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Principles of Gas Blowback (GBB)
Initial position. As the trigger is pulled, the BB is shot out and the slide starts moving back. The pistol will "nod" a little bit.
When the slide hits the rear position, it comes to such a sudden stop that it creates a feeling of "recoil" and tilts the muzzle. As the slide starts accelerating forward, it pushes the grip further against the web of thumb.
As the slide returns to battery, it comes to yet another sudden stop. If you were holding firmly, the sight picture should be as it was before you pulled the trigger.
The valve is knocked open, and gas rushes into the loading nozzle. The floating valve directs it behind the BB first.
With the BB gone and the loading nozzle forward, the pressure is directed to propel the slide back.
The Marui system works as above. Valve open -> BB out!
The gas flow pushes the spring-loaded floating valve forward, allowing pressure to build up for blowback.
Even after the slide has cocked the hammer during the rear stroke, the valve knocker remains forward, keeping the valve open for better blowback.
Only after the slide trips the valve knocker lock, the valve is allowed to close.
The blowback system of the Tokyo Marui P226 disassembled. Notice the white piston head cup inside the blowback frame.
Ever wondered why a high flow valve is able to release more gas? Well, here's why!

We've all seen the shooting games in arcades, where you use tethered pistols to shoot zombies that charge towards you relentlessly. Shooting off-screen reloads your pistol, and during game-play you might notice that the slide has sort of a "blowback" movement to it. Barely noticeable.

In airsoft, the highest level of realism is found in gas operated models with blowback (GBB). These are mostly pistols, but there are longer weapons with blowback available as well. Not only is the level of detailing and weight at a completely different level, but they operate like the real thing as well! But how does it all happen, and can you make it happen even better?

 

A SHORT HISTORY OF KICK

The first GBB systems can be traced back to Tanio Kobayashi (of Tanio Koba), a grand old man of Japanese model gun and airsoft gun designs, who was working for MGC at the time. The principle was quite simple, having the valve open directly with the pull of the trigger. The blowback system would first propel the slide to the back, and the BB was shot with the exhaust gas. Because of the slide accelerating back, the front of the pistol would "nod" before shooting the BB, causing the BBs to consistently hit low. You also had to pull the trigger decisively to open the valve every time to produce the same amount of blowback. Squeezing slowly would open the valve slowly as well, causing sluggish operation and low fps.

Western Arms created the first system that improved lots of things all at once: The hammer was used to open the valve via a firing pin, the BB was shot out before the slide would move, and the blowback system went into hiding in the back of the slide when you racked the slide manually to get more realistic looks in the ejection port. WA have protected their intellectual property, so other manufacturers can't copy the design without licence. Tanaka Works uses the WA Magna Blowback in their lineup.

Mr. Kobayashi continued to develop his system into the "pre-shoot" system which uses a floating valve to direct gas behind the BB before moving the slide, and the nozzle also retracts into the slide when it is moved back. Tanio's design is used by Tokyo Marui in all their pistols now, and many KSC models have a striking resemblance to the principle. For Maruzen, he designed another system that has no moving parts inside the loading nozzle, but the switch valve is located in the top of the magazine.

 

HOW DOES IT HAPPEN?

The magazine of a GBB pistol holds the gas and BBs. The gas is commonly HFC134a (aka. duster gas) or C3H8 (Propane, aka. Green gas, Top gas etc.). Upon firing the trigger, the hammer knocks the main valve open via a firing pin. Another term for the firing pin is "valve knocker". The valve is now open. There are locking devices to keep the valve open for a certain amount of time even when the slide goes back to cock the hammer. Western Arms puts valve locks on their magazines, while others incorporate the lock into the firing mechanism to hold the firing pin (or valve knocker) out. The slide trips the lock later during the cycle to allow the valve to close.

The main valve lets the gas into the loading nozzle. Inside the loading nozzle, a switch valve (aka. floating valve, rocket valve...) directs the gas behind the BB to shoot it out of the barrel. The gas flow causes the switch valve to move forward and block the gas flow into the barrel. As the pressure inside the loading nozzle builds up, the slide is forced back by the blowback piston. It has been confirmed with high-speed video equipment, that the BB makes it out of the barrel before the slide has time to move, so the recoil does not affect accuracy!

