Longrecoil mechanisms

The need for the entire barrel/bolt unit to recoil a considerable distance, followed by a delay in commencing reloading while the barrel returns into the firing position, gives long-recoil mechanisms a relatively low rate of fire. As a result, weapons using this system are much less common than the short-recoil type. However, the long and relatively soft recoil push puts less strain on the mechanism and makes accuracy easier to achieve while avoiding the need for the whole gun to recoil in its mounting. It has therefore been more popular in large-calibre cannon of 37mm and upwards, one of the earlier examples being the British 1 Vi pdr (one-and-a-half pounder) COW (Coventry Ordnance Works) gun of the First World War. This weapon was not particularly successful but did lead to the Vickers 40mm Class aircraft gun of the Second World War.

LEFT BARREL

ACCELERATOR

RECEIVER

LINK PIVOT POINT

LEFT BARREL

ACCELERATOR

RECEIVER

LINK PIVOT POINT

Pivot Mechanisms

LOCKING lUGS

Double Barrel Gun. Barrel Extension Recoils, Unlocking Rotary Locking Blocks.

Gast twin-barrelled mechanism (BuOrd. USN)

LOCKING lUGS

Double Barrel Gun. Barrel Extension Recoils, Unlocking Rotary Locking Blocks.

Gast twin-barrelled mechanism (BuOrd. USN)

The ubiquitous John Browning also produced long-recoil designs, most notably used for the 37mm M4 and M10 aircraft guns, the 37mm Ml AA gun (and its M9 aircraft gun derivative) and the USN's 1.1" AA gun. The Japanese 37mm Ho-203 and the scaled-up 57mm Ho-401 aircraft guns also used long-recoil mechanisms.

The British Rarden 30mm AFV gun is a current example of this system, adopted partly because the main priority was for single-shot accuracy rather than rate of fire, and partly because the much longer period before the breech opens after firing allows more of the propellant gas to escape from the barrel, minimising gas pollution within the enclosed turret. The same reasoning led to the development of the Oerlikon KDE, a member of the 35mm gun family and the only one not to use gas operation. The combination of a low rate of fire and the relatively long recoil movement (which has a recoil distance of 170mm, rather than the usual 55-60mm of the gun mounting) makes the KDE much easier to control than its faster-firing cousins.

The Russians learned the same lesson following unsatisfactory experiences with the gas-operated 30mm 2A42 cannon in the BMP-2 AIFV (armoured infantry fighting vehicle), which suffered considerable problems (especially at its higher rate of fire) with fumes and smoke entering the turret. While the 2A42 has remained in use for external mounting in helicopters, the long-recoil 2A72, with a 330mm recoil movement, has been developed for the BMP-3. Both were designed by Gryazev.

The most famous users of a version of the Tong-recoif system are the Bofors family of weapons. In this case a vertically sliding breechblock opens during the recoil phase and the spent case is ejected rearwards by a separate mechanism. During the 1930s automatic weapons from this firm were available in 20mm and 25mm (with a 37mm version adopted by Russia and subsequently China) but these were eclipsed by the success of the 40mm gun

Type Machine Gun
GSh-23 twin-barrelled gun (Courtesy: MoD Pattern Room)

Barrel moving Lock Open

Forward Barrel Release

Push Unlatch Mechanism How Works

Spent Case Bolt being Extracted Latched

Walther Gsp Bolt DisassemblyM249 Ejecting The Cycle Operation

Latch Lugs Engaged Bolt Released

Bolt

Unlatched

Return

Smg Breech Blocks

Buffer

Bolt Latch

Return

The breech block and barrel are slowed by the barrel spring and the breech block return spring. Towards the end of the stroke the breech is unlocked from the barrel

Barrel moving Lock Open

Forward Barrel Release

Spent Case Bolt being Extracted Latched

The bolt is held to the rear and the barrel moves forward under the influence of the barrel spring. As the barrel moves forward the empty cartridge case is extracted and ejected

Bolt Latch

With the round chambered, the breech block is locked to the barrel. Upon firing the breech block and barrel are driven backwards

Buffer

The breech block is released, a fresh round is chambered and the breech block is locked to the barrel

Latch Lugs Engaged Bolt Released

Bolt

Unlatched

Bolt moving Forward

Long-recoil mechanism (Courtesy:

Rarden Gun
30mm Rarden long-recoil gun (Courtesy: Ian Hogg)

which achieved fame as an AA weapon in the Second World War and has been continually updated ever since.

