“Two-tube” refers to the stacked body style, where the internal parts ride in parallel tubes. Sheridan, the company which at one time produced a series of pump paintball guns for Pursuit Marketing Inc. (PMI) produced the game’s first successful two-tube blow-back semi-automatic, the PMI-III (later renamed VM-68 when sold directly by Sheridan).  The operational system of the PMI-III was basically a modification of the system Sheridan used in it’s pump guns (which actually dates back to the company’s earlier pellet guns). After being manually cocked, the hammer in the pump system would be released and carried forward by a spring and strike a valve, which would release gas directed through the bolt to impact the paintball. In the PMI-III, the valve would also simultaneously release gas directed toward the hammer to blow it back into a cocked position, hence the name blow-back. Since the PMI-III hit the market in the early 1990’s, many other two-tube blow-back systems have come along (and some have since gone). The concept has been refined, and nearly perfected over the last decade. It’s overall simplicity, efficiency, reliability and low cost make it among the most popular systems in the industry.

The Parts
Actual specifics of the internal parts vary from manufacturer to manufacturer, but these are the basics of most modern stacked tube blow-back semi-automatics.

1.   Bolt
2.   Hammer
3.   Valve
4.   Valve pin
5.   Cup Seal
6.   Valve Spring
7.   Main Spring
8.   Bumper
9.   Rear Cap
10.  Valve Screw
11.  Trigger Spring
12.  Trigger
13.  Trigger Latch
14.  Sear Spring
15.  Sear
16.  Frame Screw
17.  Gas Hose
18.  Grip Frame
19.  Barrel
20. Front Cap

How the Valve Works

The valve system is the heart of the blow-back semi-automatic. It hold gas in the valve chamber, and when opened by a strike of the hammer, releases gas in two directions simultaneously – upward into the bolt to propel the ball, and rearward to blow the linked bolt and hammer back into a cocked position.  In most two-tube blow-backs, the gas directed toward re-cocking travels across a flat cut-out on the valve pin, however, some models such as the Diamond GT, have an extra hole in the front of the valve which allows dispersal of the re-cock gas.

How the system Works

Cocked Gun
Blow-backs fire from an open-bolt position, where the bolt is in an “open” position, with a ball in the breech, before the trigger is pulled. When the gun is initially charged up, the linked bolt and hammer must be manually pulled to the rear of the gun, which compresses the main spring, cocking the gun. The bolt hammer combo, which is under pressure from the main spring, is held in place by the sear.

Forward Stroke
When the trigger is pulled, it lifts the front of the sear. As the sear pivots, the rear lowers, releasing the bolt/hammer combo, which is carried forward by the decompressing main spring. The bolt pushes the waiting ball into the bore of the barrel. The sear spring pulls the sear back and down (note the sear not only pivots, but also moves back and forth).

Valve Action
As the hammer comes forward striking the valve pin, the pin moves the connected cup seal away from the valve, allowing gas (blue) to flow into the valve. The gas moves through the valve in two directions (blue arrows), both up and through the bolt to impact the ball, and back to impact the hammer, blowing the linked bolt/hammer combo back toward the rear of the gun (see closer detail on the valve above.)

Rearward Stroke
As the bolt/hammer combo travels back, it compresses the main spring, and travels across the sear. Meanwhile the next ball in the feed tube drops into the breech. Traveling rearward, the bolt/hammer combo impacts the rubber bumper in the back of the gun, which saves the end cap from damage. The bolt/hammer combo then returns slightly forward and is caught by the sear, pushing the sear forward. Meanwhile when the user releases the trigger, it is returned to it’s “at rest” position by the trigger spring. The gun is then ready to fire again.

Full Animation

A few notes about the above animation: For purposes of clarity, the animation runs substantially slower than an actual blow-back. Unrestricted by the sear, a two-tube blow-back can easily cycle in excess of 20 times in a single second.  The animation also features a trigger with a retractable, spring-loaded trigger latch, which allows the trigger to pass the sear smoothly as it returns to it’s rested position. The retractable trigger latch appears on guns like Spyders and Piranhas, but is not a feature on all blow-backs.

