For half a century after the demise of the Gatlings, Gardners and Nordenfelts, the externally powered weapon disappeared from service, although some thought was given in Germany during the First World War to a gun driven by an aeroengine. The reason for this was that the most successful formula for fighter aircraft was found to be a front engine with fuselage-mounted machine guns. In order to avoid shooting the propeller away, complex engine-driven mechanisms to synchronise the firing of the guns with the rotation of the propeller were adopted. It would have been a logical progression, and in principle much simpler, to achieve this by using the engine to drive the gun. Several firms worked on prototypes, but the war ended before any saw service.
It was the demands of air fighting which led to the revival of the externally powered gun, this time driven by electricity. The first trials with coupling an electric motor to a Gatling gun actually took place much earlier than this, in experiments beginning in 1890 which eventually achieved 3,000 rpm. In the early 1930s the British considered fitting a Gatling-type gun to the Westland F.7/30 experimental fighter (which also presaged the Bell P-39 in having the cockpit between the engine and the propeller). None of this led to anything until close to the end of the Second World War, when the American 'Project Vulcan' was established.
This led to the famous M61 Vulcan rotary cannon, essentially similar to the Gatling. In 20 X 102 calibre, with six barrels and a rate of fire between 4,000 and 7,200 rpm (normally set at 6,000 rpm), this has been the USAF s standard air combat gun since the early 1960s. Guns of this type have been produced in all calibres from 5.56mm to 30mm, with between three and seven barrels. The external power source can be electric, hydraulic, pneumatic or even (in aircraft pods) from a turbine driven by the slipstream.
Current large-calibre models are the three-barrel GAU-19A in 12.7 X 99, the six-barrel M61A1 in 20 X 102 (known as the M168 in army A A service), the three-barrel Ml97 version in the same calibre, the five-barrel GAU-12/U in 25 X 137, seven-barrel GAU-8/A in 30 X 173 and the four-barrel GAU-13/A version also in 30 X 173. All of these normally use external power, and the demands are considerable. The M61A1 has an engine of 26 kW, the GAU-8/A requires 52 kW - enough to drive a small car at 160 km/h. In the SUU-16/A gun pod, a ram-air turbine is used to power the M61A1, a neat solution with the disadvantages of a slower spin-up time (around one second) and a rate of fire which reduces below 6,000 rpm if the airspeed drops below 650 km/h. Guns can also be powered by gas bled from the engine, a solution adopted for the GAU-12/U fitted to the AV-8B.
Finally, Oerlikon have joined the group of rotary cannon manufacturers with the KBD, a seven-barrel gun in 25 X 184 calibre, which is intended initially for the naval CIWS (close-in weapons system) role.
There are two major advantages of the modern rotary cannon. The first (shared with the revolver mechanism) is that the cartridges are moved into and out of the chambers in gradual stages, which permits a much higher rate of fire as well as
Cannon Using External Power
Galling rotary mechanism
treating the ammunition to relatively gentle handling. The second is that with several barrels in use, a high rate of fire can be achieved without over-stressing the barrel. As a result, rotary guns tend to be remarkably reliable for the high performance they achieve.
Another advantage shared with other externally powered mechanisms is that they operate independently of ammunition quality, pressure or recoil characteristics; if a cartridge fails to fire, the gun simply ejects it and loads the next one. Such a failure in a recoil- or gas-operated gun will normally stop the mechanism until the dud cartridge has been manually ejected, although some modern self-powered weapons have complex pyrotechnic devices to eject dud cartridges automatically. The Oerlikon KCA revolver cannon has four such charges; the Russian GSH-301 uses a small explosive charge, directed through a small hole in the chamber wall, to penetrate the cartridge case and
BARREL SUPPORT RING
FIRING PIN HOLD BACK CAM
BOLT GUIDE RIBS
BOLTS IN VARIOUS PHASES OF OPERATION
BARREL IN FIRING POSITION
BOLT LOCKING LUG
LOCKING SURFACE OF THE STATIONARY BREECH
> CAM TRACK IN STATIONARY BREECH CAMS THE BOLTS BACK AND FORTH AS THEY TURN WITH THE BARRELS
Ii A 1» I II FI Uli ignite the propellant directly - a brutal but appar- lower than with rotaries: the 30mm M230 requires a motor of only 5 kW.
