Reducing Gun Recoil: Differential Recoil Systems

systems.(1,2) In fact, differential recoil is not new, is not confined to artillery, and has been successfully used in automatic weapons for many years

— even though it seems to be “reinvented” every 20 to 30 years.

Firing a weapon on the counterrecoil stroke (“out-of-battery”) reduces firing loads because the rearward momentum of the weapon (ordinarily equal to the forward momentum of the projectile and the escaping propellant gases) is also reduced by an amount equal to the forward momentum of the counter-recoiling mass at the instant of firing

— a simple application of Newton’s third law of motion. This technique has been used for along time, with various degrees of success, in some large caliber cannons and many automatic weapons.(b) In fact, it is with automatic weapons that differential recoil works best, even those of artillery caliber. In this article, we’ll review a number of weapons using the differential recoil technique, but — with two exceptions

— the weapons chosen are those that have been mass-produced, those that contributed the most to the state-of-the-art in weaponry.

Guns and Howitzers In pre-WWI Europe, a number of countries and manufacturers developed weapons using the differential recoil principle. In 1910, the Schneider Compagnie designed afield gun using this principle, as did Krupp in its 12-pdr anti-balloon gun.(3) The antiaircraft nature of the Krupp gun was an early application of differential recoil that exploited its most salient advantages: a high rate-of-fire with minimum carriage shock and disturbance. Differential recoil was used for mountain howitzers by the turn of the century.(3) Some examples included the French Canon de 65M mle 1900(4) or 1906(5), French 75mm Deport 1910,(3) Krupp 75mm M1909,(3) and French 67mm Ducrest 1912.(6) Mountain howitzers are lightweight, area-fire weapons where weight is critical; surely more important than some slight loss of accuracy due to a sliding mass in motion prior to firing. Typically, these guns are designed to break down into mule loads of no more than (very) approximately 100 kg (220 lb) each. The U. S. M204 105mm Howitzer (Fig. 1) is one of only two examples mentioned in this article of a weapon that was never produced(7). Type-classified in the mid-70s, it used differential recoil. Unfortunately, there are many in the U. S. who believe that “fire-out-of-battery” came and went with the M204. It didn’t!

Automatic Weapons Automatic weapons, such as submachine guns, machine guns (MG), and automatic cannons, usually fire from the “open bolt” position, and benefit from using differential recoil. Differential recoil not only reduces recoil loads, but it permits the high-rate-of-fire weapon to function more smoothly. This also reduces dispersion in burst fire, because:

• With blowback-operated weapons, the forward moving bolt will fire the primer before the bolt can smash into the barrel and/or receiver, and

• With recoil-operated weapons, the barrel-bolt-barrel extension combination (all locked together, with chambered cartridge) will fire the primer before the combination can smash into the receiver in counterrecoil. Reduced “kick” is a bonus in both cases above.

When describing the functioning of small-caliber weapons that fire in counterrecoil, the term “advanced primer ignition” is often used in the British and American literature.(8,9) Submachine guns with fixed firing pins represent this type mechanism. The firing pin on the forward moving bolt sets off the primer and propellant charge milliseconds before the bolt can hit the barrel rear. The term “pre-percussion firing” is used in The Machine Gun, Volume I, by Col. George Chinn, USMC.(10) Citations of weapons using pre-percussion firing (differential recoil) in this book are:

• The Kjellman 6.5mm MG, ca. 1908 (Fig. 2). The gun shown in Fig. 2, incidentally, was manufactured by Bofors. (Chinn, Pg. 248).

• The Rheinmetall 13mm Aircraft MG, Model 131. This WWII-era weapon is short recoil-operated. “.... This permitted the timing of the firing mechanism whereby the powder charge could be exploded and recoil forces set in a few hundredths of an inch before the fast and heavy moving parts collided with the rear end of the stationary receiver. In high speed weapons, especially, the act of buffing the action on counterrecoil not only ensured longer parts life but gave smoother performance.....” (Chinn, pp. 458-459).

• The Gazda Aircraft Cannon. Chinn notes that pre-percussion” was used in Reducing Gun Recoil: Differential Recoil Systems by Don Loughlin ARMOR — 19 Editor’s Note: Lettered superscripts refer to “Notes,” pg. 24.

