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What's In a Semi?

© Ravi Chopra, 1997
Printed in Action Pursuit Games, October 1996

Back in the pumpgun days, there were really only two basic types of paintgun for serious players: Sheridan-based pumpguns (the Sheridan pumps and the Budd Orr Sniper) and Nelson-based pumpguns (all the rest). And though the precise configuration of these two basic designs differed in detail, they actually functioned very similarly. Picking a marker pretty much boiled down to finding a 'gun in your price range with the features you were interested in.

Today, with the exception of the few areas where stockgun play has come back into style, the torch has been passed on to semi-automatic paintguns. The variety of styles and systems capable of shooting paintball after paintball with nothing more than a quick pull of the trigger has expanded to a bewildering array, each claiming to have a performance edge over the competition. For the new player looking to buy a new semi, trying to pick the "right" one from the crowd can be a daunting task. The best place to start is to learn what they're all about.

While the differences between semi-autos dwarfs the differences between pumps, semis still fall primarily into three basic groups differentiated by the systems they use to achieve semi-automatic operation: blowback, blow forward, and closed bolt. Each of these systems has inherent advantages and disadvantages.

Here are the basic terms involved with understanding the various semi-auto designs:

BOLT
A bolt is the moving part of the paintgun behind the ball when a ball is chambered and ready to shoot. In all instances, the bolt moves back to allow a paintball to drop down in front of it, and moves forward to move the ball forward into the barrel to be shot. It is the timing of the motions of the bolt that primarily differentiate a "closed bolt" paintgun from an "open-bolt" paintgun.
HAMMER
The hammer is a moving part of the paintgun that is hidden inside the paintgun's body and in some designs has a "cocking knob" attached to it and protruding out of the 'gun body. The hammer is typically a relatively heavy, spring-loaded metal cylinder. When the 'gun is cocked, the a spring is compressed by the hammer, which is itself locked in place by the sear. When the hammer is released, the spring launches the hammer against a valve which releases a certain amount of gas into the 'gun. Blow forward designs do not have hammers.
SEAR
The sear is a part of the trigger assembly. In the up position it catches and holds the hammer or bolt back. When the trigger is pulled, it releases the bolt/hammer and allows the paintgun to shoot.


BLOWBACK

The blowback semi-automatic design is by far the most common found in the world of paintball. It forms the basis for a tremendous number of paintguns from more manufacturers than you can count. Perhaps the most amazing thing about the blowback design is the sheer variety. If you randomly pick any two blowback semis, chances are that they look, feel, and even shoot completely differently from each other. Yet they all work off of the very same principle of operation. The first thing to know about blowbacks is that they are so-called "open-bolt" paintguns. That is to say, at rest (between shots), the bolt is retracted to leave the breech open to the feed tube allowing a paintball to drop down in front of the bolt. The bolt is coupled to the hammer by a pin or shaft. There are two basic styles of blowback. In one, the hammer moves in a channel behind the bolt (in this case a shaft connects the hammer to the bolt). In the other, the hammer moves in a channel below the bolt (here, a pin directly connects the two). Commonly you find a knob screwed into the side of either the hammer or bolt, jutting out through a slot cut in the side of the paintgun. This knob allows you to pull the hammer and bolt back against a spring into the cocked position. They lock back by catching the sear.

When you pull the trigger to shoot a blowback, the sear drops and allows the compressed spring behind the hammer or bolt to thrust the two forward (remember, they are connected). As the bolt moves forward, it pushes the paintball in front of it into the barrel and closes the barrel off from the feed tube. Simultaneously, the hammer flies forward to strike a valve component. The valve is designed in such a way that when the hammer strikes it, a spring-loaded seal is opened to allow the propellant (CO2/compressed air/nitrogen) to flow through it. Part of the gas flow is channeled into the bolt which directs the high-pressure gas into the back of the chambered paintball, shooting it down the barrel. The rest of the gas is channeled into the front of the hammer, blowing it and the bolt back (thus the name) into the cocked position. Blowback trigger assemblies tend to be the most complex of the various designs because they have to be able to catch the returning bolt/hammer even if the trigger is still pulled back.

