specifically to make identification easier; the 13.2x93 was reportedly produced in order to use the longer German 13mm cannon shells). The Italians designed the Breda M31 AFV gun around the cartridge. Even the Browning M2 aircraft HMG was chambered for this cartridge, by FN of Belgium and by the Japanese Navy Air Force, who called it the 13mm Type 3.

A more significant modification was made in Britain, which not only involved necking the case up to 13.9mm but also adding a belt, to create the .55" Boys anti-tank rifle cartridge (13.9x99B). The American 16mm Vega (16x99) was designed for an unsuccessful WW2 aircraft gun. The .50.30 (7.62x99) looks remarkably similar to the German Patrone 318 (7.92x94) cartridge for the PzB anti-tank rifles, but it was only developed for testing AP bullets at high velocity. The experimental US 10mm XM277 (10x107) was developed in the 1960s for the six-barrel GAU-6 rotary aircraft gun. The cartridge reportedly generated a muzzle velocity of 4,000 fps (1,220 m/s). It might make the basis for a good long-range varmint rifle - if varmints ever grow to elk size! The gun was later modified to revert to .50 cal. Incidentally, it bears no relationship to the current GAU-19/A apart from the rotary configuration.

With the introduction of large-calibre recoilless anti-tank guns in the 1950s, it was decided that the simplest way of ensuring a hit with these low-velocity shells was to fit the gun with a ballistically matched spotting rifle firing bullets with a "flash" compound in the nose. The gunner fired the spotting rifle until a hit on the target was observed, at which point the heavy ordnance was triggered. There was much experimentation to develop the best cartridge for this purpose, but the most successful was achieved by simply shortening the .50" heavy machine gun round to create the .50 Spotter(12.7x76). The 15x99 was an experimental spotting round. Both large RCL cannon and spotting rifles are now obsolete; the former replaced by guided missiles, the latter by laser rangefinders. The .50" Browning soldiers on!

Based on the 20 mm Short Solothurn - possibly

The 20x105B "Short Solothurn" only saw service in anti-tank rifles specifically the Solothurn S18-100 series. Several other cartridges subsequently emerged with the same case diameter (although without the belt), so it is possible, although far from certain, that the Solothurn at least inspired these developments.

The 15x96 and the 20x82 form one of the best known pairings. Early in World War 2 the Luftwaffe introduced an excellent new gun, the Mauser MG 151, firing a 15mm high-velocity cartridge. Good as it was, a clamour for more destructive power arose and led to the case being necked out to form the 20mm MG151/20 round. The extra HE capacity was considered well worth the loss in muzzle velocity and the larger cartridge virtually replaced its predecessor. Modified versions of the 20mm gun and cartridge are still in production in South Africa, under the designation Vektor GAl.

Another European cartridge of the 1930s with the same case diameter is the 15x104 for the Czech ZB vz/60 MG, made in the UK as the 15mm Besa. Again, any actual relationship with the 20x105B is unclear.

During WW2, the Japanese Army Air Force made some use of the MG 151/20, but also developed their own gun, the Ho-5 (ironically based on the Browning M2) in 20x94 calibre, which appears to be a stretched MG 151/20.

The French-designed 20mm Hispano-Suiza HS 404 (20x110) was the classic RAF fighter weapon for most of WW2 (and a decade thereafter). Again, the rim and case diameters are virtually identical to the other cartridges here, so perhaps the 20x105B also had something to do with this.

The Americans made some use of the Hispano, but were always more interested in muzzle velocity, so they couldn't resist necking the case down to .50 calibre to create the .50 High Velocity (12.7x120) of the WW2 era. This effort remained experimental. Nearly half a century later, FN were seeking to develop a more powerful replacement for the .50" Browning, better able to penetrate light armoured vehicles (in effect, a Western equivalent to the Soviet 14.5mm KPV). FN initially selected the 20mm Hispano case as a basis and necked it down to 15mm for the FN-BRG (15x115). The weapon and its ammunition had a protracted development and finally emerged as a 15.5mm, as we shall see.

