Gun automatics: recoil operated actions

Recoil is one of the earliest and most successful methods of operation for automatic firearms. It works for a surprisingly broad spectrum of weapons, from pocket-sized pistols up to heavy machine guns and automatic cannons. However, there’s a noticeable gap in the spectrum of recoil-operated firearms; to be specific, very few successful hand-held long guns (especially rifles) ever used recoil operation, and we shall see why.

Recoil is a basic property of every firearm. It’s a “product” of Newton’s 3rd law of motion, which states that for every action there’s an equal and opposite reaction. This means that when a bullet is expelled from the barrel by the pressure of the hot powder gases, the barrel (and the gun it’s attached to) receives the same impulse, but in the opposite direction. This translates into a rearward movement of the gun, which is then felt by the shooter as recoil. In fixed barrel, locked breech guns (basically, in all manually operated guns) all recoil is directly transferred into the shooter’s hands, shoulder or supporting mount.

Hiram Maxim, an American inventor living in Europe, was probably the first person who practically utilised this otherwise wasted energy of recoil, to automatically cycle the action of the gun. In his first gun related patent, which he filed in 1883 (and received in 1884), he describes the conversion of a manually operated Winchester lever-action rifle, using the recoil of the entire gun to cycle the bolt and reload the weapon from an integral tubular magazine. To achieve this effect, he used a spring-loaded buttpad, attached to the shoulder stock. Upon recoil, the entire gun moves rearwards relative to this buttpad (held stationary by the shooter’s shoulder), thus operating a series of levers which cycle the gun.

This design, purely experimental by nature, eventually led to the first and foremost – most practical, successful automatic machine gun for all time, the Maxim machine gun. In Maxim machine guns, the barrel and its extension with attached breech block are allowed to recoil inside the stationary housing (the receiver), against the tension of a return spring. This movement is used to cycle the breechblock for extraction, ejection and the loading cycle, while at the same time operating the belt feed unit.

Hiram Maxim's First Patent for a Recoil Operated Rifle, 1883
Hiram Maxim’s first patent for a recoil-operated rifle, 1883

 

Very soon this basic principle found wide acceptance among gun designers in many countries around the world. Hugo Borchardt applied for the patent of a recoil-operated pistol in 1893. The Federle brothers (of the Mauser factory) developed their recoil-operated pistol (later known as the Mauser C96) in 1894. John Browning patented his first recoil-operated pistols in 1896.

By the turn of the 20th century recoil operation was firmly established as a practical and successful system of automatic operation for firearms. It must be noted that gas operated action was also known for at least a decade before the turn of the 20th century. However, for a while it found little acceptance, being greatly overshadowed by various recoil operated systems.

Borchardt's patent for his model 1893 recoil operated pistol.
Borchardt’s patent for his model 1893 recoil operated pistol.

 

Browning's patent for his long recoil operated rifle (1900), which later became famous as Remington models 8 and 81.
Browning’s patent for his long recoil operated rifle (1900), which later became famous as Remington models 8 and 81.

 

The key reason for the much greater initial popularity of recoil operated as opposed to gas operated actions is that recoil action depends purely on the impulse of the propelled projectile. The complex (and not as yet well researched at the time) dynamics of gas pressure build-up and drop down back to ambient pressure have little to do with this system.

The great variety of early powder types and occasional use of black powder in shotgun and even rifle ammunition has also offered additional complications for gas operated actions. Recoil operated actions, on the other hand, were (and still are) much more forgiving to variations of powder loads and other internal ballistic changes as long as the overall recoil impulse of the round stays within certain parameters.

The main drawback of recoil systems compared to gas operated systems is that the former always requires a sliding (recoiling) barrel. To achieve any degree of reliability, especially under widely varying environmental conditions specific to the military and some hunting guns, the recoiling barrel must have some “play” inside the gun. This is to avoid the gun jamming too easily due to thermal expansion of the steel or the ingress of smaller particles of dust or powder residue.

This play, obviously, causes a certain decrease in accuracy. This decrease is especially undesirable in rifles that are expected to shoot at long ranges. Another minus of the recoil system is that a moving barrel has to be supported and guided in its recoil not only at the rear, but also in the middle and at the front. This requires a barrel jacket, as demonstrated by most recoil-operated firearms – from the earliest Maxim machine guns and Browning rifles to the many later successful weapons, such as the German MG-34 and MG-42 machine guns.

