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Paintgun Valves

Structure and Operation

Bill Mills
Despite the innovations with new paintguns arriving on the paintball market at a rate of 2 or more a year it seems like these days, most trace distinct liniages in their designs.

Different sources provide different accounts about who made the absolutely very first paintgun, whether it was Daisy, or Sheridan, but the first to go into any decent production numbers was Daisy making the "splotchmarker" for the Nelson paint company. At the same time Nelson was having RP Scherer encapsulate oil based paintballs for them. Both the Nelspot as it became known, and the paintballs were distributed by Nelson for forestry and cattle marking.

The original Nelspot was a bolt action paintgun that was powered by a 12 gram CO2 cartridge in the grip. The Nelson valve design became extremely popular and was used in many pump action paintguns in the following years. The nelson design is in-line, meaning the valving is in line with the barrel. The Nelspot body consists of 2 tubes, the upper tube holding the paintballs, and the lower tube housing the valve in the back, and the barrel in the front. While the barrel was not removable, later "007" Nelspot models featured a removable sleeve over the muzzle to allow for a barrel extension to be added on.

The gas is fed into the valve, and prevented from flowing out the barrel by a cup seal on the back of the hollow power tube. The valve spring, keeps the power tube and cup seal pressed forward, holding them shut. When charged with CO2, the gas pressure also provides additional force keeping the valve shut. The nelson bolt sits at the front of the power tube. At rest, the hammer sits on the rear of the power tube, and the hammer spring separates it from the bolt. When the Nelspot is cocked, the bolt is pulled back, compressing the hammer spring. At the rear of the cocking stroke, a latch on the hammer (the sear) locks the hammer and bolt together. At the same time, a paintball falls into the open breach. Then the bolt is moved forward, pushing the ball into the barrel, sealing the breach, and carrying the hammer forward with it. When when the trigger is pulled, it presses on the sear, unlatching the hammer from the bolt. Under the pressure of the hammer spring, the hammer shoots back along the power tube hard enough to press the power tube and cup seal back. When the cup seal is back, gas flows through the holes in the back of the power tube, through it's center, and to the barrel, where it propels the paintball.

Like most pumps, Nelson based designs fire from a closed bolt (at rest, before firing, the bolt is sealing the back of the breach, with a paintball chambered. The first pumps were add-ons that slid over the Nelspot and connected to the cocking bolt. As the sport of paintball developed a large number of pump paintguns hit the market using this valve structure. Some have fully interchangeable parts, while others had features like bore drop barrels which meant that not all of their parts were compatible with the original Nelspot. Some examples of Nelson based pumps are the Line SI Bushmaster, the TASO Spartan, the Razorback, the Reb Line tight bore, the LAPCO Grey Ghost, Carter Comp, SL-68, and Kingman Hammer.

The extremely popular PMI Trracer, and ACI Hornet are also Nelson based but throw in a twist. On these paintguns the main power tube is connected to the hammer, which engages a very short power tube when it hits the valve. This helps prevent "kinking", a natural twisting force put on the hammer because it only latches to the bolt at the bottom.

The original Nelspot featured no way to change velocity. Eventually, aftermarket companies started selling springs of varying pressure ratings. Putting a weaker spring in the valve, or a stronger hammer spring increases velocity because the valve is held open longer, while reversing that order reduces velocity. More complex later nelson based pumps also featured fine tuning of the velocity by adjusting a screw in the bolt changing the pressure put on the hammer spring. These were "dialed in" with an adjuster tool that ran down the barrel of the marker.

While the nelson valve does not lend itself to a semi-auto design, it can be automated using a pneumatic ram to "pump" it after each shot. This is the method of operation on the Trracer Chameleon conversion, the TS-1 select, and AT85R.

And then there was Sheridan....

Sheridan's first paintgun was based in the Sheridan EB pellet/BB Gun design. Some sources claim Sheridan made a prototype for Nelson before Daisy. The Historians can argue over which came first. The first paintgun that Sheridan produced in Volume was the PGP. It did not feature an in-line valve. The PGP receiver consisted of one tube containing the 12 gram CO2 cartridge, valve and hammer. The barrel was a tube above that. A third tube, on top held the paintballs. On some early models the paintball tube was mounted on the side. In both cases it was attached to, and parallel to the barrel.