Based on the US .60" T17 and the Soviet 14.5mm ATR.

Two groups, based on different cases, shown together here.

In 1939 the US Army issued a requirement for an anti-tank rifle capable of penetrating 1¼" (32mm) of armour at 500 yards (460m). This produced the massive .60" cartridge (15.2x114). The rifle never saw service, neither did the T17 aircraft machine gun (developed from captured Luftwaffe MG151) which was intended to use it. Different designs of .60" machine guns (including revolver and rotary versions) were experimented with but without success.

In the constant USAAF search for higher velocity the cartridge was also necked down to .50" (12.7x114), generating up to 4,400 fps (1,340 m/s) with lightweight incendiary bullets. None of the HMGs came to anything and the cartridges are merely collectors' items.

Ironically the Americans learned the same lesson as the Germans had with the MG151 and necked up the case to form the 20x102 M39 round which has been the standard USAF cannon cartridge since the 1950s. Its most famous application is in the six-barrelled rotary M61 Vulcan cannon, which also serves as the business end of the Phalanx anti-missile system.

The USN decided that the 20x102 wasn't powerful enough, and their Mk 12 cannon (based on the Hispano) could take a longer cartridge, so they simply stretched the case to for the 20x110 USN and fitted a slightly heavier projectile. This cartridge was only ever used in the Mk 12 gun (which saw considerable use in the 1950s and 1960s) plus that strange Mk 11 double-barrelled revolver used in the Mk 4 gunpod.

The Soviet 14.5 x114 cartridge was developed for anti-tank rifles in 1939, since an earlier effort in 12.7mm calibre had proved inadequate. The PTRD and PTRS rifles were formidable performers, among the best of the WW2 ATRs. After the war, the cartridge was adopted for the KPV heavy machine gun, still by far the most powerful in its class.

Before the end of WW2, the case was selected for a new aircraft gun, the NS-23, and modified by the simple expedient of necking it out to accept the 23mm shells from the VYa-23 already in service (which fired a much more powerful 23x152B cartridge). The 23x115 cartridge, in modified form, is still in service today.

The 15.5x106 was FN's second effort (after the 15x115 shown above) to make a KPV competitor in the BRG-15, appropriately by necking out the KPV's case, While it seemed to work well enough, it never found any customers and the gun effectively bankrupted the company. Not surprising, really; anyone wanting a gun in that class could have bought the KPV for a lot less cash.

Based on the 20mm Madsen

The first pair in this photo are the 20mm and 23mm Madsen (20x120 and 23x106)

The 20mm Madsen was a popular weapon interwar, being one of the first of the modern, high-performance 20mm automatic weapons, and it saw service in many countries. It was offered in AA, anti-tank and aircraft versions but only the AA model seemed to be a commercial success. The 23mm version of the gun was identical except for the calibre; the cartridge case was shortened as well as necked-out to keep the overall length the same. The 23mm was aimed solely at the aircraft gun market, and the bigger calibre was selected to increase the projectile size (it held twice as  much HE as the 20mm) at some cost in muzzle velocity. The 23mm was proposed for - and tested in - many different aircraft but does not appear to have seen service.

Based on the Swiss 20mm FM-K.

The next pair are the 20mm FM-K 38 drill round  (20x139), and the 20mm HS 820/Oerlikon KAD (20x139)

A really obscure one, this. Before WW2 the Swiss developed a range of weapons using a short-recoil mechanism based on the old Maxim design, in unusual calibers like 24mm and 34mm. They also produced a powerful 20mm version which was used as an AA gun (as the Flab.Kan.38) and (to a more limited extent) in aircraft as the FM-K 38. After the war, the 20x139 case was modified by changing the material from brass to steel and fitting a thicker rim, all in the interests of surviving the battering it would get in much faster-firing guns. It achieved commercial success in the Hispano-Suiza HS 820 cannon (later renamed Oerlikon KAD when the companies merged - also used by the US Army as the M139) and the ammunition is also used in the Rheinmetall Rh202, Vektor G1-2 and GIAT 20M693.

 

Based on the 25mm KBA.