However, it must be noted that a few of the recoil-operated guns did not have full length barrel jackets. To name a few, there’s the famous Browning Auto-5 shotgun (which had its barrel supported around its middle by the ring that slides along the underbarrel magazine tube) and Browning M2HB machine gun, as well as the Johnson Model 1941 military rifle (which had a relatively short barrel jacket supporting the barrel for about its rear third). Obviously, a barrel jacket adds to the weight and the cost of the gun, which is almost always undesirable.

As a result, very few rifles were produced with any type of recoil-operated action, and the aforementioned Johnson M1941 was probably the most successful military recoil-operated rifle (produced in several tens of thousands during the early part of WW2). The most successful recoil-operated rifles ever made were the Remington model 8 hunting rifle (designed by John Browning) and its model 81 derivative. Overall, Remington produced about 140 thousands of these rifles between 1906 and 1950. On the other hand, recoil operated pistols were and still are produced by the millions each year worldwide. Recoil operated machine guns, while less popular today than 50-100 tears ago, are still kicking and ticking in many parts of the world, although mostly in older, time- and war-proven designs such as the Browning M2HB or the MG-3. Developments in propellants and detailed knowledge of internal ballistics have gradually made gas operated machine guns more cost-effective than recoil-operated ones, but this is a topic for our next article. Semi-automatic pistols still remain the most widely used type of recoil-operated firearms. Virtually all pistols designed over the last 120 years for military-type ammunition use one form of short recoil operation or another. In most cases, these systems are based on century-plus old designs invented by the firearms genius John Browning. He also designed many other recoil operated weapons, including the first ever successful semi-automatic shotgun (Browning Auto-5 / Remington Model 11), one of the first mass produced sporting semi-auto rifles (Remington model 8), and a number of machine guns. Ironically, apart from his pistols the only other design still in widespread use is a recoil-operated Browning M2HB heavy machine gun.

There are two distinct types of recoil operated systems in guns: usually known as “short recoil” and “long recoil”.

In this context, “short” and “long” refer to the relative length of the recoil path that the barrel and breech block travel together before unlocking.
In short recoil operated guns, the breech block unlocks after only travelling briefly rearward (about 0.5 to 3 centimeters, depending on the length of the cartridge used and particulars of the gun design). Once the breechblock is unlocked, the barrel stops its rearwards movement while the breechblock continues its rearward cycle alone, extracting and ejecting the fired case on its way back and feeding and chambering a fresh round on its returning stroke. Classic examples of the “short recoil” principle are the Colt M1911 pistol, Johnson m1941 rifle and Browning M1917 machine gun.
In long recoil systems, barrel and breechblock recoil while locked together for the entire length of the cycle; at the rearmost position the breechblock is captured by a special sear and held there, while the barrel commences its return forward under the pressure of its own return spring. The initial forward movement of the barrel unlocks the breechblock from the barrel, and the fired case remains stationary at the breechblock (held by an extractor claw), while the barrel slides forward. Once the empty case is cleared out of the chamber, it can be ejected from the gun. When the barrel hits its forwardmost position, it automatically trips a bolt (breechblock) release lever, and the breechblock then moves forward under the pressure of its own spring. It then loads a fresh round and finally locks the breech for the next discharge. Examples of the “long recoil” system are really scarce, and besides the already mentioned Browning Auto-5 shotgun only the Chauchat CSRG M1915 machine rifle / light machine gun is worthy of serious note as a mass-produced weapon.

typical diagram of a short recoil operated action
Typical diagram of a short recoil operated action

typical diagram of a long recoil operated action
Typical diagram of a long recoil operated action

 

In most long guns (rifles, shotguns, machine guns etc) the barrel is noticeably heavier than the breech block. Therefore, after unlocking, most of the overall impulse “remains” within the barrel. If untamed, this energy is then wasted with no practical use. However, many short recoil operated long guns feature so called “bolt accelerators”, which, once the bolt is unlocked from the barrel, transfer some of the barrel impulse to the bolt. This further accelerates the latter while slowing down the former, before the barrel is completely stopped. In handguns, (where the barrel is usually of a comparable weight with the breechblock / slide), an accelerator is of little use. Only one military pistol worthy of note was ever produced with an accelerator – the Finnish Lahti L-35; it had a relatively short slide and was intended for use in the most unforgiving conditions of harsh Nordic winters.

As we have seen, recoil operated systems have many merits, and the most important one is relative insensitivity to the properties of the propellant charge, as long as the cartridge generates sufficient recoil impulse upon firing. The key disadvantage of all recoil operating systems lies in its sliding barrel, which requires additional support and decreases long-range accuracy.