Paintballs.  Uhg.  These evil little things.  The laws of physics never intended for an object with a liquid core and a semi rigid shell to be lobbed through the air with any accuracy.  To make matters worse, there are theoretical and practical limits to the manufacturing tolerances of paintballs which are far greater than the tolerances of even the worst grades of traditional projectiles.  Top this off with the fact that paintballs must be biodegradable means that outside factors can easily affect their performance.  Figure, for tolerance reasons, that every batch of paintballs can vary slightly in any number of their key factors.  For this reason, the paintballs that worked spectacularly in brand 'X' barrel yesterday may provide only modest performance today.  Lets look at each factor.

    Diameter

        Considered number one factor almost universally.  Is it?  Who knows?  I've made some observations about paintball diameter that don't seem to add up.  Diameter is readily affected by outside factors.  Evidence seems to indicate that having the paintball diameter closely matched to the barrel diameter results in the best accuracy.  However, this does not seem to result in the maximum range.  Observable results from my own experiments seem to indicate that oversized barrels give longer range but poor accuracy.

    Weight

        The number 2 consideration.  A heavier ball, will in theory, travel farther.  I have seen people argue against this saying that the lighter ball will have a flatter trajectory.  I don't buy into this.  A lighter projectile is known to loose velocity more rapidly and thus develop a more pronounced drop.

    Shell Uniformity

        Another major factor is the overall 'roundness' of the shell.  The rounder the better.  Extremely round shells are typically only found in costlier grades of paint.  Age, mishandling, and poor storage techniques can quickly deform even the best paint.

    Shell Elasticity

        This affects how much the ball is deformed upon firing.  This deformation is theoretical but it does seem to pass the reality check.  More rigid brands will deform less but be subject to excessive breakage in the gun.  More elastic brands break less in the barrel, but less on the target as well.

    Shell Thickness

        A linking factor to both of the above.  Thick shells are typically more rigid, but then again a thick, rigid shell is less likely to break in the gun OR on the target.  A thick shelled and uniformly round ball would offer superior stability in flight but what good does this do you if the ball doesn't break when it hits the target?  For best results a balance between elasticity and thickness must be achieved.   While closely linked to both of the above, shell thickness bears consideration.  Shell thickness and elasticity affect the manner in which the paintball interacts with the barrel's surface as well.   My personal observation here.  The 'roundest' paint I have ever used was the paint produced by Nelson many years ago.  Their paint was big, round, and rock hard.  If flew straight but bounced with maddening frequency, approaching 20% of the time.  The shell was simply too thick and rigid.  It fired spectacularly out of my old Bushmaster.  It was difficult to get this paint to fire out of a Splatmaster.  It was simply too big for the barrel.  When it did fire it almost never broke on impact.   Nelson no longer makes paint to the same specs, opting for a thinner shell and a smaller diameter.  The old Nelson paint had characteristic 'ringworm' defects in the shell.  Little wavy lines of lighter color that were places where the shell itself was doubly thick.  Improvements in manufacture have corrected this.  

    Fill Viscosity

        The thickness of the paint fill also affects performance.  More liquid fills will be more prone to being affected by fluid dynamics.  Thicker fills will be less susceptible.  But excessively thick fills are less likely to leave a mark on a glancing hit, which is the most common type of paintball hit.  Viscosity irregularities caused by separation within an individual paintball are highly detrimental to accuracy.  Everyone knows this but still I hear people claim that the liquid fill has no effect on the balls flight path.  I'm not sure how both statements can be fact.   I can state, without hesitation, that I missed first place in a tournament due to bad paint.   Because thicker fills would result in more consistent performance, it would be quite interesting to see a paste filled paintball.  Something with a consistency closer to Jell-O that would retain much of its shape even with the shell cut away but still spread under impact.  Doubtless, it would be more difficult and expensive to make such a ball.

    Color

        I'm only putting this down for the sake of thoroughness.  I have heard it said time and time again that one color outperforms another within the same brand.  I don't buy this at all.  There are only three exceptions I can find that might make it true.  One, that one color is more visible in flight than another and therefore easier to make aiming corrections with.  Two, there is a slim possibility that the pigments used in one color offer an advantage in weight over another color.  Three, it's a psychological effect.  The user thinks color 'x' outperforms other colors, so his or her perception is altered.  Perception could well be a key factor in all areas.