Numbered superscripts refer to “References,” pg 24. Figure 1. The U. S. M204 105mm howitzer, developed in the mid-’70s, employed the differential recoil principle, but never went into production. (Photo: U. S. Army, Rock Island Arsenal Museum.) Figure 2. The Kjellman machine gun, developed around 1908, employed the differential recoil principle, firing the 6.5x55mm Swedish rifle round. This gun was built by Bofors, a firm still well known for its automatic weapons. (Photo: Rolf Hjalmarsson, Eskilstuna Museer, Sweden.) Figure 3. A familiar weapon to armor and cavalry soldiers, the.50 caliber M2 machine gun employs differential recoil. By firing the cartridge just before the barrel and bolt reach their most forward position, the momentum of these heavy moving parts helps equalize the rearward force of recoil.

• The Rheinmetall 30mm MK 108 Cannon(c). Rheinmetall’s Handbook of Weaponry(11) uses the term “counterrecoil ignition” to describe this straight blowback-operated weapon. They also refer to the MK 108 as “mass-locked, blowback-operated” (pp. 558-560). Some weapons that are representative of the differential recoil principle are described in the following paragraphs. U. S Heavy MG,.50 Cal. M2 It is interesting that Col. Chinn’s Volume I omitted from the list that most ubiquitous of all short recoil-operated machine guns, the Browning.30 Cal. and.50 Cal. The.50 Cal. (Fig. 3) is a weapon that has been in production, on and off, for over 70 years. Although he passes over this weapon in Volume I, he makes up for it in Volume IV with this clear description:(12) “... As the bolt moves forward, its motion is aided by the driving spring. The bolt picks up afresh cartridge from the feed mechanism and loads this cartridge into the chamber.....” (See Part E of Fig. 4) “... Just before the bolt locks to the barrel, the barrel is un-latched so that the bolt and barrel move forward into battery while locked together. Shortly before the recoiling parts reach their most forward position, the firing mechanism is actuated and anew cycle begins....” (Part A of Fig. 4) “... Since the cartridge is fired before the counterrecoil motion is completed, the forward velocity of the recoiling parts is first checked by the initial part of the rearward thrust exerted by the exploding propellant charge and the recoiling parts are then driven to the rear. (Timing the firing in this way eliminates the need for a heavy counterrecoil buffer to absorb the forward kinetic energy of the recoiling parts.)...” (Emphasis added by this author.) Oerlikon 20mm II SS Gun A good example of a late-30s Oerlikon blowback-operated gun using differential recoil was the 20mm II SS Gun, widely used in WWII, including service with the U. S. Navy as the Mk 2 and Mk 4 (Fig. 5). Its functioning was a classical use of differential recoil to smooth operation, reduce bolt mass, and reduce operating stresses in a straight blowback-operated weapon. The different phases of the operating cycle are shown in Fig.

6, which are described in the Oerlikon Pocket-Book, English language edition,(13) as: “I. Rearward motion of the bolt is arrested by the bolt buffer and counter-recoil commences.

“II. Counterrecoil of bolt: the cartridge is fed into the chamber.

“III. The cartridge is ignited while the counter-recoil process is still under way.

Forward motion of the bolt is slowed down by the increase in pressure in the barrel.

“IV. The bolt is accelerated rearwards by the residual gas pressure and the empty case is extracted....”

Bofors Automatic AA Guns Differential recoil has been used in Bofors AA automatic gun systems since the 1930s in order to obtain a high rate of fire and low dispersion.

Representative weapons are: 20mm M43 (1943), 25mm M36 (1936), 40mm L/60 (1936) (Fig. 7 shows a U. S. WWII variant), 40mm L/70 (1948), 57mm L/60 (1954) and 57mm L/70 (1968), and the 120mm single and twin naval guns.

