Gasoperated guns

linear mechanisms a few years after maxim introduced the short-recoil gun, John Browning invented a different principle of operation for his first machine gun. After observing that the propellant gases escaping from the muzzle still had considerable force, he devised a simple mechanism to trap some of the gas and use it to drive the loading cycle. In his first practical machine gun, the gas was tapped from the barrel before the projectile left the muzzle. A prototype was sold to Colt by 1890 and the gun was adopted by the US Navy in 1895. The inventor cannot have been entirely satisfied for, as we have seen, most later Browning machine guns and cannon used recoil mechanisms.

In principle, gas-operated linear-action machine guns have worked in the same way ever since. There are variations in how far up the barrel the gas port is sited, and in whether the gas drives a piston which is connected to the action or drives the action directly, but these are detailed refinements. The only significant variation is the nature of the mechanism the gas is driving: linear, revolver or rotary. Gas-operated weapons were exclusively linear until the invention of the revolver cannon, which did not see service until after the Second World War. Gas-oper-ated rotary cannon are even more recent.

Gas-operated linear-actions have proved particularly popular in small arms, three of the most famous examples from the first half of the century being the Lewis gun, the Browning automatic rifle (BAR) and the Bren gun. The vast majority of the current generation of assault rifles are gas-operat-ed, as are most light machine guns. Gas operation has two advantages over recoil: as the barrel remains fixed to the weapon, it is more likely to be accurate; and it is easy to regulate the amount of gas operating the mechanism in order to ensure that the weapon functions properly regardless of climatic or ammunition variations. The exact means of locking the bolt to the barrel has as many variations as recoil-operated guns, with rotary bolts, lifting locking pieces and tilting or sliding breechblocks all being employed.

In heavy automatic weapons gas operation has constantly rivalled the recoil type. During the inter-war period, the French Hotchkiss in 13.2mm and 25mm, and Italian Breda M31 in 13.2mm and M35 in 20mm, were among the earliest examples. The Soviet 12.7mm DShK (Degtyarev-Shpagin Krupnokalibernyi; heavy-calibre gun designed by Degtyarev and Shpagin) and 20mm S h VA K (Shp italnyi- VIa dim iro va A via tsionnaya Krupnokalibernaya; heavy-calibre aircraft gun designed by Shpitalnyi and Vladimirov) followed in the late 1930s. The 12.7mm UB (Universalnyi Berezina; general-purpose gun designed by Beresin) also incorporated gas operation in a mechanism reportedly inspired by the 20mm Lahti cannon, captured during the Winter War with Finland, and the later 23mm VYa ( Volkov- Yartsev, named after the designers) uses the same system.

The two Soviet 12.7mm guns were direct equivalents of the .50" Browning, the DShK being intended for army use while the UB was an aircraft gun. Both fired a 12.7 X 108 cartridge. Ballistic performance was virtually identical to the Browning, but the UB in particular was both lighter and faster-firing than comparable weapons of other nations. The UB replaced the earlier and little-used ShVAK, designed around a quite different 12.7 X 108R cartridge, which later achieved success when enlarged to 20mm calibre. The UB was available in three versions: the UBK wing gun, the UBS for synchronisation and the UBT for fitting into turrets.

The Soviet ShVAK was a light, reasonably short and fast-firing gas-operated gun firing a rather old-fashioned, straight-cased, rimmed and percussion-primed 20 X 99R cartridge of modest power. In 1944 the ShVAK was replaced in production by the B-20 (designed by Beresin), which used the same ammunition and had the same performance but weighed only 25kg.

The Czech Brno firm used gas operation in their 15mm ZB vz 60 (produced in Britain as the Besa). Wartime Germany saw the development of the Rheinmetall-Borsig 3.7cm FlaK 43, as well as the unsuccessful 5cm FlaK 41 and the experimental 5.5cm FlaK 58 by the same firm.

