Why does AN-94 have higher burst fire than its cyclic rate?

Question

AN-94 is an assault rifle of Russian design chambered in 5.45 caliber. It is claimed to be state-of-art in Russian small arms development.

The most important design differences (relative to common designs like AKM) include:

• Internal receiver, chamber, bolt, bolt carrier, barrel and gas tube/piston are all integrated into one independent assembly - called firing unit.
• Stock, external receiver form what is called "Effect Envelope" and firing unit can move along the barrel axis relative to this envelope - which is supposed to increase accuracy by delaying recoil until the second round is fired.
• The firing unit includes a rammer connected to the bolt carrier by a cable and pulley system. The rammer is moving forward as the bolt moves backwards, to accomodate for pre-feeding sequence, by stripping the next round from the magazine and moving it just in behind the chamber, where it's being subsequently fed by returning bolt.

The gun specification claims that when firing full-auto the first two rounds are being fired at 1800 rpm speed, but all subsequent ones at 600 rpm. Why is the time between the first two shots shorter? I can't find anything that would differentiate between the first two and all the subsequent rounds? I would understand if the gun was either keeping the same cyclic rate or rather shoot in a continuous stream of two-round bursts with mean rate of 600rpm but neither is the case. Can someone explain this to me please?

Answer 1

The hammer mechanism is quite complex; it can "choose" to fire another round either as soon as its chambered or only in the forward position of the internal (moving) receiver.

In single fire mode, it hits the round only once, when it's in the forward position, and doesn't do it again until both the barrel with the next round is in the forward position and the trigger was pressed again.

In 2-bullet burst, the "hit as soon as the round is chambered" is enabled - after the first shot, the next one is fired when only the next bullet is chambered, but then the mechanism returns to inert forward position, awaiting next press of the trigger.

In full auto mode pressing the trigger sets the release of hammer ASAP, but it is reset after the second shot (when the barrel moves backwards) and doesn't "return" until next press of the trigger - after the first two rounds, the gun operates similarly as in single fire mode, except it fires as soon as the inner receiver reaches the forward position while the trigger is depressed, no need to release it first.

Generally, this behavior is achieved by a very complex construction of the trigger mechanism. New round is not chambered while the previous one (or just the shell) is still inside, so the mechanical "logic" that enables the hammer to fire in different position determines whether new rounds are chambered at "double rate" or only after a full cycle.

source

Another way to think of it is that AN-94 is two guns: a fairly standard Kalashnikov derivative, except its bolt carrier, chamber and barrel are replaced with a second, recoil-operated gun. While the Kalashnikov normally chambers the round on its forward movement and fires it when locked in the forward position, this second gun chambers the round on backwards movement (thus need for the rammer, otherwise it would be unable to strip it from the cartridge), and fires it as soon as it's chambered, still during its backward movement. "The Kalashnikov part" operates normally for single and burst mode, but the fire selector may allow the "inner gun" to shoot once, while moving after the first "kalashnikov" shot. (and also, the chambering of consecutive rounds after the first occurs following the "inner gun procedure" (backwards), not the "kalashnikov procedure" (forward motion), but that's merely a quirk.)

Answer 2

SF. explained nicely how the rifle achieves this behavior, but the question remains as to why you would design a rifle to do this. When firing an automatic weapon, the accuracy decreases very quickly (after a few rounds are fired) because the recoil causes the weapon to jump around. What the AN-94 does is delay the recoil of the first round from reaching the shooter's body until just after the second round is fired. This makes it possible to get two rounds on-target, both with first-round accuracy (or nearly so). There are other automatic weapons out there that do this; the generic term is "hyperburst" I believe.

However, the recoil has to eventually be absorbed by the shooter, so you can only get a few rounds off with this method. After that you need to reduce the rate of fire to some sustainable level so that the shooter can control the weapon. That's why the AN-94 reduces its rate of fire after two rounds.