The chain gun concept originated in the USA
ently effective solution.
The main disadvantage, apart from its bulk and the need for an external source of power, is that the and has been applied to weapons in a variety of cal-
gun takes a significant fraction of a second to speed ibres from 7.62mm upwards. The two most impor-
up to its maximum rate, whereas gas-operated tant large-calibre versions are the MDHC (now weapons reach peak firing rate almost instanta- Boeing) M242 'Bushmaster* in 25 X 137 calibre neously. This is important in circumstances such as and the MDHC M230 in 30 X 113B; the former for air combat in which firing opportunities may be general AFV/naval use, the latter for helicopters, extremely brief. The latest version of the M61, the More powerful versions have been developed: the
A2 selected for the F-22, has been redesigned to Bushmaster II, chambered for the 30 X 173 car-
achieve quicker acceleration (mainly by reducing the weight of the barrels) but still takes a quarter of a second to reach maximum speed.
There is also a problem in achieving high levels of accuracy with a rotary gun, not just because of the need to align several barrels to the same point of impact (the alignment of the barrels can be adjusted), but also because the projectiles are thrown sideways by the rotary motion of the gun, tridge, and the Bushmaster III in 35 X 228 (with an option to upgrade to the 50 X 330 'Supershot* calibre). An interesting side-effect of external powering is its insensitivity to ammunition type. The Bushmaster II can quickly be adapted, by exchanging a few parts, to fire 30 X 170 Oerlikon (KCB) or Rarden ammunition.
Externally powered weapons have also been developed in France. GIAT offers two such to a degree which varies as the gun accelerates to its weapons: the 25M811 gun in 25 X 137 and the maximum rate of fire. This delays the firing of the 30M781 in 30 X 113B. In Germany, Rheinmetall first effective shot. Having said that, the M61 has a developed the Rh 503 in both 35 X 228 and reputation for high accuracy once it is spinning at 50 X 330 'Supershof calibres; development of peak rate. Rotary cannon also make great demands these weapons has been taken over by Boeing as the upon the ammunition feed system, both in terms of Bushmaster III. Yet another weapon in 35 X 228 is the quantity required and rate at which the car- the South African EMAK 35. As will be seen, tridges have to be supplied to the gun. Finally, externally powered guns are beginning to dominate along with other externally powered automatic can- the AFV and helicopter weapon fields, with the non they are vulnerable to 'hang fires', or delayed main exception of Russian equipment.
A unique application of external powering is used by the Spanish Meroka CIWS. This mounting contains twelve externally powered guns, using
Duplex Chain ignition. It could clearly be disastrous if a cartridge fired after its barrel had moved from the normal firing position.
More recently still, external powering has been introduced in a range of single-barrel machine guns and cannon. Many of them are known collectively as chain guns. These are simple linear weapons, named after the chain which drives the reciprocating bolt mechanism. Unlike the Vulcan, the purpose of this design is not rate of fire but reliability, combined with the ability to vary the rate of fire depending on the tactical needs of the moment. This is particularly important in AFVs and helicopters, which might need to switch between slow, deliberate fire and a high-rate burst at short notice. Chain guns also have a very compact mechanism, a major advantage in AFVs in which turret space is always at a premium. Power demands are much Operating principle of the Chain Gun (Courtesy: d.f. Allsop)
Bolt Head Sliding Member
Heavy automatic weapons for ground forces have been developed to meet a varied range of threats. The first automatic cannon, introduced towards the end of the nineteenth century, were intended for anti-personnel use. By the end of the First World War, these weapons were being adapted, and new ones developed, to counter two quite different threats: aircraft and armoured fighting vehicles. In both cases, this led to an emphasis on high-velocity cartridges. Against aircraft, the main reason for this was to reduce aiming errors by minimising the time of flight. Against AFVs, the need was to increase armour penetration.
These concerns have remained dominant until quite recently, when they have been joined by a renewed interest in large-calibre, low-velocity automatic weapons for use against troops and unarmoured or lightly armoured vehicles.
In a form of parallel evolution, automatic cannon proper have occasionally been supplemented by shoulder-fired weapons, often firing the same ammunition and sometimes capable of fully automatic fire, which are described in Appendix 1.
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