Post-war, gas operation has featured in several linear guns in which a high rate of fire combined with light weight is required. These include two

Mk20 FlackRussian 7mm Dshk Recoil SpringsGas Operated BoltBarrel Cannon Oerlikon Kaliber

The cannon is ready to fire, the bolt is retained by the sear and the return spring is compressed. A cartridge is ready to be fed into the chamber

The trigger is released and the bolt feeds a cartridge into the chamber. The breech bolt locks to the barrel, the firing pin strikes the primer and the cartridge is fired

After the projectile passes the gas port, the gas piston unlocks the breech block from the barrel. Residual gas pressure accelerates the breech block after the bolt has unlocked. The empty cartridge case is extracted and ejected

A fresh cartridge is chambered when the bolt is pushed forward by the breech block return spring

Gas operation (Courtesy: D.F. Allsop)


HMGs - the Soviet 12.7mm NSV (.Nikitin-Sokolov-Volkov, named after the designers) and the CIS 50MG (Chartered Industries of Singapore) - Soviet aircraft cannon such as the NR-23 (Nude I man-Rikhter, the designers), NR-30 and N-37, and several light AA guns: the Soviet 23mm ZU, French 20mm GIAT M621 and M693, and various Oerlikon and Mauser models. However, for reasons explained later it is arguable whether many of the modern guns should be described as gas-operated or gas/blowback hybrids.

The ZU-23 (Zenitnaya Ustanovka, or AA system) uses a vertically sliding breechblock combined with a separate cartridge rammer and extractor, all powered by gun gas. It uses a version of the 23 x 152B cartridge which first emerged in the wartime VYa aircraft cannon, but with steel rather than brass cases, a belt of a slightly different shape and a different primer.

The French firm GIAT (Groupement Indus trie I des Armements Terrestres) has produced the 20mm 20M693 using the percussion-primed 20 x 139 cartridge. It is available with dual belt feed and, like many modern AA cannon, has a floating firing mounting to reduce recoil.

The Oerlikon KAA, KBA, KBB and most of the big 35mm KD series are all gas-operated, albeit with a variety of approaches to locking the bolt and with blowback assistance to aid extraction. Incidentally, the Oerlikon designation system has been quite logical: the first letter - K - stands for Kanone; the second stands for the calibre -A = 20mm, B = 25mm, C = 30mm and D = 35mm; and the third indicates the model of gun. The 35/1000 revolver cannon, described later, has broken away from this pattern.

The KAA of the 1950s, initially known as the 204GK (for Gurt-Kanone, or belt-fed cannon) introduced a new 20 x 128 percussion-primed cartridge and uses pivoting bolt locks. It is most commonly used in the A A role. The KBA emerged from American developments in the 1960s by TRW (Thomson, Ramo-Wooldridge), which resulted in the experimental TRW 6425, using a rotating bolt-head lock. Oerlikon took over and modified the design into the KBA, using a powerful new 25 x 137 cartridge in a compact, dual-feed gun. This cartridge has become a NATO standard and is effectively regarded as a higher-powered replacement for the 20mm, although weapons in the latter calibre remain in production and widespread use. The 25 x 137 has become available in a wide variety of weapons with every conceivable use in all three services. More recently, the powerful KBB has been developed, based on a 25 x 184 cartridge which is a lengthened version of the 25 x 137.

The largest of the gas-operated family is the big Oerlikon KD series (with the exception of the long-recoil KDE), which is effectively a scaled-up KAA based around the very powerful 35 x 228 cartridge. It combines the usual features of the modern series including floating firing and selective ammunition supply, which in this case can be via belts or a link-less feed system.

It should be noted that the full range of Oerlikon cannon is more complex than is indicated here, with many sub-types developed for specific applications.

Mauser has produced a range of guns based on a mechanism which uses gun gas not just to power the firing cycle but also to drive the ammunition feed, which may be linkless or via a disintegrating belt. Three calibres have been offered - the MK20 (20 x 139), MK25 Mod E (25 x 137) and MK30 Mod F (30 x 173) - of which the largest has so far been the most successful.

The hybrids

Many automatic cannon combine elements of more than one operating system in their design. Probably the most common of these are the gas/blowback hybrids, typified by the famous Hispano-Suiza HS 404 aircraft gun.

It should be explained at this point that Hispano-Suiza was a rather complex organisation, with (at least) semi-independent branches in France, Switzerland, Spain and (later) the UK: the British Manufacturing and Research Company (BMARCO) was established in the late 1930s to manufacture the HS 404. Post-war, the Swiss and French branches were sometimes in competition with each other, offering separate products.

