Several years ago I went to play paintball with my four boys for the first time, with a church group. I recall being mostly concerned about how to get back to the sofa and watch the Dallas Cowboys game to see if they would become "World Champions" for the first time.
I thought we might play for an hour, get done and come home. I had no idea how fun it would be, and that we would miss that Cowboy game--and many more to come--as we chose to play paintball and gave up our Cowboys tickets.
After waiting around two Saturdays back to back, waiting for my boys to play, I saw this "Old-Guy" (45 or 46) all decked out in camo with a marker. "Do you like this game?" I asked. "I love it," he said.
To myself I said, "What can it hurt?" Little did I know where this would eventually lead me, and I went for it. My boys were elated, and mom became a "Paintball Widow".
In my third week of playing paintball, during a rec ball game in the woods, I took a shot at a player with my trusty rental marker. The paintballs kept curving so much that I simply could not hit anything from more than 20 yards away. Now, I own a Johnson .22 target rifle that can hit within 1/4-inch at 100 yards, and here I was trying to thwack an opponent at 25 yards and found it impossible. This kid bunkered me.
The embarrassment overwhelmed me. I asked my son, "Why doesn't my marker shoot straight? Why couldn't I hit him?" The answer he gave me, and a field referee offered the same advice, was to buy expensive markers using compressed air/nitrogen.
I knew there had to more to accuracy than money.
My boys and I kept playing. Some liked speedball, some became focused on woodsball and scenario games. Markers in our house eventually included Tippmanns with Flatlines, Spyders, Angels, New Matrix Extremes, Phantoms, Automags, and Timmmys. We tried most all markers, with varying results. Each son believed his favorite shot the best.
Without an accurate test method, we had no scientific answer. The boys claimed boldly, "I shot out five today and that's two more than you so mine's more accurate," yet we know that wasn't proof of accuracy.
After spending way more energy and resources on paintball equipment that any sane person should, it became obvious that "more expensive" did not prove most accurate. We tried high-pressure, low pressure, short barrels, long barrels, cheap paint, expensive paint, and didn't answer the accuracy question.
Our quest continued. We tried on-line chats and literary searches, only to find as many opinions as questions. We concluded "to understand the dynamics of paintball" would take far more serious research and data collection.
Asking players and store owners produced very little data. We found reports supposedly comparing barrels, patterns of hits, and hits on a target at a specific distance. We did not find a "standard model" for barrel and marker testing. My comparison is engineering and construction standards, where there are standardized tests and methods for concrete, steel, chemicals, acidity, wastewater, etc. Nothing comparable seemed to exist in paintball.
TRADE SECRETS? We found some excellent work. For example, Tom Kaye of Airgun Designs has done excellent research. However, paintball does not have a central research facility using standard methods. Those who test independently seem to maintain their information confidentially. This is not unusual. One would expect manufacturers to keep proprietary information confidential, since they expend considerable funds to gather that data. Why give away the trade secret recipe for Heinz 57, Coca-Cola, KFC Chicken, etc?
We simply wanted to know about the variables affecting paintball trajectories, without bias.
Was it the marker, was it the barrel, the paint, the type of air? Just what was the answer? Because we didn't find a standard test method for accuracy testing, we developed our own. We had come full circle back to where we had started. We were convinced we had to collect the data ourselves.
This article describes our methods and conclusions. We spend quite a lot of time and money in testing. Still, we don't have a multi-million dollar laboratory and comparable budget to guarantee absolute certainty in results or perfection in test methods. We expect disagreement with our methods and conclusions, and welcome other studies and a healthy dialog within paintball, in search of more information about accuracy and performance.
The purpose for our testing was to find some simple basic answers to help us make decisions on what gear to use, for our paintball fun. This article does not compare marker or barrel results, and we are not speculating on what equipment is best. The hope is for players to read and learn something from all the work we did.
As previously mentioned, the purpose of the study was to learn what affects the paintball trajectory, and that remains our focus.