The most common way to adjust velocity of the paintballs fired by blow-back semi-autos is to control the tension of the main spring. This can be achieved in several ways. On modern blow-backs, a spring tension adjuster (a.k.a. velocity adjuster) is often located in the rear cap of the gun. It consist of a screw which moves the spring guide (located inside the rear of the main spring) back and forth, thus putting more or less tension on the main spring. With more tension, the hammer strikes the valve pin harder, releasing more gas and increasing paintball velocity. With less tension, the opposite occurs. If the adjustment range of the screw is not enough to achieve the desired velocity, changing the main spring may be necessary. Most modern blow-backs have aftermarket spring kits available, which include color-coded springs of varying tension. If no alternate springs are available, with the proper size shims, a ball bearing, or other objects, the tension on the main spring can be increased. On the other hand, tension can be decreased by cutting the main spring, but that should be considered a final option, since it is not reversible.

Valve spring adjustments can also effect velocity, however the valve spring is not as easily accessed as the main spring, since the gun must be de-gassed first. More tension on the valve spring means the valve closes quicker, and less gas is released, resulting in a lower velocity. Less tension on the valve spring will result in the valve staying open longer, more gas released, and a higher velocity. The options for adjustment include swapping springs, using shims, or cutting the spring. Because the area is under gas pressure, few blow-back guns have ever featured tension adjustment screws for the valve spring (although it was an option on the Line SI Promaster)

Far less common and not so easily accessed ways of controlling the velocity on blow-back semi-autos involve physically changing the valve system and other parts of the gun. For instance, the shape of the valve pin can help determine how much of the gas is used for propelling the ball, and how much is used for re-cocking the action, while the hole in the top of the valve can determine the volume of gas allowed to flow into the bolt to propel the ball. Early blow-back semi-autos actually had parts made to allow adjustments in these areas, but the concept contributed to the complexity of the gun and was abandoned in later blow-backs. In modern blow-backs, such adjustments are only for those who like to tinker.

One thing to keep in mind when tuning a blow-back is that the amount of gas being released by the valve also affects the ability of the gun to re-cock. If the volume and/or pressure of the gas being released is too low, the gun will not have enough gas to re-cock. This often results in the “dead” effect where the gun simply does not try to re-cock after firing a shot, or the “full auto” effect where the gun tries to re-cock, but cannot blow the bolt/hammer back far enough to catch the sear, causing the hammer/bolt to come forward again, strike the valve open, and repeat the process. These effects are especially pronounced when trying to use the gun to with inadequate pressure (cold weather Co2 operation, or a tank running out of gas). In most cases, getting the correct spring combination, or adequate tank pressure will address the problem.

Common Problems
From a standpoint of parts failure, the cup seal takes the honor of “most likely suspect” in two-tube blow-back semi-automatics. Cups seals are made of plastic or some other semi-soft material, and hold gas inside the valve chamber by sealing the valve. Cup seals can be damaged by debris in the gas system, or sometimes just wear out. They are generally inexpensive and replacing one is a fairly simple matter, however, often a cup seal can be repaired through cleaning. Leaking gas heard down the barrel is indicative of a faulty cup seal.

Another source of problems with blow-back semi-automatics is O-ring failure. Depending on the design of the gun, a blow-back can have in excess of a half of a dozen o-rings. Although the failure of any of the O-rings could possibly stop the gun dead in it’s tracks, there are only a few which are critical to the operation of the system. O-rings such as the ones located on the bolt are meant to help seal the gas in as it passes from the valve through the bolt, but the gun will operate without them. However, if those o-rings break, they could leave enough debris to jam up the action. The O-ring on the front of the hammer is a little more important. It helps the hammer catch the gas from the valve directed for re-cocking. With enough gas pressure, some blow-backs will function without the hammer O-ring, but having the O-ring in place makes the process more efficient. The more critical O-rings are the two that seal the valve chamber. The O-ring on the valve itself which seals the chamber usually suffers damage only during the removal or installation of the valve. If it is damaged, the result can be gas leaking down the barrel, much like a faulty cup seal. The O-ring on the front cap must be in working order or the valve chamber will not be sealed, and gas will leak from around the cap.

Although any hard part on a two-tube blow-back can wear out after heavy use, it usually takes tens of thousands of rounds before that happens. Among the most common hard parts to wear out is the sear. If the rear edge of the sear is broken, chipped, or worn down, it may not catch the hammer as it returns to it’s re-cocked position. This is usually evident upon inspection of the sear.

The Basics
The list of possible modifications to improve performance could go on for days. However, for most players, those modifications should be considered “wants” as opposed to “needs”.  Right off the shelf, modern two-tube blow-backs offer decent performance at a decent price, and are easy to use in an effective manner. When well maintained, they are probably more reliable than any other system available in paintball, and easy to repair if they fail. Taken as a whole, blow-back semi-automatics probably offer the best value among all paintball guns.