The 20mm HS 404 was developed by the French branch during the 1930s by a Swiss, Marc Birkigt, in order to improve upon the unsatisfactory rate of fire of the Oerlikon FFS guns which Hispano-Suiza was then making under licence. It is a linear-action

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25 mm Oer I ikon KBB sectioned (Courtesy: Ian Hogg)

Zastava Revolver

30mm Zastava M 89 (Courtesy: Ian Hogg)


weapon usually described as gas-operated, but that is not strictly correct. In a purely gas-operated mechanism, the gas tapped from the barrel is used to drive the entire reloading cycle; in the Hispano, this gas only unlocks the bolt from the breech and the remainder of the cycle is driven by the recoiling cartridge case, just as in a blowback weapon. In fact, the entire gun recoils about 20-25mm on firing and this energy is used to drive (via an intermediary spring) the ammunition feed in belt-fed versions, so the gun system incorporates elements of three different operating principles: gas, blowback and recoil.

One consequence of the similarity to the API blowbacks is that case extraction can prove difficult. The first HS 404s used oiled cartridges, but the British did not like this system and were able to design a cartridge case which did not need oiling. Most guns of this type now have fluted chambers, so it is surprising that the USNs Mk 16 (a modified HS 404 on a naval mounting, still in service in the 1980s) was equipped with a built-in cartridge oiler.

The HS 404 (and its post-war refinement, the Swiss-developed HS 804) fired a powerful percussion-primed 20 X 110 cartridge which bore a close resemblance to that of the Oerlikon S except that it had a sharper shoulder and did not have a rebated rim. The muzzle energy was some 15-20% higher than the Oerlikon, probably because it could accept higher pressure limits. Initially, ball rounds were preferred by the RAF as these were found to have considerable destructive effect, but by the end of the war HE/I and AP/I shells were standard.

Birkigt's gas-unlocked blowback mechanism has the virtues of light weight and relative simplicity so it has survived in the post-war Hispano range, examples of which were subsequently adopted by Oerlikon after the long-lasting rivalry between the firms was ended when Oerlikon absorbed Hispano-Suiza in 1971. The HS 404 itself was further developed in the UK and, in several forms, in the USA.

In fact, most modern gas-operated linear cannon could be described as gas/blowback hybrids. This is because their very high firing rates necessitate the breech opening very early, while gas pressure in the chamber is still relatively high, so this residual pressure is utilised to help accelerate the bolt and blow the fired case out of the chamber. It is therefore arguable as to which section to describe them under. As with other weapons using blowback extraction, fluted chambers are commonly used to ensure that the fired cases do not stick.

The first of the post-war guns in the field in 20mm calibre was the Hispano-Suiza HS 820, based on a powerful new 20 X 139 cartridge developed in the late 1940s. The gun, which uses percussion priming, is available in magazine, drum or belt-fed versions, but belt feed is usual. Since Oerlikon took over the Hispano company, the HS 820 has been redesignated the Oerlikon KAD. Hispano next introduced the HS 831, now known (in a modified form) as the Oerlikon KCB. This is a scaled-up version of the HS 820 using a new 30 X 170 cartridge (subsequently adopted in a yet further modified form for the British Rarden cannon).

The Hispano/Oerlikon family has not been the only one to use a gas-unlocked blowback mechanism. The Rheinmetall Rh202 is a particularly successful gun which uses the 20 X 139 cartridge introduced in the Hispano-Suiza HS 820. It features a floating firing mechanism to reduce recoil and a dual belt feed, enabling a rapid switch between HE and AP ammunition. It has been widely adopted in AA and AFV roles.

The British Royal Air Force, among many others, adopted the HS 404 in various marks, of which the most common were the Mk II and the (mainly postwar) Mk V. The HS 404 was also made in the USA, initially as the 20mm AN-M1/M2 and in modified form post-war as the M3 (percussion) and M24 (electric ignition). The design was further developed into the Mk 12 naval cannon, chambered for the more powerful 20 X 110 USN or Mk 100 series round.

