How Nelson Paintball Guns Work .. mostly, sort of..

The Nelspot 007, originally sold by the Nelson Paint Company for entirely different purposes other than shooting people,  was the original paintball gun. Not only was the cattle and tree marker the object of many a mythological tale of the early days of the game, it was also the very first gun with which I played the game, and about a week later, the very first paintball gun I owned.

The Nelspot 007 was a popular paintball gun, for both rentals and as first-time purchases. When compared to it’s nearest competitors, the PMI/Benjamin Sheridan PGP and the National Survival Games Splatmaster, the Nelspot spawned a much larger upgrade path. Early Nelson based guns were truly “Nelson based” – having been sold as upgrade kits for the 007. For years most of these guns – even the ones completely manufactured by other companies – had 75%+ parts compatibility with each other and the 007.  That is not really the case today.  Many of today’s popular pump guns, including the Component Concepts Industries Phantom, the Maverick/Trracer series, and current Brass Eagle pump guns, as well as the Advanced Tactical Systems AT-series select fire guns,  are based on the Nelson system, but do not use compatible parts with their predecessors.


Over the years, there have been many variations of the internal systems of Nelson based guns, including different configurations on nearly every internal part of the gun. While all Nelson based guns may not have interchangeable parts, they do function the same. The Nelson internal system consist of several moving parts including a bolt, hammer with connected sear and sear spring, valve tube and cup seal, main spring, valve spring, and trigger.  Pumping the gun requires one stroke back, and one stroke forward. Although the original 007 used a small bolt-action like screw to pump, everything since has had a wrap-around pump handle connected to the bolt.

Gas enters the gun through the valve body in the rear of the gun. The cup seal on the back of the valve tube is held against the rear of the valve seat by pressure from the gas itself, and the valve spring. The sear on the hammer pivots on a small pin and is held in it’s normal position by a small spring. On the backward stroke, the bolt moves rearward, compressing the main spring until the bolt and hammer are joined and hooked together via the sear. Note the hammer is held in place by the valve tube ledge.  At the same time, a ball drops in place in front of the bolt.

On the forward stroke, the bolt moves forward, carrying the locked hammer and compressed main spring with it, while pushing the ball into the bore of the gun. The gun is now ready to fire.

When the trigger is pulled, the rear of the trigger lifts the rear of the sear. As the sear pivots, the front end unhooks from the bolt (yellow arrow). The compressed main spring throws the hammer backwards very rapidly. When the hammer hits the valve tube ledge, it pushes the entire valve tube backwards. Gas rushes into the holes in the rear of the valve tube (small blue arrows), travels forward through the tube,  impacts the ball, sending it down the barrel (large blue arrow).  The valve spring then pushes the valve tube forward and closed. The gun is now ready to cock again.

Full Animation

There are actually a multitude of ways to tune a Nelson based gun. Most are based on valve timing – how strongly the hammer knocks the valve open, and how quickly the valve closes.  However, there are other methods that involve the internal size of the valve tube through which the gas must pass, or variations in the weight of the hammer. These last two methods are not necessarily applicable to newer Nelson based guns, since few optional parts are made for those guns.

The Main Spring
The most common way of controlling velocity on Nelson based guns is to control the tension on the main spring. The stronger the spring is, the more the hammer will open the valve.  The more the valve is open, the higher the volume of gas released, and the higher the velocity of the ball.  The tension on the spring can be controlled in several ways:

  • The spring itself can be swapped for a stronger or lighter spring. This method is somewhat imprecise, but can get you in the ballpark.
  • Existing springs which are too strong can be cut, or springs which are too weak can be supplemented by washers. Cutting the spring is obviously a non-reversable method. The washers must be a proper size to ensure no problems.
  • A tension adjustment screw can be installed (usually by the manufacturer) in the bolt or hammer.

The last of these methods is the most common on modern Nelson based guns, usually with a screw inside the face of the bolt which can be adjusted to increase or decrease tension on the main spring. This method is easy and convenient, because you simply stick a long hex wrench or other tool down the barrel (or remove the barrel) for adjustment. Phantoms, the Deluxe version of Maverick/Trracers, Hornets, ATS guns, and others come with this feature.  Some older guns like the Line SI Bushmaster had a similar adjustment method except the tension screw was located in the hammer. Adjustment of the screw required the removal of the valve body. This was a “non-cheatable” tournament feature.

Some Nelson based guns come with two main springs, commonly known as a “summer” (weak) and “winter” (strong) spring.  The reason for this is that even with a tension adjuster, the proper velocity cannot always be found in the range of a single spring. This can be important when operating on Co2, which produces lower pressure in colder temperatures. The lower pressure means the valve needs to be open longer to allow more volume of gas.