The three on the left are the 25mm Oerlikon KBA (25x137), 25mm Oerlikon KBB (25x184) and an experimental 34x184.

The development of the Oerlikon 25mm KBA cannon is a story in itself, starting with a US military project in the 1960s, being taken up on its cancellation by Oerlikon and eventually reimported into US service. The cartridge has been adopted as a NATO standard and now has a variety of guns chambered for it; gas operated (Oerlikon), chain driven (Bushmaster and GIAT 25M811) and rotary (GAU-12U, GU-22/A). There was even a revolver - the Aden 25 - and a twin-barrelled gun - the GE 225 - but both are now history.

Not satisfied with this, Oerlikon stretched the case to produce the high-velocity 25x184 KBB round, but this has achieved nothing like the same success. Actual case length varies from 173-184mm.

The 34x184 is something of a mystery, probably intended for an AFV project.

Based on the 25mm Hotchkiss AT round.

The pair on the right are the 25mm Hotchkiss (25x194R) and the German 28/20 PzB 41 (28/20x187R).

The rimmed 25mm Hotchkiss case was used in the light anti-tank and AFV gun in French service in the late 1930s. This was taken as the basis for the taper-bore experiments (conducted in 1940 by the Danish Larsen company for the French Army), which were then followed up by the Germans to create the PzB 41, the most significant of the Gerlich type taper-bore AT guns. Muzzle velocity of the PzB was in the region of 4,600 fps (1,400 m/s). It worked well, but suffered from a shortage of the tungsten needed in the projectile.

Based on the 25mm Hotchkiss AA round.

This group shows the 25mm Hotchkiss AA (25x163), Experimental British-made BMARCo 30mm dating from 1942 (30x170), steel-cased HS 831/Oerlikon KCB (30x170) and 30mm Rarden APDS (30x170)

During the 1930s Hotchkiss developed a 25mm AA gun which was adopted by both France and Japan, although it disappeared into history after the war. During the war, the British Manufacturing and Research Company (a Hispano-Suiza subsidiary) produced an unsuccessful 30mm aircraft gun using a cartridge based on the Hotchkiss case. In the 1950s, Hispano developed this idea for the 30mm HS 831, renamed the Oerlikon KCB on the merger of the two companies, which has been widely used as a naval AA gun. Originally it had a brass case, but now uses a steel one. There is something of a mystery about the origins of the BMARCo and Oerlikon rounds since a British report following a visit to HS in Switzerland in July 1945 reported that a 30mm Hispano was available to purchase. More investigations are in hand...

In the 1960s the British Army was looking for a cartridge case of about that power for its new light AFV gun, which emerged as the Rarden. The case reverted to brass to provide better gas sealing, with the aim of preventing gas from seeping back into the turret. The cartridges are not qualified as interchangeable although apparently the steel-cased rounds can be fired in the Rarden (I'm not sure about the other way round).

 

Based on the Oerlikon KCA.

An interesting variety of rounds here.

Oerlikon developed the powerful 30x173 cartridge for its KCA revolver cannon in the 1960s, shown first on the left. This was adopted for versions of the SAAB Viggen fighter, which were retired not long ago.

The USA decided to adopt the 30x173 case for the GAU-8/A rotary cannon, only the case material was changed to light alloy, the priming from electric to percussion, and the driving band to plastic. The same cartridge is fired by the Mauser MK30F AA and AFV gun, and the Bushmaster II / MK44 Chain Gun, although in AFV applications steel cases (but still percussion-primed) are becoming standard.

Mauser took a steel version of the case, fitted it with electric priming and necked it down to create the 27x173, used solely in the SCORE tank-training system. Used in a sub-calibre barrel fitted within the tank gun's barrel, it matches the trajectory of HEAT shells out to 1,000m).

The 35x173 is a mystery. Somebody took the GAU-8/A's case and necked it out to take the shell from the 35mm Oerlikon. I have no idea who, or why, or for what kind of weapon, but I would guess for an aircraft gun, possibly American. If anybody knows more than this, please tell me! The cartridge should be quite a useful performer; I estimate a muzzle energy of 230-240,000 joules, enough to propel the standard 550g shell at 915-935 m/s. And of course, it could fire the AHEAD shells (at a somewhat lower velocity) assuming the FCS problems could be solved.