The gas/blowback principle was also featured in the Italian Scotti system (which actually preceded the HS 404), except that a rotary lock was employed. Scotti designs were made (frequently by Isotta-Fraschini) during the Second World War in 12.7mm and 20mm calibre. The Japanese Army 20mm Type 97 anti-tank gun, together with its associated Ho-1 and Ho-3 aircraft cannon and the closely related 20mm Type 98 AA gun, used a different form of lock, in which a gas piston cams a locking piece down, leaving the breechblock to recoil.


Oerlikon 20mm Kaa











Section Views of the Hispano-Suiza Action.

Birkigt Type 404 20mm (Hispano-Suiza) cannon (BuOrd, USN)

30x170 Hispano SuizaFrench Cannon Breech MechanismMachine Gun Giat M621

20mm HS 820l0erlikon KAD: top - 50-round drum, bottom - 10-round box (Courtesy: Oerlikontlan Hogg)

The French GIAT 20M621 is a modern lightweight gas/blowback gun based on the American electrically primed 20 X 102 round.

Not all hybrids have used gas pressure to unlock the bolt; it is equally possible for a delayed blowback weapon to use recoil unlocking. These are much more difficult to distinguish from short-recoil mechanisms, however, as the initial rearward movement of the barrel gives some impulse to the bolt before unlocking occurs and blowback takes over. The distinction is that in a short-recoil gun, the principal driving force is the momentum of the bolt assembly, which therefore needs to be reasonably heavy. In a recoil/blowback hybrid, the principal driving force is blowback, so the bolt can - and, indeed, needs to be - as light as possible. In practice, the difference between short-recoil and recoil-unlocked blowback is one of degree rather than kind and is related to the rate of fire.

As the bolt is lightened in order to increase the firing rate, blowback becomes steadily more important.

These hybrids can achieve higher rates of fire than other linear mechanisms, due to the fact that the maximum potential rate of fire is directly linked to bolt speed, which in turn depends on bolt weight, and delayed blowbacks can utilise the smallest and lightest bolts. This is because they neither need much momentum nor carry the additional weight of a gas piston assembly. Of the types of hybrid, the short-recoil/blowback has the potential for the fastest rate of all, partly because the initial barrel movement gives the bolt a 'flying start1, partly because the design lends itself to the use of a bolt accelerator to give a further impulse to the bolt. There is another incidental advantage of the short-recoil type: it is easier to engineer the design so that the bolt is pulled slightly away from the breech face


during the initial barrel movement, providing some positive initial extraction and reducing the risk of sticking cases.

These advantages may well explain why the Russians adopted recoil-operation for the remarkable GSh-301 aircraft cannon, which has such a high rate of fire for a single-chamber gun (1,500-1,800 rpm) that blowback forms an important element of its operating cycle. Gas pressure remaining in the barrel when the breech is opened is so high that the fired cartridge case is ejected at a 'breech velocity' of 100 m/s. A 'return accelerator' is also used to return the breechblock as quickly as possible. The barrel is somewhat shorter than that of most of the Russian 30mm guns, in order to permit the quickest possible breech-opening time.

Not all hybrids have relied on blowback. One of the notable exceptions was the Rheinmetall-Borsig MK 103 aircraft gun, in which barrel recoil operated the belt feed and initiated bolt motion, while a conventional gas system unlocked and accelerated the bolt. A primary concern of the designers appeared to be to ensure that the bolt did not unlock too early, so unlocking was delayed until the chamber pressure had dropped to the point at which blowback was not a dominant feature. It seems that the main reason for the barrel recoil was to absorb the recoil pulses without the necessity of allowing the whole gun to recoil in its mounting. The designers then presumably reasoned that the recoil movement might as well be put to good use. This is an interesting example of the difficulties that can exist when trying to place gun mechanisms into neat categories.

More recently, the Russians developed the 2A42 AFV/helicopter cannon which first emerged in the early 1980s in 30 X 165 calibre. It is perhaps surprising that it is not externally powered, given the modest rate of fire requirements, but it is quoted as using a mixture of gas operation and barrel recoil in the design.

Hybrid power sources can relate to the ammunition feed as well as the gun action. Externally powered ammunition feeds are normally associated with externally powered guns, but there have been rare exceptions, e.g. the GZ 1-FF electrically driven belt feed for the MG-FF API blowback cannon.

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