The front of the lower receiver tube held the 12 gram which pressed up against a seal. The gas fed from there into the valve chamber. The valve spring pressed the cup seal toward the back of the valve, sealing it. The valve stem ran past the center of the seal and into the hammer area. The hammer, in the back, sat up against the valve stem when at rest. The original PGP was cocked by pulling back on the knurled back end of the bolt. Later models featured a pump on the front of the bottom receiver tube which linked to the bolt by a connecting rod. That concept came from the KP paintball rifle (kind of a misnomer since it had a smoothbore barrel). The KP had a wood stock (just like the Sheridan EB pellet rifles), and other than the length of the receiver, barrel, and paint tubes, operated like the PGP. Early airsmiths cut off the front of the wood stock, and placed it on a dowel, linking it to the bolt with a connecting rod so that it operated as a pump. Eventually Sheridan caught on to that idea and began shipping the PGP and the KP1 with a pump.

When the PGP's bolt was pulled back, a pin linked up with a ridge on the back of the hammer, pulling it to the rear, and compressing the hammer spring against the back plug of the receiver. At the rear of the cocking stroke, a paintball fell into the breach, and the hammer engaged a sear which protruded up from the grip frame below. The bolt was than pushed forward, sealing the breech.

When the trigger was pulled the sear released the hammer which flew forward hitting the valve pin, which pushed the cup seal forward, letting gas flow backward, where it was routed up to the barrel. The gas entered the barrel just ahead of the bolt face. Later aftermarketeers, and eventually Sheridan, produced center-fire bolts with the belief that releasing the jet of gas on the center of the rear of the paintball led to greater accuracy. These bolts were a little bit longer. The gas entered them through a hole in the bottom, and shot it out the center of the bolt face. The Sheridan design is a closed bolt design, as that is the position of the bolt at rest.

An interesting note here about paintgun evolution. The original Sheridan design evenly distributed gas around the back of the ball. The idea then became trendy that center-fire was important, and center fire bolts became prominent in paintball (Nelson designs were center fire to start with). Then Air Power released their Apex with it's "venturi" bolt (which is actually the opposite of a venturi) to evenly distribute the gas against the back of the ball, and that became the new standard. How long before centerfire becomes trendy again? We will have to wait and see.

The Sheridan valve closed bolt pump concept was used in a number of paintguns that Sheridan manufactured for PMI, the KP, KP2, KP3, PMI-1, PMI-2, and PMI Pirahna notably. The Sheridan valve was used in the WGP Sniper paintgun also. This valve concept has appeared in other custom paintguns, like the Houndstooth and Super Stocker from Palmer's Pursuit.

Adding a pneumatic ram and automation system to this design allows it to work as a semi-automatic. This system uses a two step trigger. In the first step of the trigger pull the hammer is released and the paintgun fires. As the trigger is pulled further back, it actuates a 4 way valve (referred to as a 3 way in WGP terminology) which causes a pneumatic ram to pull the bolt and hammer to the rear. As the trigger is released, the ram returns the bolt to the front, chambering the ball for the next shot. Such is the case with the Autococker, Blazer and Typhoon/Hurricane paintguns. These paintguns are closed bolt pneumatic cocking paintguns.

By expanding the hole around the valve stem slightly, the Sheridan valve fires a second jet of gas out the back each time it is actuated. The first paintgun to take advantage of this was the F1 Illustrator. The F1 featured a bolt that was linked directly to the hammer. At rest the bolt is open. When fired, the hammer is released, and the hammer spring pushes it and the bolt forward. This chambers the ball, and at the end of the forward stroke, the valve is actuated firing the paintball, and pushing the hammer, along with the connected bolt toward the back, where the sear, protruding from the grip frame below, catches it and holds it ready for the next shot. This is an open bolt blow-back design, and can be found in many paintguns including the AFT, the Viper M1, the Spyder, The Inferno and of course the F2 and F4 Illustrators.

The odd-ball in the early race of paintguns was the Splatmaster. It's bolt was connected to a power tube that went into a cup seal in the valve body. The barrel was linked to a cocking knob behind the valve. When the cocking knob was pressed it compressed the mainspring between itself and the valve body. The link rod engaged a sear which held it, the cocking knob and the barrel forward. When fired, the sear released this assembly, and the barrel's impact with the bolt released gas out of the valve.

The Nightmare pump action paintguns from Brass Eagle were produced with a slightly different valve also. The Nightmare design combines the hammer style of the Sheridan with the power tube concept of the Nelson. The Nightmare valve had a cup seal inside like the Sheridan, however that cup seal was on a hollow shaft which directed gas flow through its center and out a non-moving power tube at the front of the valve. The bolt slid on the power tube. The bolt and hammer were linked together by the pump arm. The bolt was linked directly to the pump arm, while the hammer was connected by a screw going through a slot. When the pump was pulled back it pulled back the bolt, allowing a ball to drop in, and pushed the hammer back against the hammer spring until it caught on the sear which protruded up from the grip frame. When the pump was pushed forward the bolt sealed the breech. The hammer remained in the back, as the slot in the pump arm allowed it room to move. When fired, the hammer flew forward where it impacted with the valve releasing the gas forward to propel the ball.