The Valve Spring
The next method of adjusting the velocity is the control of the valve spring. A weaker valve spring allows the valve to stay open longer, expelling more gas and increasing velocity. Conversely, a strong valve spring closes the valve quicker, releasing less gas for a lower velocity. As above, valve springs can be replaced, cut, or supplemented by washers. There were a couple of guns which had a screw adjustment of the valve spring such as the Apex Elite. This method required the removal of the gas source from the back bottle adapter which came standard on the gun.
The Valve Tube
Changing the valve tube is certainly not an option on all Nelson based guns, especially those with proprietary parts. For a number of years, valve tubes were made with different IDs (Inside Diameters). The larger the ID, the larger the volume of gas sent to the ball, and the higher the velocity.  Since using a larger volume meant relying less on overall pressure, a larger ID could also mean a reduction in noise. Another feature on some valve tubes that could make a difference in overall gas flow is the number of inlet holes in the rear. Early Nelspot tubes only had two small holes and a small ID. Later versions, such as the ones made by Lapco offer three large inlet holes and a much larger tube ID.

Random Valve set-ups
From left to right:


  • Nelspot 007 valve tube, steal valve seat, brass cup seal with urethane insert
  • Line SI #4 vavle tube, steal valve seat, brass cup seat with urethane insert
  • LAPCO #6P valve tube, aluminum valve seat, and plastic cup seal
  • ACI Maverick valve tube, brass valve seat, plastic seal with brass nut
  • Brass Eagle Tigershark valve tube, brass valve seat, brass cup seal with urethane insert




The weight of the hammer affects velocity by affecting valve timing. A heavier hammer holds the the valve open longer, etc. Unfortunately, varying hammer sizes are not an option on most of today’s guns. However, realistically, most of today’s guns probably have decent weight hammers developed from years of Nelson type experience. The hammer in the original Nelspot 007 was, for most purposes, ridiculously heavy.  Heavy hammers bounced.  In other words, due to their own weight, the hammer would knock the valve open, bounce back a little against the main spring, then hit the valve open again, then bounce again… much like dropping a basketball on the ground and watching it bounce back until it runs out of power and doesn’t bounce any more – except a lot faster of course. It would sound like a short fart, with each audible pop after the initial hit being a waste of gas.  At some point, somewhere, somebody concluded that a lighter hammer would be more efficient, because if would not carry enough weight to bounce – you would get just a simple, crisp, “pop”, when you fired the gun. Early lightened hammers were simply stock hammers with holes drilled in them. Eventually aftermarket companies purposely built hammers in varying weights to add to the overall “tunability” of the Nelson based guns.

I am aware of only one other (non-regulated) method of velocity adjustment for a Nelson based gun. The Tippmann SL-68 series has a restricter screw blocking the air passage, much like Tippmann’s semi-autos. The screw is in the side of the bolt.  This is an inefficient (yet effective) method of adjusting the velocity since the screw merely diverts away gas that has already been released by the valve system.

In Tune
There really isn’t a perfect parts combination. It can vary from gun to gun for a lot of reasons, and personal preference can also make a difference.  Once you figure all of the variables into the picture, the exact same velocity range can be reached using any of several different spring combinations and options. Some players like a smoother pumping action for faster rates of fire and opt for a light main spring, then use whatever valve spring is needed. In the past, it was not uncommon for players looking for a fast, smooth action to use the lightest main spring possible, and a heavy weight hammer. The hammer, of course, would bounce and cause a waste of gas, but the trade-off was lost efficiency for an increased rate of fire.  On the other hand, stronger valve springs give more consistency and better efficiency, so some players (especially 12 gram players) will use the heavy valve spring and whatever main spring is necessary.

Breech or Bore?
Bolts vary from brand to brand, but there are two common types. The Phantom, for instance, is what is known as a “breech drop” gun, while guns like the Line SI Bushmaster are a “bore drop”.  The most obvious noticeable difference is the length of the bolt. On the 007, the bolt was the shorter “breech” type. The main operating difference is that the ball drops into the breech area, and must be pushed up a small step to be seated in the bore. The “bore drop” type simply drops the ball into what is basically the bore of the gun. The theory behind the development of the “bore drop” was that the step on “breech drop” guns was part of the problem in pinching balls as the gun was cocked. The “bore drop” concept eliminated the step, thus eliminating the alleged problem. This was used as a major selling point for “bore drop” guns. Whether it makes a realistic difference or not is an entirely different discussion. I have owned a multitude of both types and tend to favor the “bore drops”.

The 007 could be sort of stiff to pump, especially on the forward stroke. This was caused by kinking of the bolt and hammer. When the bolt and hammer were locked together with a compressed spring between them, they would kink, causing friction against the interior walls of the gun. Several styles of bolt/hammer combinations were developed to combat the problem, each of them using a different method of holding the bolt/hammer combo straight once they are locked by the sear.

  • The top system uses a collar around the hammer which fits inside the back of the bolt. The design works well and is used on the Phantom, and was the stock design of the Line SI Bushmaster. In the case of the Bushmaster (shown), the velocity adjustment tension screw for the main spring  is inside of the hammer.
  • The middle system uses a shaft coming out of the bolt, which ran through the center of the hammer.  Later versions had the shaft also act as the velocity adjusting tension screw (mentioned earlier). This meant the shaft rotated and extended backward to increase the tension. This maneuver will also increase the level of “frictionlessness” on some guns, however, the shaft can be turned back so far as to interfere with the mating of the bolt and hammer.
  • The bottom system appeares on the Maverick/Trracer series. It involves a shaft protruding from the hammer which fits inside the bolt. This shaft also acts as the forward end of the valve tube, leaving the gun with a very short valve tube that extends only as far as the back of the hammer (see valve tubes). The ATS select-fire guns use this same method.