Author Robert Judson is the founder of Hammerhead Marketing Group, LLC., which manufacturers barrels for paintball markers.--editor TEST METHODS We needed a reliable and repeatable test method. We wanted to define and isolate the dependent and independent variables affecting paintball trajectory. We knew we needed to minimize the variables such as paintball size and shape, porting, barrel lengths, etc., and find out the effect of marker type on the results. We chose to analyze the data using statistical methods and compare the results using agreed to formulas. We assumed the simplest evaluation criteria would be mean diameter hits from a centroid with a standard deviation calculation. We also knew we needed to run an accuracy test, have immediate results, and make changes quickly to the system to optimize our study efficiency. We used Excel spreadsheets for data collection, and standard equations for determining standard deviation.
Initially, we shot paintballs into styrene insulation panels. The paintballs would penetrate the panels, and we could then measure the distance from the centroid of the target for accuracy using a clear plexiglass sheet with concentric circles with increasing radius of six inch increments. However, this process proved to be very time consuming and expensive.
We modified our testing procedure to include the use of a digitized electronics board similar to those used in the drafting business. We overlaid the board with a thin metal sheet, and covered the sheet with the plexiglass. Ball impact would be indicated on the computer screen when acted upon by a ball. The data would then be downloaded showing all strikes on a PLC (programmable logic controllers). Each time the ball hit the target, it recorded where the ball hit, and entered that on the spreadsheet. The paintball did not have to break to get a reading.
Markers were bench-mounted in a vise, and a short wooden fulcrum activated the trigger. Each test had to consist of at least 40 shots for the test to be statistically accurate. The Excel program was used to calculate standard deviation, and average diameter, and to graph probability curves.
Our mounting bench was approximately 4.5 feet high. All barrels used were of a bore size of 0.690 inches. Bores were measured using stainless bore plugs accurate to within 1/10,000 of an inch, and using a high-end digital caliper. We measured the barrel diameters at the breech side of the barrel.
VELOCITY TESTS Velocity was measured using a bench mounted chronograph with a digital readout. We set the screen at three to four inches from the muzzle. We also had a handheld chronograph to verify chronograph accuracy. The two chronographs read within three to five per cent of each other. We did not know what the repeatability or accuracy of these devices are.
The bench mounted chrono was on the bench just in front of the barrel on the table, and the hand-chrono was held above the end of the muzzle parallel with the bench chrono.
All tests were calibrated to approximately 280 ft/sec. All testing was done at 120 feet.
We set up an Interceptor brand tripod mounted radar chronograph to measure paintball velocity just before it hit the target. We set up the tripod perpendicular to the flight bath at 10, 20, 30, and 40 meters. We did not get a reading every time and had to repeat the process at each distance to get an "average velocity at those distances".
We found that a change of five ft/sec in velocity at the muzzle (as the paintball left the marker) could vary the accuracy at the target by 2.5 to three inches down.
We found that some misshapen paintballs provided a speed variance of 10 to 15 ft/sec, and affected the accuracy by as much as six to nine inches from the target center. Our definition of misshapen: not round; oblong, football shaped, or with seams that protruded at the two interfaces of the two clam shells; any defect that resulted in the paintball being not round.
We also attempted to size the paint with the barrels (see below).
We found that the velocity of the paintballs slowed down considerably (leaving the muzzle at 280 ft/sec but slowing to approximately 100 ft/sec at target impact). This velocity drop can be mathematically calculated in the abstract.
Time of travel to hit the target was less than a second. The paintballs slowed due to drag on the ball due to air resistance, as well as to gravity acting on the paintballs (see below).
QUALITY PAINT Paintball quality was a serious concern. Paintballs vary in shape, seam size, seam location, diameter, and weight, all factors which can affect velocity and trajectory. Dimpled paintballs or paintballs with air bubbles (voids) in them were not used in testing. We chose paintballs of the same weight and similar centers of gravity.
We measured diameters and weights. The sample weight was 48 grains, and we measured individual paintballs. We rolled the paintballs to check for empty pockets. The test was pretty simple. We simply put them on a V-shaped incline, and rolled them one at a time to observe if the ball would roll straight or wobbly. We also would spin them on a smooth stainless plate, and the
paint with voids in them would wobble. We used the stainless ball gauges that we have made and measure the balls in one plane and then in a plane 180 degrees from the first plane. By using the different sizes with the ball gauges, we could estimate the out of roundness.