The 40mm L/70, post-WWII successor to the L/60 still used by many countries, ARMOR — 21 Figure 5. Widely employed by the U. S. Navy in WWII as a shipboard gun, the Oerlikon 20mm II SS, above, uses blowback operation and differential recoil. Figure 6. Below, the operating cycle of the Oerlikon 20mm II SS automatic cannon. (Photo and drawing: Oerlikon-Contraves Defence). was introduced into the Swedish Army in 1951, three years after completion of the original design effort. It is, or has been, manufactured under license by several European countries as well as India and the UK. Fig. 8 shows a Bofors variant of the L/70 adapted for mounting in the CV-90 Combat Vehicle. This particular installation shows the gun mounted inverted: the ammunition feeders are mounted below the gun and the ammunition is fed upwards into the gun. This type of mounting has advantages in turrets; keeping the bulk of the ammunition magazine below the gun helps maintain a low turret profile.(14) Rheinmetall 30mm MK 108 Automatic Cannon The Maschinenkanone MK 108 (Fig.

9) was developed by Rheinmetall in 1942 for aircraft fixed installation as an air-to-air weapon. As an aircraft weapon to be produced in wartime, some principle design goals were low recoil forces, low weight, and suitable for mass production. Consequently, the design approach chosen was blowback operation for simplicity, combined with differential recoil to reduce recoil forces, and use of sheet metal forming technology to reduce costs for the receiver housing and feeder.

Description of functioning is:

After releasing the trigger (Fig. 10, Part A and Part C, Point 1), the bolt moves forward, pushed by the driving springs. On the first part of its travel forward, it operates the feeder by half a step, transporting the fresh cartridge to the barrel axis.

On the second part of its travel, it drives the cartridge out of the link and rams it into the chamber. Before it is completely rammed, the primer is ignited. (Fig. 10, Part Band Part C, Point 2). Note that ignition occurred (at Point

2) while the bolt was still out of battery, but moving forward until it reached the “in battery” position (Part C, Point 3). By the time the bolt is in battery at Point 3, the bolt’s direction of travel is reversed by the force resulting from propellant gas pressure in the chamber.

The bolt extracts the empty cartridge out of the chamber and positions it in the link again. On farther travel backward, it operates the feeder again by half a step. Rearward motion is arrested and reversed by the driving springs (together with annular springs as the recoil buffer).

The differential recoil principle was again adapted by Rheinmetall in the 20mm Rh 202, developed in 1963-1968. Fielded in the Bundeswehr and in the armies of other countries, it is a classic recoil-operated (locked breech) weapon using differential recoil to reduce gun internal shocks, reduce recoil loads and to increase accuracy.(15) U. S. 30mm M140 Automatic Weapon The second exception to the general rule of this paper — that only production weapons are considered (the first being the U. S. M240 105mm Howitzer) — is the 30mm M140 Automatic Weapon (Fig. 11). I believe this weapon is a rarity, an externally-powered gun using differential recoil. Originally intended for the AH-56A Cheyenne helicopter, the M140 was designed to produce low recoil loads compatible with the airframe limits and flight stability of helicopter gunships. It was an unusual dynamic mechanism which combined features found in both internally- and externally-powered automatic cannons. For recoil load attenuation, the receiver underwent fore and aft excursion in the stationary cradle/mount (Fig. 12). During operation, rotation of the receiver drum cam caused the barrel to reciprocate inside the receiver. Fig. 13 is a plan view, schematic diagram which illustrates one gun cycle. The cradle/mount is shown in white; the receiver and buffer tubes are shaded dark gray; the sear mechanism is in black; and the barrel is in white. The action sequence begins at Fig. 13A. At the start of the cycle, the receiver is seared to the rear with the buffer springs compressed. The barrel is in the forward (open) position, and a cartridge is positioned in front of the standing breech.

When the sear is released (Fig. 13B), the receiver starts to move, driven forward in the mount by the compressed buffer springs. Simultaneously, the barrel is shuttled to the rear by the drum cam. As the barrel moves to the rear, it Figure 7. The 40mm L/60 Bofors antiaircraft gun, a 1936 design, saw wide use in many countries. This one, displayed at Aberdeen Proving Ground, is a U. S. Army M1A1 version of the WWII era. (Author’s Photo) Figure 8. The 40mm L70 is adapted here for use in the Swedish Combat Vehicle CV-90 turret. To reduce turret height, this version feeds from the bottom. Inset shows installation on the vehicle. (Photo: A. B. Bofors) chambers the cartridge which remains seated on the face of the breech block. The round fires (Fig. 13C) while the receiver group is moving forward and the barrel is held to the rear in the closed position by the dwell in the drum cam. The firing impulse reverses the receiver motion, the recoiling receiver compresses the buffer springs (Fig. 13D), and the cam shuttles the barrel forward. The receiver is seared to the rear at the end of its rearward travel.