Gas leaking down the barrel on a Nelson based gun is indicative of a bad cup seal. The cup seal wins the “most likely to wear out first” award for Nelspot parts. That’s not to say that they will. I have some Lapco pieces that I have been using for years. However, on stock Nelspots, and a lot of the clones, the cup seals were definitely the most frequently replaced part. The only other critical air seal on the gun is the O-ring on the valve seat. I can’t recall ever having a problems out of one of those, although it is feasible that you could damage one when removing the valve.

Older valve tubes used to be bad about breaking in colder weather. The break would usually occur at the threads where the cup seal screwed on. Ever since the advent of plastic cup seals, I have not seen any break.  Other parts to watch out for are the sear, which can wear out from excessive use (I mean years of use), and the thread holes on body parts. Most Nelson type guns are made of aluminum, and repeated disassembly can take it’s toll on the threads. Other than cleaning, the guns generally require little maintenance.

A common occurrence on Nelson based guns is the “popping” sound on the backward stroke of the pump. This can happen for several reasons. First off, it’s important to understand why most* Nelson based guns cannot be cocked without gas going into the gun.  As mentioned earlier, both the valve spring and gas pressure hold the valve tube in it’s forward (closed) position. The valve tube ledge in turn holds the hammer in place so that it can be locked with the bolt on the backward pump stroke. If there is no gas in the gun, the valve tube will not be held in place, which means the hammer will not be held in place. This means the main spring will push the hammer back out of the reach of the bolt on the backwards pump stroke, and thus the gun cannot cock (although it will load balls into the bore).

The popping noise is caused by a small release of gas when the valve tube is opened on the backward stroke. The most obvious reason for this is because of low gas pressure not holding the valve closed. In fact, the “popping” noise is often a nice audible warning that you are exhausting your supply of gas.  The noise can also occur if there is too much tension on the main spring, and the spring pushes the hammer against the ledge with excessive strength – in other words, the velocity adjuster is turned up too far.  In this case the adjuster needs to be turned back down, and proper velocity should be sought with the use of a spring change.

*If a Nelson based gun has a very strong valve spring to hold the valve tube closed, it can be cocked whether it has gas or not.

Semi-automatic Nelsons
There are only a few semi-automatic guns based directly on the Nelson system. Each involves a pneumatic cocking system attached to the bolt, as opposed to a manual pump handle.  The Brass Eagle Rainmaker should probably be considered a “half Nelson” since the hammer/sear system is similar. It is a strange hybrid of a Nelson/Sheridan gun.

To my knowledge the only Nelson based semi-auto remaining in production is the ATS series of select-fire guns. They use internal parts very similar to those found in the Maverick/Trracer series. In addition, ATS guns have a lever mounted near the valve tube ledge which is tripped by the hammer and activates a 4-way valve. In a similar fashion to the Worr Games Autococker, the 4-way valve directs gas through a pneumatic ram. The ram pulls the bolt back for cocking. When the bolt connects with the hammer, the 4-way is tripped the opposite direction and the ram pushes the linked bolt/hammer forward. The unique thing about the ATS gun is the fact that the pneumatic operations are tripped by the hammer action, as opposed to trigger action. This completely eliminates the chance of “short stroking” the gun, which is a common user mistake on guns with trigger activated pneumatics.

For a short while Worr Games did produce a semi-auto conversion kit for Nelson based guns. It worked in an identical fashion to the company’s Autococker, with the pneumatic cocking system being activated by the trigger action. A lot more details on this kit and some of the problems that probably lead to it’s unfortunate demise can be seen on my Gun From Hell page.

As I was recently reminded, Lapco also made a pneumatic cocking version of their Nelson based pump guns. It was similar to the Worr Games kit in that the pneumatics were activated by the trigger action. There were reportedly only 150 of the guns sold.

There was also kit made by PMI for the Maverick/Trracer series called the Chameleon. It was a side mounted pneumatic cocking conversion which, like the others, automated the pumping process.
The design of Nelson guns is extremely reliable in nature, while offering great performance for a pump gun – but performance is a relative term. . The most impressive part about Nelson performance is, to me, the one that counts the most – consistency. When I think of consistency in velocity and consistency in accuracy, I think Nelson based pump guns. If you are looking for accuracy, Nelson based guns can certainly deliver. If you seek efficiency, they can be tuned for that too. And even if you want a good rate of fire, A nelson based gun can be surprisingly fast, when set up properly.  Back before the almost complete saturation of the semis, many teams still played competitively with pump guns. Most of the guns include an “auto trigger” which allows the user to simply hold the trigger and pump.  By tuning to use a light main spring, heavy weight hammer, and friction free internals, an experienced player could easily match the rate of fire of many of today’s entry level semi-auto’s.   Pump play has since gained the reputation of the “one shot” or “low volume” way to play. However, I think many of today’s players would be surprised to find out just how much paint was shot and just how fast it was shot during pump tourneys a decade a ago.