The Nightmare valve design saw use in the Nightmare paintguns, and the DMR series pumps. Since the Nightmare valve leaked a fair amount of gas out the back when fired, that was able to be harnessed. Brass Eagle produced the Eagle semi auto as the first gravity fed semi. It was rather like the Nightmare but the hammer was linked directly to the bolt. When it fired both the hammer and bolt flew forward, chambering the ball, and then fired it when the valve opened. The gas out the back of the valve pushed the hammer back (which pulled the bolt with it). Unfortunately the Eagle only operated in a narrow temperature range, and ended up being recalled (leaving Glenn Palmer's custom built paintguns to claim the title of first gravity fed semi). It's valve concept lived on however, and can be seen in the Stingray, Eagle 68, Z1, Mega Z, and Avenger paintguns. These are open bolt blowback semi autos.

The first semi auto on the market was the Tippmann SMG-60, a 62 caliber semi/full auto. It's valve was most similar to a double ended sheridan valve. The valve body received it's gas through a flexible hose, while the body floated free within the receiver. When the hammer struck the valve from the rear it pressed both the front and rear valve stems in. The front valve released the gas to propel the ball down the barrel, while the rear released a jet of gas that re-cocked the bolt. While most consider this to be an open bolt design because the breach did not seal, it didn't actually have a bolt. The balls were carried in a stripper clip, which formed the breach. After each shot the clip was advanced out of the magazine by the magazine spring. The limited size of the magazine (three 5 shot clips) balanced out the high rate of fire in full auto mode. The SMG-60 had another first - the first factory paintgun to use "Constant Air" - refillable CO2 tanks. While other custom paintguns featured CA tanks connected to the valves with hoses (the first being credited to Lou "Gramps" Grubb), the SMG-60 was the first to have a backbottle connection where the tank simply screwed into the paintgun. This valve structure has seen continued life in Tippmann's other paintguns including the SMG-68, 68 Special, Pro-Am, Pro-Lite, F/A and Carbine.

The Sterling arrived on the scene as a pump hybrid of the Nelson and Sheridan concepts. In the sterling design, a lower bolt performs the same function of grabbing the hammer, and holding the hammer spring compressed as the bolt in a Nelson system. However no gas flows through the lower bolt. Instead the lower bolt is linked to an upper bolt that is used to chamber the ball. This design is cocked with a rod that pushed the lower bolt toward the back where it connects with the hammer. They are then drawn to the front, with the upper bolt chambering the paintball. When the sear on the hammer is released, the hammer flies back to strike a valve that is like the Sheridan design. This valve releases gas to the upper bolt which directs it to the ball in the barrel.

Operated as a pump the Nelson/Sheridan hybrid is a closed bolt system, and can be found on the Sterling and Kingman Hammer II pump paintguns. With the addition of a pneumatic re-cocking system, it can be found in the Vector and Rainmaker semi-automatics. In these semi-auto versions the bolt rests in the open position rather than the closed position, making them open bolt pneumatic cocking paintguns. In the rainmaker the control valve is electropneumatic and controlled by an electronic firing system.

Another unique form of valve system is the blow-forward design developed by Airgun Designs. In the blow forward concept the gas is fed into a fixed volume chamber. Unlike other designs that rely on how long the valve stays open after a hammer strike, the blow forward design delivers a metered volume of gas. The theory is that the fixed volume, combined with fixed pressure gas (controlled by a regulator) will provide a precisely metered amount of force on the paintball for perfect consistency. Unfortunately, the temperature of the gas also effects the energy imparted on the ball, and keeping the temperature of the gas a constant after the compression, decompression, and friction of getting the gas into the valve makes that difficult. At rest, the bolt is in the rear, held in place by the sear. In the center of the bolt, is a piston. When the sear releases the bolt, a valve is operated (by the sear in some designs, and by the bolt in others) which cuts off the flow of gas into the chamber (this is known as the On/Off valve in the Automag terminology). The gas in the chamber expands pushing the bolt/piston forward, chambering the ball and compressing the mainspring. At the end of the forward stroke the gas is vented out of the piston through the center of the bolt, propelling the ball out of the barrel. At this point the pressure in the piston has been released, and the mainspring pushes the bolt back to its open, staring position, where it re-engages the sear, and the valve once again lets gas into the air chamber, charging the piston.

This is an open bolt design, and can be found in the Automag, Micromag, Automag RT, Equalizer, Desert Fox and Armson designs. The Armson is unique among this group in that it does not include an internal regulator to provide a fixed gas pressure, and thus benefits greatly from the addition of an external regulator.

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