When the barrel reaches the forward (open) position, the spent case is ejected out the bottom of the receiver as anew cartridge enters the receiver from the top. The cycle then repeats itself until the weapon is shut down. Type-classified for limited production in August, 1969, the M140 was never put into production. Its only application was the AH-56, and when that program was terminated it decided the fate of the M140.(16) Problem Applications Differential recoil has been shown to operate well with automatic weapons, even those of artillery caliber, where its ability to smooth high-rate operation by using existing dynamic forces to cancel out other forces provides superior performance. It has not yet been successfully applied in two areas:

• Weapons with large propellant charges, in particular, those using bag charges (“separate-loading ammunition”).

• Weapons needing very high accuracy, such as tank guns.

The difficulty with using differential recoil with large propellant charges, particularly bag charges, is that these systems tend not to have the uniform, repeatable ignition times crucial to the close timing needed for differential recoil functioning. With differential recoil, peak pressure must be achieved at a specific point in the counterrecoil of the moving parts (preferably at maximum forward momentum), and if the two do not coincide, the mechanism will be out of synchronization. This results in an increase in shock and stresses, rather than reduction. The fact that howitzers, and most field artillery, use variable propellant charges does not make this problem any easier to solve. In a tank gun, the problem with using differential recoil is that, with amass as large as a tank cannon in motion prior to firing, there could be shifts in center-of-gravity and disturbances of axes of barrels and sights which can affect accuracy — an unacceptable trade-off in a very high accuracy system. Note that the two above applications are usually hand-loaded, relatively slow rate-of-fire weapons, as compared to automatic weapons, demonstrating further that the payoff with differential recoil is with automatic fire. Differential Recoil Terminology One reason the differential recoil technique is less commonly understood than it should be is that weapons designers use different names for it, depending on the language and preferences of the weapon designers — and whether the application is a submachine gun, machine gun, automatic cannon, or artillery. This may explain why it is periodically “reinvented.” Excessive compartmentation of knowledge is not unusual in the technical world, even when the data is not classified. Previously in this article, I’ve noted most of the variations in terminology for this principle. The only term not previously noted is “dynamic cradle,” found in Bethel’s Modern Artillery in the Field (1911).(3) What is interesting about this book, other than its early 20th Century perspective, is that it is Figure 9. Above, the Rheinmetall 30mm MK 108 automatic cannon was a WWII development for installation on aircraft. Figure 10. Time-displacement curve for the MK 108 (Part C, at left) illustrates the point at which cartridge ignition occurs (Point 2). The bolt is still moving forward at this point and the weapon has not yet gone into battery.

By the time the bolt has reached Point 3 — the in-battery position — chamber pressure is already beginning to reverse the bolt’s direction of travel.

(Photo and illustration: Rheinmetall GmbH) Rheinmetall MK 108 30mm Autocannon also the oldest English-language source found (in the author’s limited research) that uses the term “differential recoil.”

Differential recoil permits use of forward momentum in the high-rate firing cycle to counteract recoil momentum, thereby smoothing operation, reducing stresses, and permitting a lighter weight and more stable weapon. It has been successfully used in many automatic weapons and deserves the attention of engineers for other applications. Perhaps it would be better understood if we could settle on a common name for it, regardless of the application. How about “differential recoil?” And, please, let’s not re-invent it!

Notes (a) “Fire-out-of-battery” has also been called “soft recoil,” for obvious reasons. But in recent years, “soft recoil" has also been used to describe the long-recoil systems now being used in some direct fire, turreted applications. Note, however, that differential recoil reduces the total rearward impulse delivered to the system, while long-recoil systems do not. To avoid confusion, “soft recoil” is not used in this article. “Differential recoil” is preferred, a simple and expressive term. (b) This article does not include other means of reducing recoil which are external to the weapon itself, such as recoil adapters which are placed between the weapon and the mount. It is a different technical principle, and the subject should be treated separately. (c) “MK” here is “Maschinen Kanone” (Automatic Cannon), not “Mark” (Mk) for Model Number.