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 AKM Rate Reducer - How does it work? (Pics, video)
the-troubleshooter  [Member]
6/9/2008 10:51:09 AM
A few weeks ago mjolnirforsworn at YouTube asked me how the rate reducer (RR) at the AKM works.
I checked the WWW and couldn’t find any useful info and even in specialist books – no explanation what way it works.
And they are all telling different stories about what this thing is supposed to do.

Here some examples:
· …a redesigned version designated the AKM (M for "modernized" or "upgraded") ... was introduced in 1959 ... a hammer retarder was added to prevent the weapon from firing out of battery (without the bolt being fully closed), during rapid or automatic fire. This is also sometimes referred to as a "cyclic rate reducer", or simply "rate reducer", as it also has the effect of reducing the number of rounds fired per minute during automatic fire.
· This does not affects the cyclic rate of fire, but allows the bolt group to settle…
· … improved trigger/hammer unit, that introduced a hammer release delay device (often incorrectly referred as a rate reducer).
· … and a delay device in the trigger group. The latter, often misidentified as a rate reducer, made certain that the bolt carrier had come to a stop prior to the release of the hammer. The main goal of this device was to prevent 'bolt bounce' which can lead to misfires.
· Other improvements include ... a rate-of-fire control alongside the trigger; ...
· It also has a cyclic rate reducer built into the trigger mechanism.
· When the AKM was introduced, the bolt’s bounce characteristics were changed by the lighter sheet-metal receiver. As a consequence, to prevent firing out of battery, which can destroy both the rifle and the operator, a five-component mechanical drag device was installed in the trigger mechanism to delay the hammer’s fall until bolt bounce had settled down to a safe degree. This device, erroneously called a rate reducer by many self-styled authorities …
· The trigger got an additional safety for the single shot position. It ensures that only one cartridge will be ignited.
· The AKM rate reducer is actually an Anti-Bounce device. It's rate reducing qualities are just a side effect.
· The unit works by catching and holding the hammer for just a split second. The purpose was to make sure that the bolt had enough time to lock into battery before the weapon fired.
· There is a little paddle type thing on the rate reducer that catches one of the side ears on the hammer and slows it down a bit. Last summer I had installed a rate reducer in a full auto AK. When I took it to test it, the gun fired correctly. Then, right in the middle of emptying the mag, the spring on the rate reducer came out of it's slot, and the guns cyclic rate jumped incredibly high.

Now I was becoming curious – probably the most famous military rifle of the world with tons of infos regarding every version and each single receiver type described, but nothing to settle this question.
Getting sick and tired by all this rumors and mostly hearsay I decided to test this on my own.

A matter of terms

There are often used different designations for a certain part of a gun or a similar part that is doing the same thing on a different gun. These terms are used quite arbitrary. Sometimes they call it:
disconnect(-or) and auto-sear (auto-safety-sear)
or sear and auto-sear
or sear and disconnector
or sear and disconnector operating sear
Sometimes they divide the sear into primary and secondary sear (yeah even at the AK).
Wow! Not yet confused?

In the following I will use disconnector and auto-sear.

The disconnector is attached to the trigger holding the hammer if the safety lever is on semi auto and the trigger is pulled. If the safety lever is on full auto it prevents the disconnector from turning upwards with trigger pull so that the hammer won’t get hold and will catch against the auto-sear.
The auto-sear in front of the hammer holds the hammer (if the gun is in full auto modus) until it is pushed forward by the cam of the bolt carrier.

AK, milled receiver, no RR
AKM, stamped sheet metal receiver, RR added

A friend of mine introduced me to a guy who did maintenance and repair for the AK in East Germany for some years.
He called that piece (RR): Winkelhebel mit Klinke = angle lever with pawl
In the german literature it is usually called: Verzögerer = retarder

I will stick with the commonly used term rate reducer (RR) since it actually has this effect. Even if only a ‘lil bit.

That guy told me that the AKM proved to be very instable in full auto modus compared to the AK. That’s why this piece was added. To better synchronize the internal vibrations of the gun.
So it seems as if they tried to compensate the lack in torsional rigidity (compared to a milled receiver) that way.

At the german wikipedia version I got the info that the hammer will be delayed for 0.002 seconds. Don’t know if this is true but seems to be much effort to gain such a slight delay. Wouldn’t it have been easier to reinforce the receiver instead of introducing this quite complex rate reducer? To me it only weakens the simple original design.

For the following investigation I used two east german AKM receivers made in the late sixties. They are both torch cut at the cartridge chamber area thus damaging the front part of the receiver.
Since rifle receivers are (still) free for sale in Germany (handgun frames/receivers are not) I could have obtained even intact receivers. But most of the east german surplus inventory has been destroyed after the reunion or given away to Turkey and elsewhere.
That’s why they turn up very seldom on the market and are on the pricey side. So these partially damaged ones were the cheap alternative.