The original 40mm gun, the L56 (now commonly known as the L60), was developed in the early 1930s and fired a powerful 40 X 311R cartridge. The quality of the design and manufacture were such that the rate of fire was a competitive and reliable 140 rpm, and the combination of virtues which this gun represented was enough to make it the supreme weapon of its class. The gun was available in both air-cooled and water-cooled versions (for army and navy use respectively) but with the exception of some USN versions was always fed by a four-round clip held in guide rails above the breech.

After the war an improved L70 version was developed which used an even more powerful cartridge, the 40 X 364R. The rate of fire was increased, initially to 240, subsequently to 300 and, in the latest Trinity version, 330 rpm. Improved mechanisms for feeding ammunition to the breech also improved the sustained rate of fire, and with powered mountings, advanced fire-control systems and the latest ammunition technology, the gun system remains highly competitive today. The 40mm guns were also joined by 57mm versions post-war. These were produced in an army version but have achieved more success as naval weapons. A Bofors 57mm gun designed for aircraft, the m/47, used a much smaller and less powerful cartridge.

Incidentally, the term L70 (or L/70) means that the barrel length is seventy times the calibre; in this case, the barrel is therefore 2.8m long. Generally speaking, the higher the L number or calibre length, the higher the muzzle velocity.

The particular type of recoil mechanism used in the Bofors has the advantage that the length of the recoil stroke is not determined by the cartridge length. In fact this, like the Oerlikon KDE, is not a 'pure' long-recoil design as the breechblock is unlocked and extraction commences before the barrel has recoiled the full distance required to chamber a fresh cartridge. It is therefore possible to shorten the recoil stroke, and this has been done (from 250 to 100-110mm) by OtoBreda in its Fast Forty gun, as part of a package of measures which have increased the rate of fire to 450 rpm. The speed of the Chinese NORINCO 37mm Type 76A, based indirectly on the Bofors mechanism, has been increased in a similar way.

The long-recoil mechanism has also been adopted for some of the new generation of automatic grenade launchers (AGLs), although most use API blowback.

Finally, it is worth mentioning one interesting American experimental gun of the 1950s, the 20mm T220, which had two rates of fire: 1,500 rpm for AA use (operating in short-recoil mode) and 500 rpm for ground fire (operating as a long-recoil gun). The idea was unsuccessful and was not pursued, although the concept of variable rates of fire survived in the service 12.7mm M85 AFV gun and is easily achieved in modern power-driven weapons.

Blowback mechanisms

The simplest form of automatic operation is the blowback. In this, the cartridge is held in the chamber by a heavy bolt, which is in turn kept pressed forward by a strong spring. When the cartridge fires, the expanding gases push the projectile up the barrel and the cartridge case back against the bolt. The inertia of the bolt holds it in place for the fraction of a second necessary to allow the projectile to reach the end of the barrel, by which time the bolt is being pushed back, ejecting the fired case and compressing the return spring as it goes. This leaves the rear of the chamber open so that the bolt, now being pushed forward again by the return spring, can pick up another cartridge from the ammunition feed and chamber it in the gun. During this cycle the bolt also recocks the firing mechanism.

The problem with the simple blowback is that it only works well with low-powered ammunition such as small pistol cartridges. Military cartridges are so powerful that the bolt would need to be prohibitively heavy to hold the cartridge case in place until the projectile had cleared the barrel. The penalty for premature opening of the breech is likely to be a burst case, with high-pressure gas escaping from the breech. Various methods of modifying the simple blowback have therefore been developed and many of these are in service today.

Some of the modifications involve engineering a delay to the movement of the bolt. This may be arranged by locking the bolt to the barrel at the instant of firing, then unlocking it as soon as pressures have dropped to safe levels and allowing the blowback cycle to proceed (delayed blowback) or arranging for the bolt to push some mechanism out of the way, engineered to provide maximum resistance for the first few millimetres of travel (retarded blowback). Delayed blowback has been frequently used in heavy automatic weapons (see The hybrids') but retarded blowback has been restricted to small arms. Even less common is the gas-retarded blowback, in which gas tapped from the barrel is used to hold the bolt against the breech face until the gas pressure in the barrel has dropped enough for the breech to open safely.

The simplest variation in heavy weapons, however, uses advanced primer ignition. API blowback is mechanically almost identical to the pure blow-back, with two principal differences: first, it fires as the bolt is moving forwards and just before the cartridge is fully chambered; and secondly, an extended or hooded chamber is used, within which the cartridge can slide while remaining supported. The effect of these differences is that the recoiling cartridge case has to arrest the forward movement of the bolt before it can begin to push it back again. The result is that a much lighter bolt is required; it is commonly stated that for a powerful API cannon, the bolt is only one-tenth the weight it would need to be if the gun used a pure blowback system. The reduced inertia of the light bolt permits a much higher rate of fire than would be possible with a pure blowback.