Top Ten Newbie Mistakes

The first things every player should understand are the rules of the game. At commercial fields the rules are usually presented at the start of the day.  Knowing what constitutes an elimination, for example, can keep a player from leaving the field prematurely. Knowing the boundaries and time limits can help a player decide on strategy.  Knowing that the people wearing orange vest are referees can eliminate wasting paint.

While it is not necessary to be able to field strip a paintball gun, every player needs to know at least three things: 1) how to cock the gun, whether it is a pump or semi-automatic, 2) how to reload the gun and make sure the ammo hopper is secure and won’t loose balls, and 3) where the “safety” is and how it works.  Failure to understand any one of these concepts can not only leave you with a completely useless gun at the most inopportune moment, it can also be quite embarrassing when you finally figure out what went wrong.

Players need to communicate with teammates, whether by very vocal means, or simple hand signals. It is not necessary to develop a set of secret codes or signals when simplicity works fine. If you see two players from the opposing team on the hill ahead, get your teammate’s attention, hold up two fingers, and point at the hill. Chances are, he/she will not mistake the signal for your opinion on how peaceful the hill appears.

Failure to operate in conjunction with your teammates basically leaves you as a group of very lonely individuals. This is especially a bad thing when your opponents are actually acting as a team and tear through your ranks one player at a time. Team players support each other with cover fire, communication, and by working for a common objective.  Even people who have never met each other before game day can work together – really.

While teamwork is a great idea, is does not imply that you have to hold hands with your teammates.  Getting too many players in one position offers the enemy a large, tasty target of opportunity, while concentrating your own firepower and defenses in a limited area. It doesn’t matter if the tree or bunker is big enough for three of you – piling in behind it only offers an aggressive opposing player an opportunity to ruin your day times three.

If the tree is smaller than the diameter of your thigh, you might want to reconsider the location. If the bunker has lots of holes a ball can get through, don’t think an opponent won’t spend his/her time and balls trying to make it happen.  Got a solid bunker? Good. Now stop looking over it and presenting your opponents with the big target of your forehead and skull. Look around the side of the bunker.. peek out with one eye while keeping the rest of your body covered. Pull that elbow and that knee back in there – some people will shoot at dang near anything.

There is a whole game out there despite the fact that the player shooting at you has a seemingly endless supply of paint. While defending yourself from that player is important, also realize the player’s buddies are using your fixation as an opportunity to move on you. You must stay aware of your surroundings at all times. Fixating on one target only serves to distract you, and create a smile on the face of the player sneaking up on you.

Paintball guns have a limited range.  Most commercial fields have a target range where you can check a gun’s performance before you play. Whether precipitated by panic or over-zealousness, one of the most common newbie mistakes is to fire at an opposing player long before there is a snowball’s (or paintball’s) chance in hell of hitting him/her. This is an especially bad thing when the opposing player hasn’t even seen you yet. If an opposing player isn’t aware of your position, wait until he/she is as close as possible before firing. The chance of hitting targets increases exponentially with every yard the distance decreases.

Did you come to play or merely to survive? There is no point in playing a totally defensive position in paintball. Besides, the best defense is a good offense and all that stuff. In paintball, there is usually an objective, like capturing a flag. You can’t do that from way back there.  Having trouble reaching your opponent with paintballs? You can’t do that from way back there either. Move up. Get on the trigger. Take the game to them. After all, it’s only a game, and the worst thing that will happen is you’ll have to come back and try again next time.

Shiny paintball guns DO NOT perform any better than their dull-looking counterparts. Anyone with money can buy a flashy jersey.  Get honest with any experienced player and he/she will tell you that a lot of the flash is exactly for the effect it has on you – intimidation.  Don’t be fooled. The most effective player on the field may be the guy standing next to you with a rental gun – but that doesn’t matter either. What should concern you is what you do when the game is on.  Now, what are you going to do? Learn the rules, understand how your equipment works, communicate and work with your teammates, look for good cover but don’t bunch up, don’t fixate on one person, don’t fire too soon, but be aggressive and use firepower when you need it – and look at the player with the shiny gun and just say to yourself “you’re mine!”

For more tips check out this article.

Expansion Chamber/Regulator FAQ

What is an Paintball Expansion Chamber?
An Expansion Chamber is a device attached to a paintball gun designed to reduce occurrences of liquid Co2 getting into the valve area. Expansion Chambers accomplish this by routing Co2 through a passage or series of passages, which allows liquid an opportunity to boil, or expand into gas.