The 40x180 (not yet in my collection) is the new "Super 40" round intended for rebarrelled versions of the ATK MK44 Chain Gun. Compared with the 30x173, the HE shell (designed for timed airburst) is 50% heavier and the APFSDS performance significantly improved. It is the closest rival to the 40mm Cased Telescoped Ammunition system ordered to replace the British Army's 30mm Rarden in light AFVs, but has so far attracted no orders.

Based on the 30mm MK 213.

The Mauser MK 213 aircraft revolver gun was developed around the end of WW2 and subsequently copied in both France and the UK.  Sadly no photo of the MK 213 round, but it was a 30x85B almost identical to the 30x86B Aden LV shown here.

The French went for a slightly longer case with the 30x97B for the DEFA 540 series (also MK 213 copies) before both countries settled on a case of around 111-113mm for the DEFA 550 (subsequently GIAT, now Nexter) and Aden HV series. Subsequently the US joined the party with the LW (lightweight) variant for the  M230 Chain Gun carried by the AH-64 attack helicopter. Despite common dimensions the three ammo ranges are quite different, the Aden using a brass case, the DEFA a steel one and the M230 light alloy. The Aden and DEFA guns are restricted to their own ammo, although the M230 can use all three types. I'm not sure if the Nexter 30M781, also a power-driven helicopter gun used in some versions of the Eurocopter Tigre, can use anything other than the 550 ammo.

The USA toyed with the revolver cannon and this cartridge in the 1950s, producing variations on the case within the same overall cartridge length, the best-known being the 30x126B T182 shown here.

The provenance of the Swiss 25x116B experimental is unknown, but it was based on the same case.

Incidentally, the British also briefly played with a 20mm cartridge for the Aden in the 1950s. This used the same belted case as the 30mm, unlike the 20mm version of the original Mauser MG 213C, which had a rimless 20x135 case.

The 30x100B WECOM was developed by the US Army Weapons Command for a new helicopter gun, the XM140. This was intended to arm the unsuccessful AH-56 Cheyenne attack helicopter. Later, they had a rethink and adopted the Aden/DEFA case for the M230. Despite the apparent similarity with the Aden/DEFA family, the WECOM cartridge was in fact fractionally different in all dimensions.

Assorted similarities.

The first two cartridges share 23x152B dimensions but are made of different materials and have other differences in primers and loadings. The first was used in the WW2 era VYa aircraft cannon mainly fitted to the II 2 Shturmovik ground attack plane ; these all had brass cases. In the 1950s the cartridge was worked on for the faster-firing ZU series of anti-aircraft guns, the case material being changed to steel. Four of the guns were used in the ZSU Shilka self-propelled AA system. Some towed and naval AA guns remain in service today, and a version is also used in light AFVs. However, brass-cased ammunition (like the one shown) remained in production for a long time, for use in sub-calibre tank gun training devices.

There is no claim of a direct relationship between these and the other two, but they are so similar in case design that it makes one wonder…

The 27x145B for the Mauser BK 27 revolver cannon was designed specifically for the Panavia Tornado strike fighter, although it is also used in Germany's version of the Alpha Jet, in the SAAB Gripen, and the Eurofighter Typhoon. A four-barrel naval CIWS mount (the Mauser Drakon) was developed but not used, but the MLG 27, a remote-control single-barrel mounting, has been adopted by the German navy, is attracting export sales, and is now being offered in a land mounting.

The final round is the 30x150B used in the GIAT 30M791 which is installed in the Dassault Rafale. Capable of 2,500 rpm, it holds the record as the joint fastest-firing revolver cannon (with the Rikhter R-23, in case you were wondering). The rim, belt and head dimensions are different from those of the original 30x113B DEFA case.

