In this customization discussion, I'd like to boldly introduce the operating principle of gas blowback airsoft guns.
Incidentally, with gas blowback systems, they all essentially use an external tank or some kind of gas such as CO2 (fixed gas systems are a different matter).
As of the time of writing, I decided to share this in the hope that it will be helpful for everyone considering purchasing, maintaining, or customizing their gas blowback pistols before the peak of the gas blowback season this summer.
Another reason I decided to introduce this is that I recently acquired a gas blowback handgun that is easy to explain the operating principle of and easy to disassemble.
First, let's look at the operating mechanism of the slide side of a gas blowback gun. Incidentally, the operating principle is almost the same even for long gas blowback guns; the only difference is that the function of the slide part of a handgun is replaced by the bolt.
*This article aims to provide educational explanations from a mechanical engineering perspective regarding the safe hobby use of airsoft guns.
This does not promote acts of violence, the use of weapons, weapon modification, practical evaluation, or violation of laws and regulations.
Basic operation of gas blowback
Let's take a closer look at the operation of a gas blowback gun, which you're all familiar with.
Before that, let me introduce the names of the various parts of the gas blowback slide.
This is a simplified diagram and not very well done, but please take a look at it just to get a general idea (parts not related to operation have been omitted).
Based on this, let's look at how it works.
I want to see the ejection port, so the orientation is reversed compared to the diagram above, but it's the same situation.
When you pull the trigger, a bullet is fired and the blowback begins.
The key point here is that the blowback distance is still small, and the nozzle remains in the chamber.
Next, the blowback mechanism will be fully retracted.
The key point here is that the nozzle that was still in place in step 2 has also retracted.
The key point here is that the rod-shaped part located below the nozzle, which moves forward with the slide, pushes the bullets in the magazine and loads them into the chamber.
As shown in the diagram, the bullet is pushed by a part of the nozzle, climbs up the slope of the chamber, and is held in place by the hop-up packing.
This series of actions will return you to the same state as in Step 1.
This is the basic sequence of operations for a gas blowback gun (firing - blowback recoil - blowback forward - loading - blowback forward complete).
Next, let's look at the operation by focusing on the flow and movement of the gas.
Gas flow and operation
When the trigger is pulled while the gun is ready to fire, the hammer cocks and the valve knocker pushes against the magazine valve (this mechanism will be explained next time).
The magazine valve is pressed, and the gas from the magazine enters the slide.
I will explain this in a little more detail from here.
The mechanism I'm going to introduce is what's called a negative pressure system, and most current gas blowback guns use this negative pressure system (actually, it operates more on differential pressure than negative pressure; a certain WA company's mechanical system is a bit different).
The process of firing a bullet
The gas that enters the slide flows from the inlet through the nozzle to the position shown in the following diagram.
Incidentally, the yellow flow valve at the gas inlet has the following shape and changes the direction of the gas flow.
Actual flow valve
The gas flow in an actual airsoft gun is as shown in the following picture.
In this state, the gas pressure on the bullet becomes the same as the pressure inside the magazine due to Pascal's principle (the pressure inside a closed space is constant), and that pressure is applied to the bullet held in the hop-up.
Since the inside of the barrel is basically at atmospheric pressure, the gas pressure is greater than atmospheric pressure, so the force acting on the bullet is equal to the gas pressure multiplied by the surface integral of the bullet.
That force is what fires the bullet.
At this point, gas is still being supplied from the magazine, so the pressure conditions before the bullet leaves the barrel are as follows:
However, the blowback function will not work in this state.
The reason is that the flow valve inside the nozzle blocks all passages except the one leading to the bullet, preventing gas from flowing.
With this cover closed, the mechanism that is the core of the gas blowback system kicks in and the blowback begins once the bullet has fully exited the barrel.
In reality, the pressure distribution is much more complex, and the blowback begins while the bullet is still inside the barrel.
Switching the gas route from the firing side to the blowback side.
First, let's look at the pressure distribution when the bullet has completely exited the barrel.
Since the bullet has completely left the barrel, the gas supply from the magazine is released because the "lid" (the bullet) that was blocking it is gone, causing a rapid drop in pressure.
Naturally, since the area outside the gas flow path is much larger, the pressure drops to the level of the outside air, or atmospheric pressure.
You might find this hard to believe, but no matter how high the pressure inside the magazine is, it's still a tiny amount compared to the volume of air, so even if you inject gas, the pressure drops instantly.
However, the gas supply from the magazine has not yet stopped, so gas will continue to be released.
The pressure, distribution, and gas flow in this state are illustrated in the following diagram.
Here, an interesting property of fluids is utilized, which causes a change in the pressure distribution according to Bernoulli's principle.
Bernoulli's theorem, simply put, states that in a given flow, the sum of the pressure and half the square of the flow velocity is always constant.
$ P + \frac{V^2}{2} = constant (where P is pressure) $
In other words, if gas continues to flow inside the barrel, the gas flow velocity will be faster than the air velocity in the atmosphere, causing the pressure in the gas flow path to decrease.
In this case, the pressure on the piston cup side of the green part is almost the same as atmospheric pressure, so the flow valve, which is now under low pressure, starts to move due to the pressure difference (actually, gas leaks into the piston cup, making the pressure higher than atmospheric pressure).
The shape of the flow valve and the role of the flow valve spring determine the timing of this initial movement.
If the flow valve spring is too strong, the flow valve won't move and the blowback won't occur.