We chilled paintballs that rolled unevenly or ones where their mass (weight) was considerably less than the sample and dissected them because we wanted to see how much void was present in those paintballs.
We hypothesized that paintballs with voids would have a center of gravity that was off-center, and could lead to ball trajectory variances.
We noted that paintball seams vary, and paintballs can have various size and shape dimples. Seams and dimples can affect the aerodynamics of the paintball in flight, and could cause spin or erratic flight paths. Also, paintballs can be out of round (be oblong football- or egg-shaped), which will affect the paintball's flight because pressure differentials on the back of the ball could cause the paintball to move in erratic and non-liner paths. We did not want these variables to affect our conclusions.
We tested for velocity differences caused by variances in paint size and weight. We found slight variations in paintball ball size, mass, and shape could vary the ball velocity by as much as 15 ft/sec, with a corresponding effect on accuracy of approximately eight to 10 inches off the center of the target.
While we could spend weeks analyzing paintballs, after a reasonable amount of testing and evaluation, we chose to conduct all of our tests using PMI Marballizer Green/Black, with a consistent weight of 49.0 grains.
BALL SIZERS To minimize variation due to ball size, we made stainless ball sizers. These are similar to sizers used in sizing sand and gravel. We made stainless plates with holes "lasered" in them that varied from 0.679 to 0.693. We sized the paint by placing the paintballs through the larger screens, which passed subsequently through the smaller screens, until the paint would not fall to the next screen. We then used paintballs measuring between 0.688 and 0.690 inch in diameter.
Our test shooting was done at temperatures between 72 and 78 degrees Fahrenheit. We did not observe swelled or soft-skin paintballs. (Heat leads to softened paintball skins; humidity leads to swollen paintballs.) We also did not observe paintballs splitting open inside the barrel (cold can make paintball skins brittle).
The accuracy tests we ran in cold weather of 30 to 40 degrees F. were not repeatable in the laboratory under our test temperatures, which leads to the conclusion that temperature could have an effect on ball dynamics inside the barrel. At colder temperatures, the balls may become harder, and react with the barrel differently than balls at higher temperature, and the density of air is greater with cold air, affecting the ball drag.
CO2 ENTHALPY We found that CO2 (carbon dioxide) absorbs the heat from the barrel during rapid shooting. We observed a drop in barrel temperature of 25 to 30 degrees Fahrenheit and a velocity drop of as much as 60 ft/sec. Basic gas laws including PV=nRT and the subsequent equation of P1/T1=P2/T2 calculate that the pressure change from a five to 10 degree change in temperature can result in a velocity change of 10 to 15 ft/sec. However, we saw much larger swings in velocity than the ideal gas laws predicted. Evaluating CO2 enthalpy (a measure of its stored energy) with the ideal gas calculation could in fact show much greater velocity differences. Because CO2 is not an ideal gas (a gas that follows the ideal gas laws; see http://scienceworld.wolfram.com/physics/IdealGasLaw.html), we believe the ideal gas comparison does not apply. At the 120 foot distance, the 60 ft/sec velocity difference was producing a miss of as much as three to four feet, sometimes more. Because of this, we decided to use compressed air for our testing, with the source a 150 pound air tank.
VIDEO For video, we used a digital Sony high-speed video camera with a strobe. The camera has night vision capability with infrared. This let us evaluate the effects of ball rotation. We used the
camera to capture the effects of air transfer at the muzzle as the paintball approached and passed through the porting to the muzzle exit. We also used the video to observe the paintball rotation when shot through rifled barrels with varying rifling.
CLIMATE CONTROL We ran most of our tests inside an air-conditioned building, where we could hold a temperature of approximately 78 degrees with approximately 60 per cent Relative Humidity (RH). Since the density of air changes with changes in temperature and dew point, it was important to attempt to have the same atmospheric conditions for each test. Air density is affected by altitude and is a component of the air drag equation, but for our tests this was not a factor since we did all testing at the same location.
VARIABLES 1 What affected trajectory and accuracy most? We found that there were accuracy differences in marker types. We found accuracy to be equally affected by other factors including the barrel types, paint quality, barrel to paintball matches, etc. The entry level markers could not shoot nearly as fast (balls per second) as the higher end electronic markers; however, the entry level markers shot well as long as the barrel and paintballs were good and the paint was sized to the barrel.