I picked them up from a bunch of approx. 20 similar treated receivers choosing the best ones. They were all “made” of factory new, never issued AKM’s. Internally they were all untouched but externally they showed dents, scratches, some were slightly bend etc. After cutting they were simply thrown into a box altogether.
To my surprise with both the rate reducer doesn't work as I thought it would. At the first one the hammer doesn't even touch the forearm of the RR (so why would it be there anyway?) and at the second one the hammer gets jammed by the RR and doesn't go forward at all. Additionally at the second receiver there is also missing the disconnector spring. Depending on which position the disconnector accidentally takes it will fire full auto with the safety lever on semi auto! Remember – factory new, never issued…




Bolt consisting of bolt carrier [1] with cam [2] and bolt head [3]
trigger [A] with front hook [a]
disconnector (B) and disconnector spring [C]
RR consisting of angle lever [D] with pawl [E] spring [F]
auto-sear [G]
hammer [H]
safety lever [K]

In the following pictures you see a look from the right side at the trigger group and the bolt. There are shown different phases of the firing mechanism with the pictured parts in their respective positions.
For all pictures: Trigger is pulled and disconnector (mostly concealed) in the downward position as it happens with the safety lever in the full auto position.
The right ear of the hammer is making contact to the RR.
Bolt carrier, hammer and RR are pressed in the forward (right) direction by their respective springs. Auto-sear is pressed rearwards (left) by its spring.
Safety lever and springs are not pictured for better clearness.

The delay function of the RR acts between the two positions - hammer catched by the auto-sear and hammer catched by/passed the front hook of the trigger. After the hammer is released by/passed the front hook the hammer mustn’t be delayed anymore because this would reduce the force that hits against the firing pin.
A side effect of the delayed hammer move is possibly less stress to the front hook cause the hammer will bounce against it at a slightly lower speed – cold comfort for having just one front hook.

Phase 1



Hammer is kept by the auto-sear, no contact to the RR so far. Bolt carrier 8.5mm before its front position, cam gets contact to the lever arm of the auto-sear. Bolt head has already completed its locking rotation.
No out of battery firing possible. The hammer can reach the rear end of the bolt head and thus the firing pin only if the bolt head is completely locked. Otherwise the hammer would hit against the rear end of the bolt carrier. So the RR has nothing to do with preventing out of battery firing, slam/double fires a.s.o. – otherwise you wouldn’t get any civilian semi auto AK without it due to liability reasons.

Phase 2



Bolt carrier 3mm before its front position, has turned the auto-sear forward so that the hammer is released and can start to turn forward. Hammer hits against the pawl causing a rearward turn of the whole RR against the pressure of the RR spring.

Phase 3



Bolt carrier in its front position. Hammer slides along the pawl and hits the forearm of the angle lever.

Phase 4



Hammer is pressing against the forearm of the angle lever and sliding upwards inside causing the RR to turn forward at the same time pushing the pawl against the pressure of the RR spring slightly rearwards (left).

Phase 5



Hammer in its front position hitting the firing pin.
Angle lever and pawl returned under spring pressure to their starting position (Phase 1).

Phase 6



Bolt carrier moves rearwards together with the bolt head after unlocking it. Bolt carrier pushes the hammer rearwards causing it to turn downwards. Hammer hits the pawl.
Auto-sear is released – not pushed forward anymore by the cam – and is pressed rearwards against the lower part of the hammer.

Phase 7



Bolt carrier moves further rearwards turning the hammer down. Hammer pushes away the pawl against pressure of the RR spring.

Phase 8



Bolt carrier moves further rearwards turning the hammer down. Hammer has almost passed the pawl. Angle lever may have made a short slight turn rearwards but the hammer is able to pass the pawl without that. Auto-sear has turned rearwards to its starting position (Phase 1), can’t move more rearwards and slipped under the hammer notch.

Phase 9



Bolt carrier moves further rearwards turning the hammer down. Hammer has reached its most rear-/downward position. Hammer passed the pawl which returned under spring pressure to its starting position (Phase 1).

Well, I couldn’t resist to design my own RR (3mm sheet metal).



It is basically doing the same move but without the separate pawl. That’s why the RR has to turn rearwards (Phase 7+8).

The basic design you can already see at the Browning/FN Auto 5 trigger (even if there is no delay in the above-mentioned sense is given or intended).

During my military service I’ve seen and fired several AKM’s (most of them east german manufacture) and never experienced any malfunctions. So I was pretty surprised at the insufficient function of these receivers.

I hope there is somebody here who can provide additional info.

