In the last couple of posts, we studied the basics of firearm safety mechanisms as well as some manual safety mechanisms. In this post, we will study a particular type of safety mechanism called theGrip Safety. This is a popular mechanism that was first seen on the classic John Browning designed Colt M1911 pistol and later seen on other pistol models as well. It is also found in the Israeli Uzi submachine gun.
Like the name implies, a grip safety device is a lever located in the grip of the firearm. The user's hand naturally depresses the safety lever when he or she grips the firearm and this disables the safety device, thus enabling the user to pull the trigger and operate the firearm. When the user releases their grip on the firearm, the safety lever automatically pops out again and is re-enabled.
Public domain image. Click on image to enlarge.
The above image shows a Colt M1911A1 pistol. The grip safety lever is at the back of the hand grip and is automatically depressed when the user holds the pistol. There is also a manual safety lever on the firearm, at the rear of the slide, which you ought to be able to spot easily, if you've read the previous post.
Believe it or not, the original John Browning design didn't actually have a safety device, but the US Army insisted on adding a grip safety and a manual safety for the original M1911 pistol design, before they would accept it. Hence, John Browning added them for the M1911, which stayed in service from 1911-1924. The changes made to the M1911A1 model (which has been manufactured from 1924 to the present day) were relatively minor: Longer grip, wider front sight, shorter spur on the hammer etc., so it still has a grip safety and manual safety.
Public domain image. Click on image to enlarge.
The firearm depicted above is the Israeli made Uzi submachine gun. The grip safety is labelled in the above image and is pretty easy to see.
The nice thing about this design is that it is automatically enabled or disabled as the user holds or releases the firearm's grips. Therefore, a firearm with this safety device will only fire if the user is actually holding the firearm and intending to discharge it. Thus, if the user were to accidentally drop the firearm, the safety automatically enables and prevents the firearm from discharging accidentally.
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.
The blowback action is one of the more common actions used in modern firearms today. It is mostly seen in smaller caliber automatic and semi-automatic weapons such as the Uzi, Sten gun, Ingram MAC-10, Walther PP series (the Walther PPK is usually what fictional spy James Bond usually uses), Makarov etc. Actually there are many different systems that utilize variants of the blowback principle, so the next few posts will be devoted to studying these many variants.
The main principle behind a blowback action is that, at the moment of firing, there is a block of metal called a "bolt" that holds the cartridge in place. This bolt is not locked down, but is held in place by spring pressure. This is a very important point and is the main feature of a blowback action. When the cartridge is fired, the bullet flies out via the barrel and the expanding gases also push against the cartridge case which is still in the barrel. The cartridge case acts somewhat like a piston and in turn pushes on the bolt, which then moves backwards and recocks the weapon. At about the same time, an opening on the top or the side ejects the old spent cartridge case.
The following diagrams will make the concept clear.
In the above figure, we have the bolt in blue. The bolt handle A is used to pull the bolt backwards initially against spring pressure from recoil spring B. This moves the whole bolt (the blue piece) backwards. C is a cavity in the bolt, where hammer E (in rose color) can move). D (in red) is the firing pin. When the gun is initially loaded, a magazine containing cartridges is first inserted into the weapon. Then, handle A is used to pull the bolt back as shown in the figure above. As a result of this, the hammer E rotates about its axis and is cocked. At the same time, due to the pressure of magazine spring F, a new cartridge is pushed upwards into the receiver. When the bolt handle A is released, the front face of the bolt (the blue part) pushes the cartridge forward so it is pushed into the chamber in front of barrel H. The hammer E still remains in its near horizontal position because it is held in place by the trigger bar (which is orange) connected to trigger G.
Now the user pulls the trigger and the following happens:
The user pulls the trigger and the trigger bar releases the hammer E, which rotates about its axis inside cavity C and strikes the firing pin D hard. The other end of the firing pin strikes the base of the centerfire cartridge, which detonates its primer and then ignites the main propellant material in the cartridge. The expanding gases drive the bullet through the barrel H and also push back on the empty cartridge case left behind in the chamber, which in turn pushes back on the bolt.
The entire bolt assembly (the blue part) now moves backwards due to the pressure from the gases acting on the (now-empty) cartridge case. As a result of the bolt moving backwards, the hammer E is now rotated back down until the orange trigger bar holds it down Meanwhile, the opening on top of the receiver is now open and the old cartridge case is pushed out through here by the action of magazine spring F. The bolt moves backwards and compresses the recoil spring B. When the bolt has moved backwards to its utmost, the recoil spring B then expands and pushes it forward into place. While moving forward, the bolt picks up the new cartridge and pushes it into the chamber and the gun is now ready to fire again.
Obviously, for best results, the bolt should not start to move backwards before the bullet has left the barrel and the pressure in the chamber has dropped to a safe level (you don't want a lot of high pressure gas to flow back into the magazine area and out the ejection port and blow out everything in between). In a straight blowback action, this is achieved by making the bolt much heavier than the bullet mass. Therefore when the cartridge is first fired, the bullet shoots out the barrel, but the force acting on the empty case doesn't push the bolt back instantly due to its inertia. By the time the bolt starts to move backwards, the bullet has left the barrel already and pressure of the gas in the chamber has already dropped to a safe level and the bolt and empty cartridge case are moving back on pure momentum alone. Given the larger mass of the bolt and the stiff recoil springs, blowback weapons are usually a bit harder to cock initially. That is why, for most practical purposes, blowback weapons generally tend to use smaller and lower powered ammunition.
The following video shows a MAC-10 submachine gun, which uses a straight blowback action.
And now, a girl firing a Walther PPK pistol. Notice in the beginning of the video about 10 seconds in, when she loads the pistol and pulls the slide back the first time to cock it.
As we alluded to above, in a straight blowback action, the bolt assembly has to be heavy so that it doesn't move back the instant the cartridge fires. As a result of this, straight blowback weapons tend to be heavier than other similar sized counterparts that use different mechanisms. Some manufacturers, such as Hi-point Firearms, try to get around this by making the other parts of the weapon out of lightweight polymers. While using plastics does reduce the weight of the firearm a bit, they are still heavier than, say, recoil action firearms. On the other hand, the mechanism is very simple and easy to manufacture than a recoil action.
In the next few posts, we will study other blowback actions that use different ways of delaying the bolt assembly from moving backwards when the cartridge ignites.