When do you need an Expansion Chamber?
The introduction of liquid Co2 into a valve system can cause pressure fluctuations which lead to velocity spikes. This occurs most frequently on paintball guns which use a “bottomline” set-up. Since liquid Co2 follows gravity, the horizontal orientation of the bottle in combination with the various angles at which the gun is held by the user allows the erratic introduction of liquid Co2 into the system. Any gun suffering from this problem can benefit from an Expansion Chamber, although like any anti-liquid system it is not always 100% effective.
Since remote lines serve a similar purpose and are most often connected to bottles in a vertical position, guns with remote Co2 set-ups may see no real advantage from the addition of an Expansion Chamber. As in other cases, an Expansion Chamber should only be used in remote systems that are experiencing continued problems related to Liquid Co2.
Guns using HPA (High Pressure Air, a.k.a., Nitrogen) as a power source do not need an Expansion Chamber since the entire “liquid” issue is nonexistent.

What is a Paintball Gun Regulator?
A Regulator is a device which controls the output pressure of the gas power source on paintball guns. The Regulator can only maintain or reduce the pressure of the gas coming from the source tank – It cannot increase the pressure. Aftermarket, or on-gun Regulators are adjustable through a certain pressure range as determined by the manufacturer – not all Regulators work with all paintball guns. For example, some Regulators are adjustable for 0 to 600 psi, which would negate the use of the Regulator with a gun requiring more than 600 psi to operate.
Co2 tanks are generally not sold with built-in Regulators, all though several companies manufacture replacement valves which include a built-in Regulator. For most Co2 powered paintball guns, regulators are placed in the gas line somewhere between the tank and valve.
All HPA tanks have a built-in regulator, since tank pressures are 3000 psi and above, which far exceeds the operating requirements of any paintball gun. The built-in tank Regulator is often used in conjunction with an on-gun secondary Regulator, especially in cases where the tank Regulator is not adjustable.

When do you need a Regulator?
Anyone wanting to control the pressure at which their paintball gun operates needs a Regulator. Whether Co2 or HPA is the power source, there are numerous advantages to controlling the operating pressure of paintball guns such as maintaining the consistency of the velocity, and more finite control over velocity adjustment. There are also some advantages to operating at lower pressure, which requires a Regulator.
A Regulator can also be used to help eliminate the erratic introduction of liquid Co2 into the system. Since liquid Co2 exist at higher pressures (at a given temperature), the lower the operating pressure, the less likely Co2 will exist in a liquid form. Like the Expansion Chamber, the Regulator alone will not totally eliminate liquid Co2 and should be used in conjunction with another device, such as an anti-siphon tube in the Co2 tank.

Should a Regulator and Expansion Chamber be used together?
Depending on how the system is set-up, a Regulator and Expansion Chamber can be used together, but in most cases, the use of both devices is excessive.  More likely than not, the combination will adversely affect performance.
An Expansion Chamber can be used to help eliminate liquid Co2 before it reaches the Regulator, but is more cumbersome than other methods such as the use of an anti-siphon tube in the Co2 tank.
A set-up where the Expansion Chamber is placed between the Regulator and valve system may partially negate the effectiveness of the Regulator, since it allows the regulated gas an opportunity to warm and expand. Such a set-up is also unnecessary in a properly organized system, where efforts should be made to eliminate liquid Co2 before it enters the Regulator.

Which is better, a Regulator or an Expansion Chamber?
With price not being a consideration, a Regulator far outperforms an Expansion Chamber. While offering the same anti-liquid advantages, the Regulator also offers actual control over the operating pressure of the paintball gun. An Expansion Chamber should only be considered if liquid Co2 is a problem, and a Regulator is outside of the user’s price range.

Regulator vs. Expansion Chamber vs. Vertical Bottle
The following test was conducted using a stock Avalon Diamond GT, which is a two-tube blow-back paintball gun. The purpose of the test was to check the differences in velocity consistency when using a standard vertical bottle set-up, a 6-stage expansion chamber, and a regulator. Every effort was made to keep the parameters as close as possible during the test. The three steps of the test were performed back to back, using and the same brand/batch paintballs. A standard 12 ounce Co2 tank was used (no anti-siphon tube), and both the expansion chamber and regulator were used in the vertical bottle position, and fed from a standard bottom-line adapter secured to the grip frame.