In the WA system, the rear of the loading nozzle is the piston, and the blowback-frame in the rear of the slide wraps around the piston head. In other systems such as Tokyo Marui, the loading nozzle itself is the cylinder, and a piston is fixed to the rear of the slide. In either case, gas pressure is pushing the loading nozzle and slide in the opposite directions. Because the loading nozzle can not move forward, the slide moves back. It's a common misconception that the loading nozzle has to "stick" forward when you rack the slide manually. Don't worry: The pressure holds it forward.

 

The loading nozzle has a mechanical limit as to how far it can extend, and the slide moves a longer distance than that. After the blowback piston has given the slide all it can, the slide continues to move rearward with inertia, pulling the loading nozzle along. This allows the next BB to move up to the feed lips of the magazine. During the rearward movement, the slide resets the trigger mechanism, cocks the hammer and compresses the recoil spring. The loading nozzle has its own return spring(s) to retract it into the rear of the slide, but the purpose of this is mainly aesthetic. Some models don't have this spring at all, and sometimes a piston head with a tight seal makes the spring unable to retract the nozzle completely.

As the slide returns forward pushed by the recoil spring, the loading nozzle picks up a new BB from the magazine and chambers it. The pistol is then ready for another shot.

 

IMPROVEMENT WORK

As pretty much all mechanical systems, the blowback system can be upgraded as well. The parts can be swapped for more durable ones (being as many models are designed for HFC134a, not the high pressure Green Gas), but you can also tune the performance with different valves and springs. Here's a short rundown of the effect of various parts:

The recoil spring is often changed to any pistol upgraded with a metal slide. The main purpose of a recoil spring is to push the heavier slide forward more quickly, to make the action more snappy and reliable. It's a common misconception that a stronger recoil spring would equate stronger recoil: It actually works against the slide rearward movement, which means that the shock of the slide stopping at the back is reduced.

High flow valves are a replacement for the main valve of the magazine, which means you need one for each mag. There are various models around, and you can also port the original valves if you're related to Blue Peter. The principle is quite simple: They let out gas at a higher flow rate from the magazine into the loading nozzle. It increases the muzzle velocity of the BB, rearward speed of the slide (and thus recoil as well), but also gas consumption is increased and it can lead to cooldown issues.

Piston heads are typically a reinforcement part, commonly equipped with a simple and durable O-ring instead of skirted Y-ring or cup designs. They are more resistant to malfunction at higher pressures. The PDI Winter type piston head extends further into the loading nozzle, which means that the effective stroke length is increased. You can use it the whole year around for the best performance if you have a metal slide.
The Nine Ball Dyna piston head features ports to expand the O-ring against the inside of the loading nozzle, when it's pressurized. These are popular piston heads, although the cost is many times the price of a PDI head. Dyna piston heads come with a spring for the floating valve, which improves muzzle velocity quite nicely.

Floating valves of various kinds are available, such as the Airsoft Surgeon Power Up floating valve for the KSC Glock series. Metallic ones are more durable than original plastic ones, and brass-made valves are heavier, so the gas flow behind the BB is improved, resulting in a higher muzzle velocity.

A largely misunderstood part is the hammer spring. While it does improve muzzle velocity, the main purpose is to reliably open the valve in the first place. When higher pressure gases are used, the valve becomes harder to open. If the hammer spring tension is insufficient, the valve does not open fully, and the blowback will be sluggish. This is easy to spot in a pistol, when you insert a full magazine and start shooting. The operation will be erratic for a few shots (with proper cycling and half-arsed strokes), but as the gas cools down lowering the pressure, the pistol starts operating properly. If the original hammer spring is not opening the valve reliably, it should be changed to a stronger one. But remember that a harder hammer spring is harder to compress, so it slows down the slide as it moves back. The trigger pull weight is also increased, and the wear to the sear as well.

Tuning a gas blowback pistol doesn't have that many variables, but it takes a lot of experience and testing to find the perfect combination for you. Whether you like to maximize the velocity, get the most recoil and realism, or just a fast-cycling stable race gun for practical shooting, the options are available!