Blowbacks have many new strong points, most significantly as a result of recent developments. There's been a real revolution in blowback design. New designs feature lighter weight, improved trigger design, and superior gas efficiency. In addition, a large number of upgrade parts have become available for older designs, including performance bolts, hammers, and valves, allowing them to be more competetive with the new generation. As always, the biggest advantage of the blowback continues to be robustness and reliability. They really only have one critical seal: the valve seal. Because of this, blowbacks tend to be much more tolerant of liquid CO2. In fact some are designed to run on liquid exclusively! The simplicity of the design also makes them very easy to clean and maintain. Usually all you need to do is rinse it off and run some oil through the ASA (air source adapter) to keep a blowback paintgun operating smoothly and reliably. Blowbacks tend to shake a bit with each shot. This movement is often enough to keep your feeder from jamming and makes an expensive motorized loader largely unnecessary. Finally, on average, blowbacks hold a significant price advantage over other designs. Street prices well under $200 are not uncommon for blowback semi automatic paintguns. This is MUCH less than even the least expensive paintguns utilizing other semi automatic systems.

The blowback design also has several limitations. One you'll definitely hear about relates to the fact that the open-bolt designs shove the paintball forward into the barrel and simultaneously smack it with the high-pressure gas to launch it down the barrel. Some think that this has more chance of putting a spin on the ball and reducing accuracy. Additionally, open-bolt designs always have some degree of "blowback" wherein excess pressure escapes around the bolt tip and up the feed tube. If your feeder is running near empty this can push balls up the feed tube and cause misfeeding during rapid shoot such that you get half-fed paintballs chopped by the forward-moving bolt (the so-called "bingo ball" effect). The mass of the bolt and hammer slamming back and forth with each shot also has a tendency to make the 'gun shake or kick with each shot, also reducing accuracy. Finally, despite significant improvments, blowbacks still tend to have longer, heavier trigger pulls.

Some examples of blowback paintguns are the Sheridan VM68 series of paintguns, the Tippman Pneumatics semi-automatics, the Kingman Spyder, the Brass Eagle Stingray, the FASTech F2 Illustrator, and the Montneel Designs Icon-Z.


BLOW FORWARD

The blow forward design was introduced by the wizards of Airgun Designs. Recently another player has entered the blow forward market (Sheridan's Equalizer) with a somewhat different design which works off the same basic principles as the '68 AUTOMAG, but has it's componenets positioned somewhat differently. The specifics of this section will focus on the Airgun Designs-based blow forwards.

The blow forward is similar to blowbacks in that it also is an open-bolt design. Beyond that, it is almost the inverse of a blowback. Rather than having a bolt/hammer combination thrust forward by a spring and returned to the open position by gas pressure, it has the bolt blown forward when the 'gun is shot and is returned to the resting back position by a heavy spring.

To understand how this works, you first have to understand the layout and gas path through the blow forward. The gas feed enters a blow forward at a pressure regulator which makes up the back of the marker. This pressure regulator is used to bring the pressure of the gas down to a level far below tank pressure and hold it at a steady level. The regulator can be adjusted to allow through higher or lower pressures to increase or decrease velocity. After passing through the regulator, the gas is fed forward to an on/off valve which only allows gas through when the valve pin is released. From the on/off, the gas goes forward into a fixed-volume reservoir behind the bolt. The bolt, pushed back by a heavy spring, has a stem which fits down into the end of the reservoir (into the power-tube at it's fore-most end) and seals against an o-ring.

The sear of the blow forward actually has two important functions. At rest, the front of the sear catches the bolt and holds it back against the o-ring sealing off the gas reservoir. When the trigger is pulled, the sear is rotated around it's center. The back of the sear rocks up and pushes against the pin of the on/off valve, closing off the gas flow to the fixed-volume reservoir. The front of the sear rocks down and releases the bolt. The high-pressure gas in the reservoir thrusts the bolt forward against the return-spring, pushing the paintball forward into the barrel. At the forward limit of the bolt's travel, the gas escapes around the end of the bolt stem, through the bolt, and shoots the paintball down the barrel. At this point, the pressure behind the bolt is relieved and the heavy return spring snaps the bolt back into the full-back position.

When you release the trigger, pressure on the top of the on/off pin provides the return force. The front of the sear first catches the bolt, then the on/off pin drops far enough to open the valve and allow refilling of the reservoir.

This is a truly elegant system with many significant performance advantages. First and probably foremost is the extremely fast shooting that this system will allow. The trigger pull is extremely short and "snappy." No matter how fast your finger is, you're not going to come anywhere close to the 'gun's cycle-speed limit. The whole system fits into a very small, compact, well balanced package. Additionally, the lack of a weighty hammer slamming back and forth makes it very stable while shooting, especially with longer barrels which help eliminate gas-related "kick". Because of this, it is much easier to hold a target from shot to shot and accuracy is somewhat improved.

On the downside, blow forward designs share the same potential accuracy-degrading open-bolt characteristics that blowback designs have. Additionally, they have MANY critical seals which can easily freeze up and cause the 'gun to leak or shut down completely if you dump any liquid CO2 into it. As well as being a source of liquid CO2 and cold weather problems, all of these seals undergo some degree of wear during normal use so it is important to know which seals can cause which problems, and to have replacement seals so you can continue to play when these problems do crop up. Furthermore, the design is very stable when shot. This intrinsic stability makes an agitated loader mandatory to ensure reliable feeding during rapid shooting.

