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by Bill Mills
Backspin – a catch phrase amongst some airsmiths and aftermarket bolt makers in the early 90s, has resurfaced with a new following in the 21st century. Paintgun manufacturers and players alike are faced with some certain limitations. Paintguns fire a round ball at a maximum of 300 feet per second for safe use in the sport. Over the years, many companies have made claims about range of their paintguns relating stories of rapid acceleration, deceleration, balls “still accelerating” across the chronograph, etcetera. The reality is that once a paintball leaves a paintgun it no longer has gas pressure pushing on it, so it no longer accelerates. All things being equal – wind resistance, aerodynamic effects, gravity, mass of the paintball, angle at which it is fired – a paintball traveling 300 feet per second will only travel as far as another ball traveling at the same speed.
That is of course with all things being equal. Gravity is not an easy parameter to change outside of a science fiction movie script. The ASTM is setting standards on what a paintball should weigh, so there isn’t really room for change in either of those factors, but what about how the paintball interacts with the air around it?
When a projectile travels through a fluid, such as air, pressure builds up in front of it, where it is pushing air forward and out of its way. As an object travels through a fluid it also leaves an empty space, or low-pressure region behind, and the fluid must flow to fill in this area. Different shapes do this differently. Sleek shapes tend to “slice” through a fluid, not causing much of a pressure boundary in front, and allowing the fluid to smoothly fill in behind them. As a result, sleek projectiles have little wind resistance. A sphere, like a paintball is a different story. The boundary layer of air pressure around a flying sphere ripples off chaotically into the void behind the ball. The combination of high pressure in front and low pressure behind slows, or decelerates a spherical projectile. This is commonly referred to as wind resistance. It and gravity are the two forces that prevent a paintball from shooting in a straight line forever until it hits something or was lost in space.
Gravity, unchangeable with current technology, accelerates a paintball toward the ground. If you hold a paintgun level and fire, you will notice the ball curve downward until it hits the ground. It falls at roughly the same rate as if it had been dropped. The only difference is that it is moving forward at the same time. In order to shoot a paintball further, we need to aim upward, shooting the ball higher than our target so that by the time it has traveled forward far enough; it has dropped to the right height to splatter our opponent.
Putting a backspin on the ball tackles the wind resistance that is slowing the ball, and takes a poke at gravity in the same shot. Physicists call it the Magnus effect. When a cylinder or ball rapidly spins in a fluid stream, the boundary layer of air pressure around it is disrupted, and it behaves differently. The pressure remains in the front, but the pattern of turbulence behind the ball is thrown downward. The low-pressure region moves to a different position relative to the ball’s center. How much the spin effects the flight of a ball depends on the direction and speed of the spin as well as the size of the ball and its surface texture. The rougher surface a ball has to “grip” the air, the stronger the Magnus Effect it creates when spun. Various balls used in sports, notably tennis balls, baseballs, and cricket balls are built with the aerodynamic effects of spin in mind. Smooth balls can also create a strong effect, but they need to spin at higher rates to do so.
One consequence of moving the low pressure out from the center behind the ball is that the pressure difference between the front and back is decreased, so the ball will not decelerate as quickly from drag. The other, more directly visibly effect is that the flight path of the ball will change. If the ball is spun clockwise, the low-pressure void will be shifted to the right. The same pressure differential that used to slow the ball down will now push the ball to the right, causing it to hook. Undoubtedly players who have broken a ball in the barrel have seen this happen unintentionally before.
If the spin is forward on the top, the same forces are at work, but now, they are fighting against gravity. The Magnus Effect tries to hook the ball upward while gravity is trying to hook it downward. If the two are properly balanced, they will cancel each other out and the ball will fly in a straight line. Too little spin and the ball will still curve toward the earth in an arced path. Too much spin and it will hook upward.
Over the years, assorted airsmiths have tried to harness backspin in an effort to shoot a paintball straighter and further. Backspin bolts, which jet gas out on the bottom of the ball, are available for many paintguns. Their effectiveness has often been debated. The main problem they face is developing the right amount of spin on a ball that still must travel through a barrel where it may be effected by contact with the barrel walls.
Enter the Galactic System Z Body from National Paintball Supply in South Carolina. The Z Body is an aftermarket receiver for the Air Gun Designs Automag. It is designed to create a backspin on the paintballs it shoots. The Z Body was shown at the 1999 Zap International Masters tournament trade show, where National’s founder Rick Fairbanks provided a sample unit to Paintball Magazine for review.
After a brief explanation of its operation, we packed it with our gear and brought it back to the workshop to prep for testing. At first glance, the body looks like little more than a slender block of metal with some polished screws and protrusions. This part alone does not look very gun-like. Once assembled as part of a full paintgun, the story changes, and it’s angled lines compliment the shape of the Automag grip rail to blend together in a sleek shape. The Z Body sight rail extends up over the Automag’s AIR valve toward the rear. Rather than the standard 3/8” rail typically seen in paintball, it is a wider Weaver style, capable of accepting larger firearm scopes and sights.