        Oh boy.  The first paintguns were strictly smoothbore.  I don't know who came up with the first rifled barrel.  Armson perhaps.  I believe Smart Parts lead with the first ported.  Don't quote me on either of those.  I really don't know for certain.  How much of these improvements are gimmicks?  How much of them are fact?  It's a reasonably safe assumption that the barrel that comes on a factory paintgun is perhaps not the best choice.  Due to the manufacturing expenses incurred while producing a production paintgun tolerances of a particular stock barrel are more likely to vary than the tolerances of an $$$ after market barrel.  This is not a law though.  Brand 'X' paintgun maker may have state of the art facilities capable of mass producing barrels to extremely tight tolerances.  While Brand 'Y' after market barrels may be built in someone's garage using crude hand tools.  Quality might vary from one example to the next.  Paintgun barrels at least do not vary from day to day like paintballs.  A barrel that worked fine yesterday will work fine today, assuming that the paintballs themselves have precisely the same qualities.  This is true with only a slim chance of interference from some outside factors.

    Internal Diameter

        Is it the number one factor, or number two?  There is little doubt that matching the internal diameter of the barrel to the outside diameter of the paintball results in superior performance at short ranges.  Some evidence, witnessed by myself and by persons unknown to me seems to indicate that matching paint and barrel diameters does not improve and may even degrade range and possibly even accuracy at range.  It would appear, for the time being, that you can have either a barrel that is accurate or a barrel that is far reaching, but not both.

    Material Composition

        The contender for number one spot.  Brass, Stainless Steel, Aluminum, and new Composites- Ceramic and Carbon Fiber.  Each has different properties.  Foremost being the coefficient of friction that their surface provides.  There seems to be a trade off in weight versus friction with barrels.  Stainless Steel is the heaviest, but also the slickest.  With brass in the middle and aluminum bringing up the rear.  Aluminum barrels still retain a mirror finish and are quite light.  The new Carbon Fiber barrels leave me under-whelmed.  I tried one on a 'Cocker and was impressed only with it's astonishingly light weight.  Accuracy was piss poor.  Among the worst I have ever shot.  I have not had experience with a Ceramic barrel yet.  Other important factors in material composition are dimensional retention, thermal stability, and self cleaning.

    Internal Finish

        Strongly related to composition but not the same.  The degree of internal finishing varies from maker to maker.  It is a key factor in the cost of a barrel.  A more highly finished internal surface dramatically increases the cost of a barrel.  It also reduces the coefficient of friction which is the greatest single factor of barrel performance.  Lower internal friction results in less deformation of the paintball and a greater tendency for the paintball to behave in a manner according to the design characteristics of the barrel.  That is to say, to follow the rifling without skidding against its seam or otherwise behave in an unpredictable manner.

    Design Features

        Some barrels have tapering internally diameters, ports, muzzle brakes, rifling, and other factors that affect their performance.  There are dozens of port designs, muzzle brakes, rifling styles, and taper factors.  Sorting out the ones that work best is difficult because many different factors are at work.  Realistic comparison's often cannot be made of individual features because they are often combined into a single barrel while no barrel exists that has the exact same characteristics minus any particular one.  The result is, that if an improvement in performance is noted, one doesn't know which feature is contributing to the improvement the greatest amount.  One also doesn't know if the features are building on a whole as improvements go, or hindering one another.

    Self Cleaning

        The ability of a barrel to purge itself of paint by simple firing after a paintball has broken inside.  No one will deny that having paint in the barrel affects accuracy.  With some barrels it becomes utterly pointless to keep firing after you have broken a ball.  With others, breaking a ball is not noticeable to begin with.  While still others will clean themselves from firing and return to a relatively normal state.   Stainless steel generally has the best self cleaning properties, paint seems to bead up and slide right off of it's surface.  While with brass and aluminum paint may adhere quite vigorously to the surface of the bore making accurate shooting wholly impossible until the bore is thoroughly cleaned.

    Ports

        The theory behind ports is two-fold.  One, that they allow air in the barrel to vent from in front of the ball and prevent turbulence which would readily affect accuracy.  It seems plausible that a barrel with linear ports that were laid out as such that they did not impart a spin but still allowed gas to escape the barrel might provide improved performance.  Two, that spiral porting imparts a spin to the paintball which improves stability.  I cannot agree whole heartedly that imparting a spin to a paintball will improve its stability in flight.  Due to the vagaries of a paintball's liquid core and its semi elastic shell, spinning a paintball may not necessarily result in the desired effect.

    Muzzle Brakes

        They provide the same effect that porting a barrel is said to provide in regards to preventing turbulence.  They do this by venting the gas away from the muzzle just before the ball exits.  Some designs are more effective than others.  Lapco's muzzle brake is one that I believe offers a slight performance edge.  It has a series of ports near the muzzle and an extended, oversized mouth that prevents the swirling air from interfering with the ball until it is safely away from the muzzle.  Other designs may be less effective at keeping turbulent air out of the balls flight path.