Some examples of Airgun Designs-based blow forwards are the '68 AUTOMAG and MINIMAG, the Pro-Team/Gun F/X '68 MICROMAG, the Smart Parts Smart-Mag, and the Fox River Games HyPerMag. The only design on the market not sourced from Airgun Designs is the Sheridan Equalizer.


CLOSED BOLT

The closed bolt design differs from the blow forward/back designs in that the bolt is fully forward, sealing a paintball in the barrel at rest. Because of this, the paintball is shot from a stationary position rather than at the same time it's thrust forward into the barrel. Cycling of the bolt/hammer occurs after the ball is shot.

Functionally, the closed bolt paintgun is a pumpgun with a pneumatic mechanism to pump it for you. As a system, it consists of two separate parts, both controlled by the trigger, and both of which must be synchronized perfectly to ensure reliable functioning.

The first part of the system consists of a relatively standard pumpgun-like sear-hammer-bolt-valve relationship. Like the blowback design, a hammer is retracted against a compressed spring and is held back by the sear. The bolt, which in this case is not connected to the hammer, is in the fully-forward position, sealing the paintball in the barrel and closing off the feed-tube. When the trigger is pulled, the sear drops and the hammer is propelled forward into the valve, metering out gas. In this case though, gas is only channeled into the bolt and launches the paintball.

The second part of the system cycles the bolt to allow a new paintball to drop into the breech, and retracts the hammer to prepare the 'gun to shoot the next ball. Part of the gas flow into the paintgun is directed into a pressure regulator. This regulator allows an output pressure of only 100 psi, the pressure at which the cocking components are designed to function optimally (full tank pressure would blow out their seals). The output from the regulator is fed to the input of a 4-way valve (one input, two outputs, and exhaust). Depending upon its position, the 4-way directs gas to either the front or back of a pneumatic cylinder (commonly referred to as a "ram"). The ram is connected to the bolt. A rod connects the trigger to the 4-way valve.

At rest, the 4-way directs the low pressure gas to the back of the ram, pushing its piston forward, thus pulling the bolt forward. When the trigger is pulled, the 4-way is moved to direct air to the front of the ram, pushing its piston back. When the ram piston is thrust back, it pushes the bolt back to allow a new paintball to drop into the breech. On the way back, a part of the bolt assembly catches the hammer and pulls it back.

Upon release of the trigger, the sear rocks up to catch the newly retracted hammer, gas is directed to the back of the ram, and the bolt is pulled back forward to seal the next paintball in the barrel.

The biggest advantage people see in the closed bolt design is accuracy. Since the ball is shot from a stationary position, it is not going to pick up any spin or deformation from a moving bolt behind it. While there is certainly no consensus on this, it is believed by many that this characteristic provides closed bolt designs with better accuracy and long range performance. The trigger configuration of the closed bolt design can be tailored to a player's particular tastes by a good pro-shop. The popularity of the closed bolt design has led to a great deal of innovation in changing it's various performance characteristics. Most closed bolt paintguns offer a truly enormous selection of available upgrades and modifications.

Closed bolt paintguns have some significant disadvantages. First and foremost is the fact that the shooting and cycling phases of the trigger pull must be in perfect phase. If the timing of these events is not just right, it can lead to velocity inconsistency at best, and major malfunctions at worst. Owners of tournament-tuned closed bolt paintguns should know how to maintain and adjust their paintguns to ensure long-term reliable functioning. The pneumatic cycling system of closed bolt designs tends to be liquid sensitive as well. While it isn't as likely to shut down as a blow forward if you get liquid CO2 into the 'gun, it is still important to take measures to keep liquid out of the 'gun to ensure reliability. Finally, most closed bolt paintguns are HEAVY. While this weight does contribute to accuracy by stabilizing the paintgun, they can be far more massive than some players care to lug around the field.

Examples of closed bolt paintguns are the Budd Orr's Autococker and Minicocker, and the Palmer Typhoon/Hurricane/Stroker series of paintguns.


CONCLUSION

The next time you go out to play paintball, ask a few players which system they think is best, and why. I promise you, they'll each give you a completely different answer. The moral of this story is that no one paintgun stands head and shoulders above the rest of the pack. Each one has characteristics tailored to suit a particular kind of player. Your job as player and consumer is to find the 'gun best suited to your tastes and style.

Good luck.

All material at this site is © Ravi Chopra, 1999