Balanced action in automatic firearms. An overview

Balanced action in automatic firearms can be traced back to pre-WW1 era, to various attempts to create low-recoil weapons. In these attempts, designers tried to compensate for recoil impulse of reciprocating parts of gun mechanism by providing a counter-recoiling weight, mechanically linked to the recoiling group. General idea here is to neutralize the additional recoil impulse, generated by heavy bolt group, slamming back and forth in the receiver during every firing cycle. This system could decrease felt recoil and neutralize additional vibrations and off-balancing effect of the moving parts, thus improving accuracy and controllability of full automatic (burst) fire at the cost of increased complexity of the gun.

One of the earliest patents, found by present author, dates back to 1908. Designed in UK by someone Ludwig Mertens and registered in USA as US Patent number 891,778, invention is titled as “Non-recoiling firearm”. In one of its embodiments, Mertens invention is described as a belt-fed machine gun that has barrel and bolt movable in opposite directions inside single housing. Movement of the barrel and bolt is synchronized by a rack and pinion system.

 

Early patent for balanced action machine gun by Ludwig Mertens
Early patent for balanced action machine gun by Ludwig Mertens

 

In 1966, Swiss gun designer Edwin Rohr, working for Hammerli AG, applied for a patent that covered several methods to reduce recoil and muzzle flip of a self-loading firearms, target pistols in particular. In his patent Rohr describes simple small-bore, blowback-operated target pistol, which can be fitted with counter-balancing weight that cycles in direction opposite to the slide movement. Operation of the counter-weight was ensured either by powder gases, driven from the bore through special channels, or via rack-and-pinion system. It is not known if Hammerly ever produced a target pistol to Rohr’s patent.

 

Patent for small-bore match pistol with balanced action (one of proposed versions) by Hammerli
Patent for small-bore match pistol with balanced action (one of proposed versions) by Hammerli

 

In Soviet Union “Balanced action” came to existence during mid-1960s from similar attempts to decrease recoil of the rapid-firing weapons, in which rearward and forward movement of the action parts (such as bolt group) created additional forces that tend to disrupt proper aim of the weapon. This is especially true if the bolt group hits stationary receiver on the end of its recoil stroke. A classic example of the gun that is known for high impact velocities of its moving parts is Kalashnikov AK and its descendants. Mass of the moving bolt group of an AKM rifle is about 500 gram, and under normal conditions it hits the rear of the receiver with velocity of 3.5-4 m/s. This creates additional recoil impulse of about 2 kg*m/s, which adds another 20% to overall recoil impulse (generated by the bullet and powder gases). This extensive impulse of the moving parts is an intentional feature, which greatly increases overall reliability of the gun, but in this case reliability comes at a price of increased dispersion during rapid or automatic firing.

During early 1970s Soviet gun designers developed several “balanced action” assault rifles using then-new 5.45×39 ammunition. Among these, there were designs from Izhevsk such as AL-4M and from Kovrov such as SA-006. The latter, designed by Konstantionov, became the main rival to the new 5.45mm Kalashnikov rifle in Soviet army trials of 1974. In fact, SA-006 slightly outperformed future AK-74 in terms of hit probability, mostly when firing from unsupported and off-hand positions or on the move. It also performed adequately well in terms of reliability and durability. However, Soviet army adopted Kalashnikov design, which was simpler, more robust, cheaper and already familiar to the industry and troops.

 

Konstantinov SA-006 balanced action assault rifle partially disassebled
Konstantinov SA-006 balanced action assault rifle partially disassebled, ca.1970

 

Experimental AL-7 balanced action assault rifle by IZHMASH with covers removed, ca.1974
Experimental AL-7 balanced action assault rifle by IZHMASH with covers removed, ca.1974

 

It seems that AK-74 was considered as a stop-gap measure, as in late seventies several R&D programs were well underway, to achieve marked superiority of individual assault rifle fire effectiveness over existing designs, such as AK-74 and M16A1. These R&D programs, among other approaches, extensively pursued “balanced action” principle. As a result of these programs, soviet army held famous “Abakan” trials that featured several balanced-action rifles. However, this trial has been won by a system featuring different and more effective approach, the Nikonov AN-94 rifle. Despite apparent failure during “Abakan” trials, two major small arms development and manufacturing centers (IZHMASH and KMZ) continued their work on balanced action systems.

During 1990s, Russian designes from IZHMASH and KMZ attempred to develop 9x19mm submachine guns with various versions of balanced action, but none went past prototype stage due to the inherent complexity and excessive rates of fire (1000 rounds/min or more).

 

PP-27 "Klin-2" experimental submachine gun
PP-27 “Klin-2” experimental submachine gun by IZHMASH. It featured Martens-style balanced action with moving barrel , counter-synchronized with the bolt

 

Despite formal adoption, the AN-94 turned out to be a practical failure, with relatively few made and issued to Russian army during early 2000s before it production stopped in 2008 for a variety of reasons. In the mean time, Izhmash balanced action rifles evolved into 5.45mm AK-107 and 5.56mm AK-108, and KMZ continued to refine its AEK-971 in 5.45mm and AEK-973 in 7.62×39. During early 2000s KMZ delivered unknown, but relatively small number of AEK-971 rifles to Russian Internal Affairs Ministry (MVD) Spetsnaz troops, and, apparently, those elite units liked AEK-971 more than AN-94 or AK-74M. As a result, KMZ continued development of the AEK line until 2006, and after 2006 this development was transferred to another factory in the city of Kovrov, known as ZiD (“Degtyarov plant”). Successor of the AEK-971, initially known as AEK-971M and later as A545 (official index 6P67) was entered into recent Russian army trials codenamed “Ratnik”, to compete against severely updated but still “unbalanced” Kalashnikov design, know as AK-12. In 2018 both systems were recommended for adoption by Russian army, with much more complicated and expensive balanced action rifles intended for Special Forces and classic rifles approved for general issue. It must be noted that during the “Ratnik” trials overall effectiveness of the balanced action weapons was found to be only about 10% higher (in terms of the number of sucessfully engaged targets), compared to classic weapons, and only at ranges shorter than 300 meters. At longer ranges, classic system proved to be more effective than balanced action system, by the same margin of 10%.

Since 2018 Kalashnikov concern produces an unique balanced action sporting rifle, intended for top-tier IPSC shooting, the Kalashnikov SR1 (formerly known as Saiga 107).