And here you get the video:
www.youtube.com/watch?v=gKhpy7NNQgc
Stahlgewehr762  [Member]
6/9/2008 8:22:22 PM
It seems to me that you have an excellent grasp of the mechanical functioning of the rate reducer. You are also correct that the AKM will not fire out of battery if the hammer should fall while the bolt is unlocked. The projection on the bolt carrier would preclude that possibility. However, if the hammer falls and contacts an unlocked bolt and carrier due to "bolt bounce", the AKM would cease firing, with a live round in the chamber and the hammer down. This would be an unacceptable situation, compromising the reliability rifle and possibly getting a soldier killed.

The rate reducer does not mechanically "lock" the hammer at any point in its cycle, it simply "drags" on the hammer during its initial release from the auto sear. Remember that the AKs stated cyclic rate is 600 rpm, or 10 shots per second. Under those high-speed circumstances, it only takes a fraction of a second delay to substantially affect the automatic rate of fire.

As for the original intent of the rate reducer, if it was designed as a "band-aid" to cover for "bolt bounce" or rather, if it was added to stabilize the rifle to improve its "firing characteristics" we might never know. Functionally, the device works to delay the fall of the hammer which reduces the automatic firing rate, so it "solves" both issues, and in that context it could be called a successful design.
Monkey-man  [Member]
6/10/2008 7:34:25 AM
I built an M92 clone on a milled receiver and fit the rate-reducer since it was in the M92 parts kit. I have not fired it extensively but it is rather easy to pull singles when on FA. Thanks for the post, now I know what it does; after wrestling for ages to get it fitted correctly. The incorrect position of the springs was causing my disconnector to malfunction. The parts do take some prying and twisting to get them all into place.



Just watched your vid. After you noted that neither test receiver allowed the hammer to contact the angle-lever after passing the pawl I inspected/tested my gun. I can find no marks on the hammer that would indicate contact with the angle lever but the gun has not even fired 200rds since being built. I ran the hammer forward through the RR system as slowly as possible and it looks like just the bare tip of the angle lever is contacting the hammer.

You mention the lack of AKM malfunctions despite the lack of function of the angle-lever; obviously the ROF is being reduced primarily by the pawl dragging on the hammer as it moves forward to strike the firing pin. This would lead me to believe the angle lever is simply a reliability measure to ensure the pawl moves back into its correct position to catch the hammer on the next cycle?
the-troubleshooter  [Member]
6/11/2008 4:44:28 AM

Originally Posted By Stahlgewehr762:
It seems to me that you have an excellent grasp of the mechanical functioning of the rate reducer. You are also correct that the AKM will not fire out of battery if the hammer should fall while the bolt is unlocked. The projection on the bolt carrier would preclude that possibility. However, if the hammer falls and contacts an unlocked bolt and carrier due to "bolt bounce", the AKM would cease firing, with a live round in the chamber and the hammer down. This would be an unacceptable situation, compromising the reliability rifle and possibly getting a soldier killed.

The rate reducer does not mechanically "lock" the hammer at any point in its cycle, it simply "drags" on the hammer during its initial release from the auto sear. Remember that the AKs stated cyclic rate is 600 rpm, or 10 shots per second. Under those high-speed circumstances, it only takes a fraction of a second delay to substantially affect the automatic rate of fire.

As for the original intent of the rate reducer, if it was designed as a "band-aid" to cover for "bolt bounce" or rather, if it was added to stabilize the rifle to improve its "firing characteristics" we might never know. Functionally, the device works to delay the fall of the hammer which reduces the automatic firing rate, so it "solves" both issues, and in that context it could be called a successful design.


Thanks!

And to the matter of bolt bounce and resulting misfires:
The hammer is able to hit the firing pin if the bolt carrier is about 10mm (may vary with manufacturer) before its front position. So the bolt would have to bounce backwards for more than that to cause a misfire.
Imagine the bolt carrier bounces back for 12.7mm /0.5inch. Exactly at that moment – bolt carrier stopped its rearward move and will immediately go forward again under pressure of the bolt spring - the hammer hits against the rear end of the bolt carrier. Since the hammer strikes forward with much more power (functional reserve) than necessary for igniting the primer it would push the bolt carrier forward for the necessary 2.7mm (to reach the firing pin) and still be able to ignite the primer. Well just a guess, but can’t imagine the bolt carrier on this weapon bouncing back that much.
Anybody experienced such misfires at the AK?

Astonishingly I’ve seen the rate of fire for both the AK47 and AKM specified at 600 rpm everywhere. ???

In the following video you can see the hammer being slightly delayed right at the beginning of its forward move:
de.youtube.com/watch?v=_sNDTdKQNVU
I like stuff like that!
the-troubleshooter  [Member]
6/11/2008 7:22:03 AM

The parts do take some prying and twisting to get them all into place.