 

Based on the 37mm Hotchkiss one pounder

The 37x94R Hotchkiss round is the daddy of them all in terms of automatic cannon development. Following the Declaration of St Petersburg of 1868, which banned explosive shells weighing less than 400g, it was calculated that 37mm was about the right caliber for a weapon firing shells at least this big. Hotchkiss first developed a 40mm six-barrel rotary gun (based on the Gatling but using a refined mechanism), but rapidly switched to a five-barrel 37mm and achieved huge commercial success. It was the first fast-firing shell gun in the world. Larger weapons in 47mm and 53mm followed, but they never matched the 37mm in popularity. So it was natural for Maxim, when he decided to scale up his machine gun to make a shell-firing "pom-pom", to choose the same 37x94R ammo. It was also used in a variety of small manually-loaded guns, including the ones fitted to the French FT light tanks. It was still used in some small French tanks in 1940, albeit with a better loading with a hard-core projectile with a much higher velocity.

Next up is a British variation from WW1, the shortened 37x69R cartridge for the Vickers 1 Pr Mk III aircraft gun. This was a smaller and lighter weapon, but still a belt-fed automatic. It saw little use. The example shown is an HE round fitted with the super-sensitive No. 131 fuze, intended to detonate on airship fabric.

Next in line is the lengthened 30x136R "Heavy One Pounder", the USN's equivalent of the RN's 1½ Pr described below; an effort to produce a more powerful cartridge for the Maxim automatic gun. It was also adopted for the American MacLean gas-operated automatic developed before WW1, named after the man who designed the original version of the Lewis gun.  Both weapons rapidly disappeared into history, although various MacLean guns are still around (one - unlabelled - at the Heugh Gun Battery Museum in Hartlepool) despite only a couple of hundred being delivered to Russia, where it was known as the Maklen gun. It wasn't much good, so the Soviets subsequently distributed them all over the place, including to Spain during their civil war.

A further lengthening of the same basic case produced the 37x145R,  developed by Browning for one of his own cannon designs. This was produced by Oldsmobile as the T9 and adopted by the USAAF as the M4. It was mainly used in the P-39 Airacobra, plus initial batches of the P-63 Kingcobra (later batches getting the M10 which used the same ammo) but quite a few were liberated from the USAAF and ended up on USN PT boats.

Based on the 37mm Vickers 1½ pounder

The second group of cartridges in the photo above used a slightly wider case. The original was the Vickers 1½ Pr (37x123R), developed for the RN just before WW1 to give more range and hitting power than the 1 Pr Vickers-Maxim. However, it saw very little use, being rapidly superseded during the war by the 2 Pr in 40x158R, which was the same case necked out to take the larger projectile. The 2 Pr saw much service in WW2 as a naval AA gun in an improved High Velocity loading, and remained in the fleet until the mid-1950s. The same cartridge case was also used in Vickers Class S aircraft gun. Just before WW2, the RAF became interested in a large-calibre gun for defending bombers. They calculated that a two-pound shell would be able to destroy any contemporary fighter with one hit, and specified a 40mm weapon capable of being fitted into a turret and aimed with the aid of director control. Vickers duly built the Class S gun based on the old 37mm COW gun but chambered for the 40x158R naval round. A turret was also built and fitted to a much-modified Wellington bomber, but the experiment was considered unsuccessful (a conclusion doubtless influenced by the effect of the muzzle blast in stripping the fabric from the Wimpey's structure). Shortly afterwards, the RAF began to realise that it might be helpful to have some means of destroying enemy tanks from the air (this time their minds were probably concentrated by the sight of massed ranks of Panzers waiting to invade across the Channel) and after various experiments (and some competition from Rolls-Royce) the Vickers S gun ended up being fitted to the Hurricane IID and IV, in which role it enjoyed modest success. The AP loadings were specially developed for the gun The Rolls-Royce guns in the same calibre but adapted to manual loading did see service in WW2, fitted to harbour defence launches and the like. Its tendency to explode won it few friends.

