maintanence and repair part1-GBB

Principles of Gas Blowback (GBB)

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!