AIR SOURCE 2 Once you pull the trigger on the marker, pressure is exerted onto the paintball by the expanding gas or air that you are using as your power source. This pressure acts upon the backside of the ball in the form of a pressure wave and accelerates the ball. This pressure wave has a definite shape for each type of marker, and can affect how the ball is accelerated in five to six thousands of a second. If you study Boyles law of partial pressures, you can calculate how far inside a barrel the gas will expand before the pressures on both side of the ball are theoretically equal. Where the pressures on both sides of the ball are equal marks the effective useful length of the barrel. Depending upon the barrel diameter and marker type, the effective length of a barrel is somewhere between six and eight inches. Porting on the breech side of the barrel that is less than the effective barrel length reduces air efficiency.
Air efficiency was measured by taking an air tank, and filling it to 1000 psi (pounds per square inch). We counted the number of shots we could get out of the tank before the velocity began to drop off by 30ft/sec on a consistent basis.
Barrels longer than the effective length can at some point have a negative effect on acceleration though practically speaking a longer barrel can help some players with aim, be used to push an inflatable bunker out of the way a bit, and can add additional porting or rifling.
BARREL LENGTH 3 We found barrels of 8.5 inches provided more consistent velocities than barrels of 14 or 16 inches, and accuracy at shorter distances (under 120 feet) was excellent with the 8.5 inch barrels. However, for the 120 foot tests, with the longer barrels we found that a change in velocity of five ft/sec at the muzzle would change the distance from the target centroid by approximately two and a half to three inches. We observed these differences but can only speculate that perhaps there is greater turbulence at the muzzle with shorter barrels or more ball orientation with the longer barrels. We found the shorter barrels were approximately eight per cent more air efficient than the 14 inch barrels.
SIZER-INSERTS 4 We made a series of back-end inserts to put into barrels so that we could test for the effect of this ball-sizer insert on accuracy. We made a series of sizer-inserts varying from 1/4 to six inches long. We varied their bore sizes and then focused on sizers between 0.688 and 0.690 inches straight through, to match the paintballs used in testing. We found that the shorter sizer-inserts that held three to four paintball diameters improved accuracy over those sizers that were four to five inches long. The shorter ones also improved air efficiency by about eight per cent.
The improved accuracy was more noticeable with paint that had larger seams using the short sizer. Some paint has seams that almost cannot be seen and do not provide a lip when you run your fingernail over the seam. We considered a larger seam to be one that protrudes and can be felt with your fingernail and can be measured with a caliper. We believe the larger seams have the tendency to grab a longer length sizer and spin out of control much more easily than a shorter sizer.
We believe the paintball begins to spin inside the sizer. The test results were empirical, as we found more accuracy at 40 meters with the shorter sizers than with the longer sizers. We attempted to use high speed video to see what was happening to the paintballs but found that extremely difficult, perhaps because the camera shutter speed was not fast enough to capture every ball rotation. With a $10,000 camera better images for analysis could have been obtained.
During the sizing process, we believe friction results causes the gas losses. The shorter sizer had a more pronounced improvement in air efficiency with the seamed paint. The smoother paints with less dimples provided improved accuracy and less differences in velocities with both lengths of ball sizers. The shorter sizer shot more accurately than the longer sizer in general.
We did not study the effects of paint that did not match the bore. We believe that the lesser the match, the greater the velocity differences from shot to shot; slop between the barrel and the paintball can cause the ball to spin out of control and allow air to escape around the ball, causing changes in velocities.
PORTING 5 We found that porting and counter-boring significantly affected accuracy. Properly ported barrels and counter-bored (enlarged internal diameter near the exit end) barrels performed better than conventional barrels consisting of perpendicular, small opening porting. Our studies found pressure differences in the barrel and the atmosphere exist and vary with the porting design and shape. We noted that the angle of the porting, and the cross sectional area of the porting, affected the accuracy.