    Rifling

        Two styles I know of are J&J's 'rifling' which actually has no twist to it.  And Armson's Polygon Progressive which does twist.  The J&J barrel actually prevents the ball from spinning.  The one J&J rifled I have ever used was extremely accurate.  Enough to win me a couple of target shoots.  I tried an Armson on the same marker and found it to shoot farther and flatter but not nearly as straight.   I am not a believer in the idea of putting a spin on a paintball.

    Taper

        Barrels of this sort have a gradual change in their internal diameter.  The general idea here is to prevent deformation of the paintball.  Most tapered barrels start out at a larger diameter and gradually neck down.

    Finish Coatings

        Some barrels have a special plating or finish applied.  Generally to improve performance by reducing the coefficient of friction.  Nickel plating and Ceramic coating are the two most popular forms.  Both offer a smooth finish that can be built on top of a lighter barrel frame.  Nickel gives the advantages of Stainless Steel without the weight.  Ceramic is even slicker than Stainless but costly.

    Length

        Length is a factor in barrel accuracy but it is difficult to see exactly where.  Chances are that barrels with less than two inches overall difference in length but the same characteristics in other regards will perform the same.  It takes a more noticeable change in length to affect performance.  Longer barrels appear to grant longer range.  Seemingly because a paintball accelerated to a set velocity in a long barrel retains momentum better than a paintball accelerated to the same velocity in a much shorter one.  It is very difficult to achieve high velocities with short barrels as the excessive acceleration required bursts the paintball.  Long barrels require more gas to achieve a set velocity.  Length does not seem to correlate entirely with accuracy.  Especially at shorter distances.

    Dimensional Retention

        Polishing operations and forming can result in visibly imperceptible deformations of the material due to heat warping or material compression.  This will occur on a case by case basis and result in performance of one barrel varying from the performance of it's siblings. The softer Brass and Aluminum barrels are also more likely to see minute dimensional changes due to temperature than Stainless Steel.   Proper stress relief will reduce the presence of these internal forces which can cause the impact point of a barrel to shift when temperature changes.  Paintgun barrels can go rapidly from a hot to cold state due to chilling caused by expanding gas.  This can lead to a change in performance.  Unfortunately truly stress relieving a barrel is an expensive operation and I doubt any manufacturers go to the trouble.

        There is more that can affect performance than readily meets the eye.  The design of the marker itself is also critical.  Notably in the area of the Valve and Bolt.

    Valve

        Two general types.  Inline, and offset.  Inline valves appear to offer greater accuracy.  Inline valves are positioned directly behind the bolt.  Mags, Tippmann's, and most pump guns have inline valves.  In an inline valve, the firing gas passes through a valve tube and into the bolt without making any turns.  This seems to result in less turbulence and a more even 'push' on the ball.  Offset valves are positioned somewhere other than behind the bolt.  Usually below.  'Cockers, Spyders, and PMI pumps have offset valves.  When the gas exits the valve of an offset gun it must make a sudden turn before or upon entering the bolt.  This results in turbulence and an uneven push.

    Bolt Action

        There are different types of bolt firing actions in paintguns, this action may affect accuracy.  Closed bolt guns fire with the ball pushed into the barrel of the gun and the bolt itself stationary.  Closed bolts are considered the most accurate though there is no hard evidence to back this up.  Open bolt guns push the ball forward out of the breech and fire upon sealing the breech.  This is considered to harm accuracy as the ball is vigorously jostled at the instant of firing.

    Bolt Pattern

        The design of the bolt face also affects accuracy.  The two main styles are a standard port and Venturi.  The theory behind the Venturi bolts is that they disperse the gas more evenly than conventional bolts and this results in less ball distortion and a more even push.  However, this does not seem to be the case with offset valve markers.  Rim Fire bolts offer the same advantages and may have a less pronounced tendency to impart a spin to the ball in offset valve markers.

    Mechanical Concerns

        If a paintball gun is not mechanically capable of consistent velocity, then accuracy will never reach it's peak.  A paintball gun must be able to control and measure out during firing a set amount of gas.  If it's valve system is not able to do this, accuracy suffers.
 

    Temperature

        This may or may not be the most critical factor.  No one really seems to know how thermally stable paintballs and paintgun barrels are.  Does heat expansion have an effect on their sizes?  Who knows?  What about CO2 consistency?  How is it affected by temperature?  A variation of a single Foot Per Second (FPS) in velocity will result in a change of the paintball's impact point.

    Humidity

        Humidity is death to paintballs.  Moisture penetrates their skin and causes them to swell.  Diameter increases, shell rigidity decreases, fill begins to separate.  All of this is bad.

    Air Density

        This will uniformly affect all paintballs but it bears mentioning.  Denser air is more subject to turbulence related accuracy problems and also reduces range.  Thinner air will be more conducive to accuracy.  This is good news if you are playing paintball in Denver.