The Z Body is constructed mainly of milled aluminum, which is in contrast to the stainless steel of the Airgun Designs standard Automag body. We decided to install the Z Body on a Minimag. The standard Minimag body includes a powerfeed and is slightly longer than the basic Automag. Even though steel is more dense than aluminum, the weight difference between the AGD body and Z Body is not enough to be felt by hand because the Z Body has additional metal in a more square external shape and in its sight rail.
Internally, the Z Body features a steel insert to brace the bolt spring, and take the impact of the bolt as it strikes forward, something the aluminum alone would not do well. The power-feed is held in place by two cap screws and is dovetailed into the body. This construction method means that it can be removed for access to clean its nooks and crannies, unlike the stock welded powerfeed. Below the powerfeed is a threaded steel insert where the front grip frame attaches to the body. Again, this is a point where steel is stronger than aluminum, and the insert protects from the danger of stripped threads.
The barrel attachment is completely different than that of a normal Automag. The Z Body is threaded to accept Worr Games Products Autococker compatible barrels rather than Automag barrels. On the left side of the receiver, a chromed nut holds a nylon ball bearing style ball detent, which prevents double feeding of paintballs. Atop the receiver, between the breech and barrel threads is what makes the Z Body especially unique, the backspin nubbin.
The nubbin is made of a flexible rubber like material approximately a centimeter in length and backed by a bar of rigid plastic. It is capped with chrome finish cover and spring locked thumbscrew. When the screw is backed out, the face of the nubbin is flush with the interior of the mid-breech area. As it is adjusted in, the front of the nubbin is pressed down so that it enters the paintball’s path in a wedge shape.
The theory of operation behind the backspin nubbin is simple. It is meant to grab at the top of the paintball, forcing a backspin as the ball rolls free of it and into the barrel. What sets the Z Body solution apart from other backspin devices is that its effect is directly adjustable from outside the gun. A player can dial more or less spin while on the field.
After a look over, it was time for the install. In theory installing the components should be no more difficult than performing a general cleaning and inspection of the ‘Mag. We started by unscrewing the rear grip frame screw, and disconnecting the gas line, which ran from the Minimag’s vertical ASA connector to the AIR valve. On a basic Automag this hose would have run from a back-bottle adapter to the AIR regulator and simply flexed as needed, rather than having to be removed. With a counterclockwise twist, the AIR valve assembly unlocked and slid out the back of the Minimag body, bringing the bolt and bolt spring with it.
A hex wrench through the hole in the bottom of the trigger guard reached to the front grip frame screw to remove it, and the Minimag body slipped easily off of the grip rail. At this point we separated the grip frame from the rail. This was not a necessary step, but it was a good idea while the gun was apart to inspect and clean the sear and make sure it was properly lubricated. Reversing the disassembly procedure, we guided the sear pin into the grip frame and aligned it with the grip rail, stacked the Z Body on top, and installed the front grip frame screw. After a similar cleaning and very sparing application of Autolube oil to the power tube and bolt spring, we slid the AIR valve assembly into the rear of the Z Body, and prepared to re-connect the air hose.
This is where we ran into a problem. On the standard Minimag body, the powerfeed comes from the left side, and puts the hopper on the right. On Z Body, the power feed goes from right to left, so the main bulk of the powerfeed was right in the way of the stiff braided steel air hose. On the basic Automag configuration the air hose is going to a backbottle, or bottom line, or as some players opt straight to a compressed air system or remote. In those cases the air hose would not be an issue. It was only the stock Minimag hose that faced a problem. In the South Florida climate CO2 stays warm enough that chilling and liquid are not a problem for the ‘Mag as long as the tank is mounted vertically (which keeps the liquid CO2 down in the bottom of the tank). More shots out of a smaller tank makes a good argument to use CO2 over compressed air when possible, so we opted to stick with the vertical tank arrangement.
The solution to our problem was fast and simple, we pulled a micro-line off of another paintgun and installed it in place of the braided steel hose. The micro-line was long enough to easily loop over the powerfeed. We slid the AIR valve assembly into the Z Body, twisted it to lock it in, then installed the rear grip frame screw to secure it, and we were done. Even with changing the gas hoses and wrapping their connectors in PTFE tape, the entire process took barely over 5 minutes. The Installation of the Z Body is fast and simple, requiring no unusual tools (just the hex wrench that comes with the ‘Mag) and requires no more knowledge or experience than the average Automag owner has from cleaning their ‘gun..
National Paintball Supply’s staff recommended the use of a large bore barrel to minimize the chance that contact with the bore would dampen the paintball’s spin. With the assembled Galactic System Z Mag in hand, we packed up a barrel bag, loader, CO2 tank, a few bags of assorted brands of paintballs, and headed out to Mike’s Paintball in West Palm Beach, Florida.
