PDI has come up with an amazing piston head design that not only allows air to flow into the cylinder on the piston’s backstroke, but also allows more air in after the piston stroke to prevent the vacuum problem associated with long barrels. This series of vacuum pistons is available in multiple pistons for all major sniper rifles (APS, VSR, M40). The secret is the vacuum chamber in the rear of the piston head.

How does it work?

Well, in the center of the piston is a small port, which leads to the vacuum chamber. The chamber consists of a small cavity and a small ball bearing like sphere. The ball bearing is free floating and works on inertia and pressure.

When the piston is at rest or on a back-stroke the ball bearing sits in the cavity and air can pass freely around it.

When the piston goes forward the bearing is pushed to the rear where it closes the chamber and seals the air in the cylinder. An increase in air pressure from the piston pushing forward will seal the bearing against the rear of the vacuum chamber. The air is then forced out the front to propel the bb down the barrel.

On excessively long barrel systems the volume of air may exceed the piston volume (or stroke volume). When this happens the bb continues to travel down the barrel from inertia but the air pressure behind the bb drops and can cause turbulence. The vacuum piston head prevents this pressure drop. As the bb travels down the barrel and air pressure lowers back to normal the bearing falls forward and opens the seal on the piston head, preventing any vacuum effect from the bb traveling through long barrels. This means you can have very long barrel lengths without worrying about if cylinder and stroke volume are sufficient.

How does this compare to other pistons and piston heads?
Many AEG piston heads have ports on the sides of the piston head to allow air to flow into the cylinder on the backstroke, but they do not have any means of preventing barrel vacuum. For bolt action pistons the ports do nothing on the backstroke, but rather direct the air to expand the o-ring during the forward stroke and offer a better air seal, but again these ports do nothing to compensate for the vacuum effect. Many aftermarket pistons offer the ports but only PDI offers a vacuum chamber.

Additionally the PDI bolt action vacuum pistons come in many different styles, such as the polymer Barikaru for easy pull and high power output at low tension springs, Vacuum Hard piston for durability with heavy springs, and B-up for the most stroke volume and power. Many other brands offer only one type of piston as an all around unit which may not give you the performance or fine tuning capabilities as the PDI selection.

For bolt actions these vacuum pistons are a means of using many extra long upgraded barrels, such as the PDI 554mm 6.05 barrel for the Tokyo Marui VSR-10 (which is 30% longer than stock and almost double the G-Spec length). These pistons result in the longest range and best accuracy. Tyler has been able to shoot 2 liter soda bottles at 100 yards using a vacuum piston.

Any drawbacks?
There will be a very slight drop in velocity compared to a piston without a vacuum chamber. This drop is only 5-8fps and unless you chronograph each shot you won’t notice the velocity difference. What you will notice is better long range performance. The vacuum chamber pistons are one example where velocity isn’t the only factor for range and accuracy. These pistons are awesome. These pistons are made for long barrel guns, so if you are using a short barrel gun like a G-Spec or APS-2 SV you may be better with a standard piston.

Overall these vacuum pistons perform great. Airsoft guns can have super long length barrels for amazing range and accuracy without worrying about barrel vacuum from not enough air to push the bb all the way. I would recommend these pistons in conjunction with long barrels to achieve the longest shots possible for your gun.