Slow motion video showed that the air would exit and re-enter reverse porting as the ball would pass by the porting. We believe the ball is most likely pushing air out of the barrel as it moves down the length of the barrel, and is drawing air in behind it as the ball passes the reverse porting. As air enters in the porting behind the ball, the pressure adjacent to the ball comes closer to atmospheric, allowing the ball to regenerate to its original shape before the ball exits the barrel if the angle and the size of the porting is adequate.
We believe that the pressure differential between the barrel exit (the end of the barrel; the muzzle) and the atmosphere at the ball exit (where the ball exits the barrel)can affect accuracy and the porting affects this differential. We believe the turbulence at the muzzle exit significantly affects the accuracy as a result of porting. Our videos showed the effects of porting on gas escape. We did not compare gas porting effects of short to long barrels to determine if turbulence was a factor in the long barrel-short barrel accuracy discussion above.
HPA-CO2 6 After completing our theoretical gas evaluation of CO2 versus compressed air (see above), we decided to use compressed air as a power source in testing. We noted that compressed air systems shot more consistent velocities than CO2, especially with rapid shooting. During rapid shooting with CO2, the barrel temperature dropped by 10 to 30 degrees or better. We found velocity reductions of 25 to 30 per cent when the barrel temperature dropped from 80 degrees to 50 degrees; for example, a velocity drop from 280 ft/sec to 223 ft/sec. At the 120 foot test distance, with the 30 degree change in barrel temperature, the target centroid was missed by as much as 3.5 to four feet. The temperature differential was due to the temperature changes from the CO2 gas expansion. We found that with a five or six per cent drop in temperature of the CO2 tank, the pressure would drop by almost the same percentage. This was most likely due to the endothermic process of carbon dioxide expanding and absorbing heat as it expands, which cools the barrel. We found that compressed air produced markedly more consistent velocity and accuracy results.
FLIGHT PATH 7 Once the paintball leaves the barrel, multiple forces act on the ball. Besides the variables that can affect ball trajectory, external forces such as gravity and drag affect the paintball (drag coefficient and Reynolds number and ball velocity).
Other forces that act on the paintball include the Magnus effect, or the aerodynamic effects of spinning. As the paintball passes through the air, gravity pulls the ball downward. Further, the ball is pushing against air, which adds drag. As the air moves over the ball, turbulence and a wake are generated behind the ball. This wake can change location on the back of the ball, causing the paintball to fly erratically like a baseball player's knuckleball.
ROTATION 8 A paintball has a "hard" shell and a viscous liquid inside. The liquid and the shell do not necessarily spin at the same rates. Difference in rotation rates between the shell and the liquid are affected by the viscosity of the liquid and temperature. We did not go into depth regarding the rotation and spinning of paintballs, nor into what happens inside a barrel from the ball drop to the muzzle. We did rotate paintballs at varying speeds from 1,000 RPM to approximately 10,000 RPM. We rifled barrels with varying degrees of twist and grooves to determine those effects on ball trajectory.
The theory behind our study was based on the assumption that we had a relatively smooth ball that we could rotate. With ball rotation, a gyroscopic effect could occur, improving the paintball flight. The paintball flight path could improve due to the rotation effects on the drag behind the ball which could be evened out. Another theory was that with the rotation of the ball in one direction, even if that rotation is slight, the ball could have a less of a tendency to change its rotation or direction after being placed in motion.
We found the rifling needed to be done with the precisely correct depth and width of grooves and lands. We also found the rate of rotation and the velocity was critical to rifling performance. We finalized the rate of ball rotation with the targeted ball velocity of 280 ft/sec. Once the ball rotation exceeded a specific RPM, the ball tended to wobble at the longer distances. At the higher rates of rotation, we believe the shell may have been spinning much faster than the paint, and the paint could have absorbed the energy of the spinning shell. Rifling with the correct porting and ball sizer provided approximately 20 feet of increased distance over most of the non-rifled barrels.
To test for distance we measured the average distance for 100 shots by measuring where each paintball hit the ground. Muzzle height was between four and five feet for this test.
We can only speculate that by rotating the ball, the drag behind the ball was made more uniform, and the overall drag on the ball was reduced. Reduced drag could have possibly contributed to the increase in ball travel. The ball rotation tests showed improved accuracy with a "specific rate" of rotation in conjunction with proper porting and ball sizing. Above or below the "specific rate" of rotation, ball accuracy improvement was not observed.
