Tuesday, August 24, 2010

Actions: Blowback Action: Advanced Primer Ignition

In our last few of posts, we've studied the basic principles of a blowback action and the reason why we want to delay the blowback action. In some of our last posts, we saw that mechanical means can be used to delay the movement of the bolt, as well as a method that uses friction and one that uses some of the generated gas to slow down the bolt. In this post, we will look at another way to delay the movement of the bolt -- the Advanced Primer Ignition Blowback or API Blowback method. This is a method that is used on a number of submachine guns, the most famous of them being the Israeli Uzi.

The history of the Advanced Primer Ignition mechanism started in the middle of World War I, where it was used for the Becker Autocannon (invented by Reinhold Becker) used on several German aircraft. It was later used in World War II in such guns as the MP-38, MP-40 and MP-44 and afterwards by Suomi M31, Uzi etc.

To understand the action, first we must realize that the word "advanced" has many connotations in the English language. However, the context in which "advanced" is used here is in the sense of "ahead of". So "Advanced Primer Ignition" must mean that the primer is ignited ahead of some other event happening. So what exactly happens here?

To answer that question, let us revisit what happens in a straight blowback action. In here, the cartridge is initially in the chamber and the bolt is holding it in place via spring pressure. When the user pulls the trigger, it releases a hammer, which strikes the back end of the firing pin at the end of the bolt. The front end of the firing pin strikes the cartridge, which then ignites the primer and propellant. The generated gases then push the bullet out of the weapon and also try to push the bolt backwards. However, since the bolt is much heavier than the bullet, it does not move right away because of inertia and only moves by the time the bullet has already left the barrel. The bolt then travels backwards along with the fired cartridge case, which is ejected in a side port. The backward moving bolt also recocks the hammer along the way and moves back till it reaches its backward-most position. After that, it is pushed forward by a spring and it picks up the new cartridge from the magazine on the way forward and pushes it into the chamber and it is now ready to fire the next cartridge. Such a mechanism is called a closed bolt because the bolt is normally holding the cartridge in place in the chamber before the trigger is pulled.

Now consider a slightly different mechanism. In this mechanism, the bolt is already held in its backward most position by a sear spring and there is no separate hammer. The bolt itself has a fixed firing pin. Such a bolt is called an open bolt because the chamber is open by default. When the trigger is pressed, the bolt is released and moves forward due to force from the spring in the back. On the way forward, it picks up a cartridge from the magazine and moves it into the chamber. When the cartridge is rammed into the chamber, the firing pin detonates it and the force of the explosion pushed the bolt back whereupon it moves back to the very back of the mechanism and is held in place again by the sear, ready to be fired again. Again, if the weight of the bolt is heavy, it will not move back immediately after the cartridge is fired, due to the inertia of the bolt.

Of course, in both these situations, the bolt is much heavier than the bullet to ensure that the bolt doesn't move back immediately after the cartridge is fired. Also, the cartridge is lower powered because if it was more high-powered, the bolt and recoil spring would need to be correspondingly heavier and therefore make the whole weapon impractical to use by most users.

Now imagine a slight variation of the open-bolt scenario we described above. What if the cartridge is ignited before it is fully pushed into the chamber by the bolt. In that case, the generated gas not only has to push the bullet out of the barrel, it needs to stop the forward momentum of the bolt completely before it can push the bolt backwards. This means the bolt is delayed from moving backward for a little bit more time. By the time the expanding gases start to push the bolt backwards, the bullet has already left the front of the barrel. This means that the bolt and recoil spring can be much lighter in this scenario than if it was using straight blowback. This is the advanced primer ignition method (i.e.) the primer of the bullet is ignited before the bolt has stopped moving forward completely.

Therefore, we answer the question posed a few paragraphs above: "advanced ignition" refers to the fact that the cartridge is fired in advance of being chambered fully.

In most submachine guns that use this principle, this effect is achieved by making the firing chamber's length very slightly shorter (typically, a few thousands of an inch) than the overall cartridge length. This causes the firing pin to ignite the cartridge a little before the bolt slams into the face of the chamber.

In larger caliber guns (such as some anti-aircraft cannon and anti-tank rifles), this effect is achieved by making an "extended" chamber (i.e. one that is longer than the cartridge length), which allows the cartridge to slide within the chamber and supports the cartridge during firing via the chamber walls. The cartridge often has a rim that is smaller than the overall diameter of the cartridge (vs. firearms using other principles, where the rim is usually the same diameter as the cartridge case), in order to allow the extractor to hook to it within the extended chamber.

There are some advantages to using API blowback. Because the bolt can now be much lighter, it makes the weapon easier to manage than one using straight blowback. API blowback also lessens the recoil as well as the muzzle climb of the weapon. The weight savings can be recycled to make a heavier barrel which means it can fire more powerful cartridges than a straight blowback action as well.

There are also some downsides to this action. The moment of ignition of the primer is more critical in API systems because if the primer is ignited too early (i.e.) before the cartridge is adequately seated in the chamber, the cartridge case could burst. If it is ignited too late, the weapon and cartridge case may be damaged, especially when firing higher velocity cartridges. Also, API blowback can only be used with open bolt weapons. Unfortunately, open bolt weapons are more inaccurate than closed bolt weapons, just by the nature of how they work. API blowback also makes the weapon very dependant on strength of cartridge, weight of bolt, length of chamber and rate of fire. In an API blowback design, the variables "rate of fire" and "muzzle velocity" are generally mutually exclusive of each other, so if you want a high rate of fire, the muzzle velocity of each bullet must be slower and vice versa.


  1. I suspect, in API SMGs (like STEN) the bolt is still moving forward at the moment of ignition, but the case is fully seated in the chamber. I may be wrong, but I also suspect that the momentum absorbed by the case is divided in two parts:
    1)when the bolt is crushing the primer, less than one milimeter from its most forward position, wich is very small;
    2)after the bolt stops against the breech, some part of the bolt momentum is transfered to the weapon´s body and the whole weapon goes forward. At this moment, the bolt velocity is smaller (due to the shared momentum with the remaining weapon mass), but it is still going forward.
    I may be wrong, I just suspect.

  2. really excellent explanation, thank you for posting this.

  3. why more time is taken by the bolt to move forward when firing first shot in advance primer ignition system