Two short-case variations also saw limited service. The 40x79R was used only in the Vickers-Crayford 1.59" gun of WW1. It was a light, manually-loaded weapon intended for use as a portable "trench gun" (the kind of role which the 40mm grenade launcher fulfils today) and also as an aircraft weapon. This was known as the 1.59" after the 40mm calibre. Although usefully light, the gun was too slow-firing to be a success. Its main claim to fame rests in confusing generations of aviation writers with its nickname of "rocket gun" - a misnomer which was due to an early misunderstanding of its nature, probably caused by the spectacular flare emitted by the incendiary/tracer shell when fired at night.

The 40x107R was developed for the 2 Pr HA (High Angle), a purpose-designed sub-calibre barrel insert for low-cost training of RN AA gunners in the interwar period. The cases were usually made from already-fired 40x158R and have the original headstamps as well as "HA". During WW2 a chronic shortage if guns for inshore patrol craft led to some of these inserts being given a proper breech mechanism, a mounting and sights, and packed off to war. Whether any of them ever fired a shot in anger is not known.

Based on the 35mm Oerlikon KD

 

The 35x228 Oerlikon KD series was developed in the 1960s to replace the Swiss 34mm Flab Kan 38 AA gun. The AA gun has been exported to many different countries and remains in wide service today, including being produced in China. It is always found in twin mountings, towed, mounted on vehicles or fitted to turrets (the best known being the German Flakpanzer Gepard). In addition, a slower-firing long-recoil Oerlikon KDE using the same ammunition was developed for the ground-firing role in light AFVs; this has only been adopted by Japan. The most recent Oerlikon gun development is the KDG revolver cannon which fires at no less than 1,000 rpm. This is offered in single mountings for both land and naval use.

The ammunition has developed too, the latest type being the KETF (Kinetic Energy Time Fuzed), formerly known as AHEAD, which is an advanced shrapnel type. Different versions of this have been developed for AA/anti-missile and anti-personnel roles.

During the 1980s Rheinmetall started developing the Rh 503, an externally-powered weapon available with two barrels; one to take the standard 35x228 ammo, the other to take a 50x330 "Supershot" round. The reason for the 50mm is to provide the biggest possible "piston area" for firing saboted rounds, particularly the APFSDS, with the bonus of firing a bigger HE shell. It was intended for the next generation of German AFVs (starting with the 35mm caliber, with the 50mm available as a later upgrade if required) but the end of the Cold War put a stop to it, Germany having selected the cheaper option of a 30mm Mauser cannon for their new light AFVs.

The overall length of the two cartridges is the same; only a small part of the sabot protrudes above the neck of the 50mm case, so little more than a barrel change is required.

ATK (or to be precise one of their earlier incarnations - anyone who can keep track of the endlessly shifting paternity of military weapons deserves a medal) has developed a version of the Chain Gun family - the Bushmaster III - to use the same 35x228 ammunition. This has been adopted by Denmark and the Netherlands in their CV9035 MICVs.

Based (or not) on the 20mm Becker

The Becker automatic cannon was developed during WW1 as the first to use the Advanced Primer Ignition Blowback system (this is explained in more detail HERE)  This used a 20x70RB cartridge. After the war the rights were bought by SEMAG of Switzerland, who continued to develop the weapon and also introduced a longer-cased version, the 20x100RB FFL. The rights were then acquired by another Swiss company, Oerlikon, who not only further developed the guns (the small FF now using a 20x72RB case) but also introduced the 20x110RB Type S, which used an altogether bigger case. Most use was made by Japanese Navy aircraft of the 20x72RB and 20x101RB (slightly lengthened, as shown here) weapons, but Germany adopted the MG-FF as an aircraft gun, modifying it and lengthening the case to create the 20x80RB.

Incidentally, if you see what appear to be Becker or Oerlikon FF cases check them carefully as they may be for the much rarer Ehrhard cannon of WW1. This also used a 70mm case length, but with a slightly wider case with a squarer base.

 

Conclusion

If there is any moral to the above examples, it is that in the military context "necking out" a cartridge to a larger calibre seems to be generally more successful than "necking down". When it comes to hitting power, there is no substitute for calibre!

The assistance of the staff of the Ministry of Defence Pattern Room is gratefully acknowledged.

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