The situation at this time can be illustrated as follows:
Once the flow valve has started to move and is fully extended, its role as a gas seal for the blowback mechanism is released, and it changes to a seal that stops the flow of gas towards the barrel.
Let's take a look at the flow valve actually installed inside the nozzle from the back. The flow valve moves when you push it with a rod.
The situation at this time can be illustrated as follows:
As shown in the diagram, when the flow valve moves all the way, a closed section is created by the blow valve and piston cup, so Pascal's principle comes into play and the red area becomes the same pressure as the gas inside the magazine.
This pressure is now applied to the piston cup, and the force generated by the pressure begins to move the piston cup and the entire slide assembly.
The force generated on the slide at this time is Force = Gas pressure P × Piston cup area, and in order to make the blowback stronger, each company is striving to increase the diameter of the piston cup.
While recent models from Tokyo Marui and KSC's System 7 use a 15mm diameter circular piston cup, overseas manufacturers often use irregularly shaped piston cups to try and increase the surface area.
As you can see, currently there are no other ways to make the blowback stronger and more stable than increasing the gas pressure or making the piston cup larger.
Especially in Japan, where the types of gas that can be used are mostly limited, the evolution has basically been towards making the piston cup larger (for CO2, the gas pressure increases).
Actual overseas-made blowback-related parts
If you prefer a sharper blowback action, choose one with a larger piston cup.
In reality, most airsoft guns don't list the piston cup diameter, but it might be fun to look at the shape of the rear of the slide and try to find one that looks like it has a large piston cup (generally, a smaller slide width and height is a disadvantage, and the CZ75 is a typical example of a difficult one, but KSC has managed to fit it in, which is why it's prone to breaking).
Blowback recoil operation
Returning to the topic, the slide moves as follows due to the force acting on the piston cup.
Here, the gas pressure keeps the flow valve acting as a cover in the barrel direction, while the piston cup moves due to the force generated by the pressure, causing all the parts attached to the piston cup, i.e., the entire slide assembly, to move.
At this time, the nozzle does not move because the pressure on the flow valve acts in the opposite direction to the piston cup, so the nozzle return spring extends.
The outer barrel also moves slightly due to the cam mechanism, but it doesn't move much because it's basically connected to the chassis (short recoil mechanism).
Consequently, the recoil spring located between the slide and the outer barrel compresses.
The next step is the final one.
Blowback forward operation
If the slide continues to move along with the piston cup, the piston cup will protrude beyond the rear end of the nozzle. Also, the gas supply from the magazine will stop at this point due to a mechanism on the chassis side (the mechanism will be explained next time).
Let's illustrate the situation at this time with a diagram.
At this point, all the gas pressure inside the slide is released, and it becomes atmospheric pressure.
This is what the actual item looks like.
The spring, which has been compressed and expanded by gas pressure, tries to return to its original shape.
The flow valve returns to its original position as the flow valve spring extends.
The nozzle returns to its original position when the nozzle return spring compresses.
The slide moves forward and returns to its original position as the recoil spring extends.
This completes the blowback process, ending the series of actions and returning the device to its initial state.
This is how the slide of a gas blowback pistol works. Long guns use the exact same principle; this mechanism is located in the bolt section, not the slide, and that's how it operates.
Summary
This concludes our explanation of the basic operation of blowback airsoft guns.
To summarize the sequence of the mechanism's operation:
In reality, each step doesn't operate in a clearly separate manner; some operations overlap and occur simultaneously. Moreover, gas leaks from the flow valve through the gaps between parts even while the bullet is being fired, flowing towards the piston cup and slightly increasing the pressure.
In that case, even while the bullet is passing through the barrel, the pressure distribution inside the barrel can cause the flow valve to start moving, creating a pressure difference with the piston cup, which can switch the valve and cause blowback to begin before the bullet leaves the barrel.
Therefore, even though there is still a bullet inside the barrel, the flow valve starts to move due to the differential pressure.
When the flow valve moves and the hole crosses the gas inlet, gas flows into the piston cup, the pressure difference increases further, and the flow valve moves rapidly, switching the valve on.
This is the mechanism by which blowback begins before the bullet is fired.
In situations like this, slightly strengthening the flow valve spring delays the switching timing, allowing gas to continue supplying to the firing side until the projectile leaves the barrel, which increases the muzzle velocity (be sure to follow the law).
Conversely, if the initial velocity is high and fuel efficiency is poor, weakening the flow valve spring will reduce the amount of gas supplied to the firing side, lowering the initial velocity and slightly improving fuel efficiency.
Incidentally, the flow valve precisely switches between the gas routes for firing and blowback, so the strength of the flow valve spring does not affect the blowback.
The changes will be in initial speed and fuel efficiency.
The parts that act as covers are intentionally not airtight (because they are moving parts and require gaps) to allow for smooth operation, which makes the pressure distribution inside the gas blowback engine more complex.
Ensuring proper airtightness around these moving parts improves performance, but be careful not to make them too tight, as this can actually make the movement stiffer.
A quick and easy way to remove the seal is to try using a slightly larger third-party piston cup.
The blowback adjustment largely depends on the recoil spring; strengthening the recoil spring increases blowback resistance, which slows down the slide's recoil speed and worsens its efficiency, but it also speeds up the slide's return.
Conversely, if you weaken the recoil spring too much, the slide's return speed will become too slow, and it might even stop midway, so be careful to find the right balance.
This concludes the slide presentation.
Next time, I'll introduce the frame and chassis mechanisms.
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