All simple and API blowbacks have in common a bolt which is unlocked (i.e. not mechanically locked to the chamber) at the instant of firing, and all the blowback family suffer from a common problem: a weakness in the positive extraction of the fired case. This is blown out of the chamber by gas pressure in the barrel rather than positively pulled as with most other mechanisms, and there is a risk that the pressure will instead stick the case to the walls of the chamber. At best this stops the gun from firing until the case has been cleared by the charger mechanism; at worst, the case may be pulled apart by the tension, leaving a section of case in the chamber and putting the gun out of action for some time.

The most effective solution to this problem is to wax or oil the cases before they are fired, but this carries its own problems such as collecting grit and carrying it into the mechanism. A later approach which works in some circumstances (actually developed in the 1930s) is to engrave shallow grooves or flutes into the chamber, leading from the case neck back to the body of the cartridge case. This provides a route for a thin film of propellant gas to seep back around the case, thereby preventing it from sticking.

Another characteristic of blowback weapons is that some cartridge shapes work better than others. Straight cases work well because they remain fully supported by the chamber walls as they slide backwards through the chamber after firing. Tapered or bottlenecked cases are far less effective, as the first rearward movement leaves a part of the case unsupported by the chamber. The gas pressure will instantly expand that part of the case to fill the gap, and may split the case or pull it apart. The only successful API blowback cannon with a (slightly) bottlenecked case is the 20 X 110RB Oerlikon S. Even so, the neck and shoulder of the fired case are considerably expanded from the unfired shape.

API case designs also need to take account of the fact that the cartridge is deep within the extended chamber when it is fired. All the cases in cannon calibres therefore have a rebated (reduced diameter) rim to enable the bolt, with extractor claw hooked over the rim, to follow the cartridge into the chamber. The cases also often feature a considerable thickness of metal near the head, to reduce the risk

Advanced Primer Ignition Blow back (seller) ra tic)

of the unsupported head of the case splitting open as it is extracted.

movement of the bolt before it can be thrust to the rear. As a result there is little surplus energy to drive

A related characteristic is an unusual sensitivity a belt feed without slowing down the rate of fire, so to different types of ammunition. It is particularly most examples have been drum-fed. The main difficult to balance the pressure characteristics of exceptions have been those such as the MK 108

different cartridge loadings with the mechanism, whose design permitted the use of a very light bolt, and in extreme cases the need to fire new loadings the energy required to drive the belt feed being bal-

required the gun mechanism to be rebalanced with anced by a reduction in bolt weight. The light recoil different bolt weights and spring strengths, as in the push also means that the complication of recoiling change from the German MG-FF to the MG-FFM (described in more detail in Chapter 5).

A requirement of the API system is that the gun can only fire from an open bolt, i.e. with the bolt all sub-machine guns (although the relatively low held fully back. In some respects this is desirable pressures and velocities mean that extended cham-

mountings can be avoided in all but the largest calibres.

The API blowback principle is used in virtually because it means that the chamber is left open between bursts of fire and therefore has a chance to bers and rebated-rim cartridges are not required) but its use in heavy automatic weapons has mainly cool down, so there is no risk of a cartridge kcook- been restricted to a chain of development originat-

ing-ofT in a hot chamber. However, it also means ing in a German gun, named after Reinhold Becker, that there is a significant delay between pressing the which was developed during the First World War.

trigger or firing button and the first round being This simple weapon of modest performance, which fired. This system was therefore never suitable for synchronised mountings in piston-engined lighter used a 20 X 70RB cartridge, saw limited service in both aircraft and anti-aircraft roles towards the end aircraft, which required each shot to be precisely of the war, but its significance lies in its post-war timed to avoid shooting off the propeller.

descendants. After the First World War, severe

A result of the API principle is that the recoil restrictions were placed on German armaments forces generated are relatively low, because much of and few new developments were possible, but some the recoil energy is absorbed stopping the forward German firms avoided the restrictions by working

CARTRIDGE CASE WITH REBATED RIM

a. ready to fire

Rebated Rim Pistol Ammunition

b. instant of firing

Cargridge Bolt

BOLT TRAVELLING

FORWARDS AT MAXIMUM SPEED

CARTRIDGE ENCLOSED WITHIN EXTENDED CHAMBER

b. instant of firing

CARTRIDGE CASE WITH REBATED RIM

a. ready to fire

BOLT TRAVELLING

FORWARDS AT MAXIMUM SPEED

CARTRIDGE ENCLOSED WITHIN EXTENDED CHAMBER

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