OUR CONCLUSIONS Ball quality. Ball quality is paramount to accuracy. Buy good paint if you want to shoot straight. Even the best marker with the best barrel is going to have a tough time shooting dimpled, swelled, or out of round paint.
Barrel type. The barrel is critical to marker accuracy and second only to ball quality. Critical aspects in our opinion are internal barrel finish, porting, and rifling. Hardness of the barrel material could affect the longevity of the internal finish, but was found to have little effect on accuracy. (Do not confuse barrel smoothness with Grinnel Hardness of the barrel material.)
Porting. The barrel porting is very important. Holes in the barrel for porting must be sufficiently sized and angled to allow ball regeneration and atmospheric balance prior to the ball exiting the barrel. Porting of the barrel closer than five or six inches to the breech wastes air. We found counter-boring of the barrel with reverse porting provided optimal results.
High-end markers. High end markers shoot well when you match the barrel and paint. (Electronic markers can typically shoot faster than the lower end mechanical markers.)
Entry level markers. Entry level markers shoot well when you match the barrel and the paint, but several could not achieve the consistent accuracy obtained with the high end markers.
CO2 or compressed air. Gas laws predict compressed air to be a more reliable gas for accuracy than the CO2. Our tests showed this to be true.
Rifling effects. The rifled barrel performed extremely well with lower rotational speeds in conjunction with proper porting and ball sizing. We found we could get approximately 20 more feet of ball travel from the rifled barrels with reverse porting and with counter boring.
Had we had more time and funding, we could have delved into the Magnus effect of spinning paintballs, discussed Reynolds Numbers, coefficients of drags, dimpled vs. smooth paintballs, Newton's First and Second Laws of Motion, boundary layers, laminar flow, etc. However, that was not the purpose of this paper. We wanted some simple basic answers that would assist us in making decisions regarding markers, tanks, and barrels, so we could have more fun. We believe we have them.
We do not want to speculate as to which barrel, marker, or barrel system is best. We are not going to compare the marker or barrel results (to avoid manufacturer bashing). That was not the purpose of this study. We do not want to take anything away from the many good manufactures of markers or barrels.
We do hope the average player will read this report, and learn something from our efforts.
We do not have the energy, time, or incentive to fight over what we have printed. As previously mentioned, the purpose of the study was to learn what affects the paintball trajectory, and that remains our focus.
A major learning for us was the recognition of the lack of a standard testing method for comparing analytical tests for the Paintball industry. It would be beneficial to players for the industry to standardize testing methods for markers, paint tanks, and barrels.
For future tests, possibilities include finding ways to minimize paintball variables for much cleaner methods of testing; examining temperature effects on accuracy; further studies into barrel length and the variables that affect it; trajectory of flight using CO2 and HPA; wind tunnel experiments; measuring drag on paintballs in flight; and a system for quality labeling of paintballs.
Markers used in testing included the Angel LCD Fly, Works 'Cocker, PMI Piranha, Tippmann 98, Kingman Spyder, New York Matrix Extreme, Bob Long Intimidator. Barrels used in testing included OTP (with inserts), Angel (stock), Titanium Boomstick, Smart Parts Freak Kit, J&J Performance Stainless, Lapco Auto Spirit, Hammerhead Pro Series (rifled), Hammerhead Battlestikxx (rifled and counter-bored)
Robert Judson is a Registered Professional Engineer, with a BS and Masters in Engineering. In his early days as an Engineering Manager with PepsiCo, he conducted R&D testing and wrote technical papers. He has written articles for the AMI, Baking and Snack, Engineering News, etc. He is presently Executive VP with CMT, Inc., in Dallas, Texas, responsible for Food Plant Design and Construction, and president of Hammerhead Marketing Group LLC. Robert and his wife Donna have four sons and a daughter; his son Paul is VP of Hammerhead Marketing Group, LLC. Colin Dennehy is Robert's business partner and owns Official Paintball of Texas. Much of the data collected during this study led to the development of the Hammerhead Rifled Pro-Series, and the rifled, gun-drilled, and counter-bored Battlestikxx Recon barrel. Contact: sales@hammerheadpaintball.com; phone 800.908.9060. 