For installing the trigger assembly I use a 5mm pin that is slightly longer as the trigger is wide (at the top) as some kind of assistant tool. I assemble all the parts (like at the beginning of my video) before putting it into the receiver. If the trigger assembly is in the correct position I take the original trigger pin and push it through (from the left side), the assistant pin will drop out at the right side.
To fiddle them together in the receiver itself may cause some difficulties. And might require some assistance of different kind (vodka, beer...)


I can find no marks on the hammer that would indicate contact with the angle lever but the gun has not even fired 200rds since being built. I ran the hammer forward through the RR system as slowly as possible and it looks like just the bare tip of the angle lever is contacting the hammer.


Checked this during my initial test at the second receiver using permanent marker to paint the forearm of the angle lever inside (contact area to the hammer). No signs/scratches at all. Even slamming the bolt forward as fast as possible – nothing. Because there might be a difference in the behavior of the RR if tested slowly in contrast to fast/live firing modus. Could you check this on occasion?


…the angle lever is simply a reliability measure to ensure the pawl moves back into its correct position to catch the hammer on the next cycle?


Hmm… interesting thought. But if the hammer doesn't contact the forearm of the angle lever on its forward move it won't on its rearward move either…
Well at my RR the forearm contributes to the delay.
GMBs123  [Member]
6/11/2008 11:27:09 AM
Good job ,but it's more a hammer retarder than rate reducer.

Stahlgewehr762  [Member]
6/11/2008 6:00:06 PM

Originally Posted By GMBs123:
Good job ,but it's more a hammer retarder than rate reducer.



Anything which retards (delays) the fall of the hammer will also reduce the cyclic rate of fire of the weapon. Obviously, the bolt carrier speed will remain unchanged, but the increased time required for the hammer to fall will slow down the rate of fire by that specific amount of time (shot-to-shot).
Stahlgewehr762  [Member]
6/11/2008 6:16:26 PM

Originally Posted By the-troubleshooter:
And to the matter of bolt bounce and resulting misfires:
The hammer is able to hit the firing pin if the bolt carrier is about 10mm (may vary with manufacturer) before its front position. So the bolt would have to bounce backwards for more than that to cause a misfire.
Imagine the bolt carrier bounces back for 12.7mm /0.5inch. Exactly at that moment – bolt carrier stopped its rearward move and will immediately go forward again under pressure of the bolt spring - the hammer hits against the rear end of the bolt carrier. Since the hammer strikes forward with much more power (functional reserve) than necessary for igniting the primer it would push the bolt carrier forward for the necessary 2.7mm (to reach the firing pin) and still be able to ignite the primer. Well just a guess, but can’t imagine the bolt carrier on this weapon bouncing back that much.
Anybody experienced such misfires at the AK?

Those are very good points, but it must be remembered that the heavy weight of the bolt and bolt carrier would do a very good job "robbing" the hammer of its inertia, should the hammer strike the rear of the carrier rather than the firing pin. It would likely depend on how far out of battery this happened as to whether the round would fire or not. Also, the hammer accelerates more rapidly than the bolt carrier (on forward travel, and particularly if moving forward due to bounce), so the hammer would "catch up" with the bolt carrier, even if the carrier is only somewhat out of time.

Astonishingly I’ve seen the rate of fire for both the AK47 and AKM specified at 600 rpm everywhere. ???

The AKM bolt and bolt carrier are both slightly lighter (due to machined-in lightening cuts) than the corresponding AK-47 parts, which could translate into a slightly higher rate of fire, absent the hammer retarder. The hammer retarder might have been developed to equalize the rate of fire between the AKM and AK-47, thereby adhering to established standards for this parameter. Also, <speculating> there might be a size difference in the gas ports of the two rifles, but I don't know if this is the case.

In the following video you can see the hammer being slightly delayed right at the beginning of its forward move:
de.youtube.com/watch?v=_sNDTdKQNVU
I like stuff like that!
the-troubleshooter  [Member]
7/22/2008 6:32:19 AM
Here you see an AK74 trigger assembly I received yesterday. The seller couldn't tell me anything else about its origin - probably russian ...
In the following you see on the left the regular rate reducer as described in the original topic above and on the right my new acquisition.



What caught my eye was that odd looking "rate reducer leftover" part.
Looks like it's coming right out of the mold that way, no signs of parts cutt'n away or other tool marks.




Trigger assembly shows 6mm holes and rides on a separate sleeve that fits on the usual 5mm pins.

If somebody wanted to remove the rate reducer for whatever reason they could have used a simple distance sleeve to fill the gap (as pictured on the left).


IMO a bit too complex for what it is doing.
It seems to increase weight only and rattles in the receiver since it is not under spring pressure.

Does anybody have more info on this?