After getting a fresh gas fill, we went to the chronograph and dialed in to 280 feet per second with a Smart Parts All American 16 inch barrel and the standard field RP Scherer Premium Paintballs. In past experience, on other paintguns this barrel has been very tolerant of assorted grades of paint, so we figured it to be a good choice for having a reasonable sized bore. After setting the velocity we began adjusting the backspin nubbin. On the short distance of the chronograph range we didn’t see a difference but we were close enough to the targets that there was not a lot of ball-drop anyway. We did notice that the velocity dropped anywhere from five to fifteen feet per second with the backspin nubbin extended to various degrees. That is an important factor for field owners to notice. Z Body Automags must be chronographed with the backspin screw backed out all the way. In this manner adjustment on the field means the velocity may only be lowered, and not raised potentially above the field limit.
We then stepped on to one of the speedball fields and braced the ‘Mag against a wire spool to keep the barrel steady while firing. The flat surface on the edge of the spool kept the barrel nearly level and parallel to the ground, but easy to pivot left or right. We pointed it toward a low horizontal pipe bunker, located just at the distance where the balls hit the ground. This way a flatter trajectory would be clearly visible, the balls would start hitting or flying over the top of the pipe if the Z Body was doing its job. We started shooting and then began twisting the backspin nubbin screw in a little more with each shot.
Unfortunately we did not see the results we were hoping for. The only noticeable difference came with the adjuster screw all the way in. About half of the paintballs flew high enough to clear the pipe and landed a good ten to fifteen feet further downrange, while the other half hit the ground in front of the bunker. While it was an increase in distance it was hardly what one would describe as phenomenal. Our next step was to test with some different types of paint. Some Pro Ball and Diablo went though the gun with similar results. No change there, so the next area we decided to explore was barrels. We began fitting the paintballs into our selection of barrels and found them rather snug in most. The Brass Eagle two-stage barrel was the least snug, but it could hardly be described as a loose fit.
Using that barrel, and switching to a freshly opened case of RP Scherer Marbalizer we found a range difference of fifteen to twenty feet. It was better, but not what we were after. The field owner offered his brand new DYE Boomstick, which he had purchased mainly because of its large bore size. With a test fitting, we found it to definitely the loosest paint to barrel fit of the group, though it was still tight enough that the ball would not drop straight through with gravity alone. Back through the chronograph and out to the field we went. We took the first few shots, lining up with the same pipe and began dialing the adjusting screw. When it was nearly three-quarters of the way in, one ball zipped not only over the pipe, but also past the next bunker and into the brush pile that bordered the field. The next ball however did not. With further adjustment we found roughly half of the paintballs fired picked up an excellent spin and flew straight, far and true. The other half flew like normal paintballs.
Screwing the nubbin all the way in, we ended up breaking a ball in the breech. The first 5 or 6 shots after the ball break hooked wildly, but surprisingly after that the ratio of flat to arced shots went up from half to roughly three out of five. Performance got better with a slightly fouled barrel and breech.
With this odd bit of information in hand, we decided it was time to put the Z Body to the test where it really counted, on the field in a game. Local player Todd Harris went into a walk-on game with it, firing from a mid-field bunker. After the game he reported that when it shot straight it shot very straight and accurate. The problem was that almost half of the shots didn’t pick up the spin, and it wasn’t possible to know which ones would arc and which would be straight to know where to aim. As a teenaged player on a tight budget every paintball is important, so he felt it wasn’t right for him.
Another young player, Mackey Voss tried his hand at shooting the Z Body ‘Mag and felt differently about it. He commented on the positive balance and lightweight as compared to his Automag. Even though the Galactic System receiver weight is close to that of the Minimag, the Autococker threaded barrels it uses have thinner walls and do not have an extended breech area, meaning they contribute less overall weight to the gun. Mackey liked the feel and the styling and saw the potential advantage in the straight shots the backspin provided when it kicked in.
While our field-testing did not show the Galactic System Z Body to be perfect, it would not be fair to say that it failed either. When it did spin the ball, it was very effective. Getting that spin one hundred percent of the time will definitely make a difference on the field. Far bunkers become easier to hit, and shots in the woods, where branches block lobbed paintballs become possible. What was evident from our field trials was that the fit of the ball to the barrel is critical to the success of this product. As we moved to larger barrels and more consistently round, smaller paint, the straight shots brought about by the Magnus Effect became more pronounced and regular. Making the most out of this receiver will involve a very careful barrel selection, and some time spent in finding the optimum velocity and nubbin depth adjustments. For the shots where the spin did not kick in, the performance provided was still at the level of a normal Automag, so no accuracy or range was lost by using the Galactic System Z Body. The Body itself gives the Automag a distinctively different look, and since it is aluminum, unlike the AGD Automag body it can be custom anodized. The ability to dial in the level of spin on the field is a first for backspin products and will be key for players optimizing its performance. As with any product the player will need to decide if the cost matches the benefits they will get. Building a custom Automag around this body could start at the component level, buying a Retro Valve kit, trigger frame and rail followed by a good large bore barrel.
The Galactic System Z Body started shipping
in fall of 1999 and is currently available at dealers nationwide. Dealers
looking to stock the body can purchase it from National Paintball Supply,
in South Carolina (www.nationalpb.com).
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