The reason why adding too much spin to a paintball will cause it veer off course is:
if and when a paintball shell spins, after the initial inertia of the paint fill is overcome, the fill will also begin to spin. Once the paint fill is spinning, the fill will spin and cause changes in the center of gravity of the entire paintball resulting in random flight patterns. These random flight patterns are caused because of how a spinning object rotates around its center of gravity. If the center of gravity is always changing (due to the spinning paint fill), the axis at which the paintball rotates will be constantly changing.
This effect can be observed with a raw egg. Spin the entire egg shell very quickly, once the egg is spinning, use your finger to stop the shell, immediately let your finger off the shell and (if the egg was spinning fast enough before you stopped the shell) the shell will begin to spin again (due to the momentum of the egg yolk inside). This shows how the fluid inside of an object can and will affect the motion of the "shell"
Another explanation to having too much spin is a result from the gyroscopic forces of a spinning object. A military example is called "bullet spin drift" and one can read more about it here: http://www.appliedballisticsllc.com/...iolisDrift.htm
. A more practical example is a spinning top. The same principal applies, a top spun to the right will drift slowly to the right, a top spun to the left will drift to the left. Try it out if you would like. (make sure your surface is completely flat and is very very smooth). So an object spun at higher velocities will have a greater drift, and an object spun at lower velocities will have a lower drift.
Hopefully this helps answer a few questions before one sits down and thinks about all of this information and several dozen new questions are formed: like...
-which force has more impact on a paintball, the spinning fill or the "spin drift"
-since a paintball has significantly less velocity than a bullet, does the spin drift principle have that much of an effect?
-since a paintball is an imperfect spherical projectile (dimples, the seam, the shell is flexible), does spinning it really have that much to do with the accuracy? would a perfect none spinning paintball behave better than a perfect paintball spinning or vis versa?
-does the rifling of this barrel actually spin the paintball in a ballistic manner?
-does the reduced velocity required to improve accuracy with these barrels, affect the breakablity of the shell at farther distances? (they could travel farther but have less momentum bc their spin could be keeping them in flight, not 100% on this physics topic, but i am sure i remember something about a spin on a projectile can generate a kind of "lift")
-does this rifled barrel really help improve accuracy? or is it just better quality paint flying better than crappy paint? OR does the rifled barrel actually make your shots more accurate at lower velocities due to less spin of the paint fill and less spin drift? It would be awesome if someone could test what the velocity range was of these barrels, but just thinking of all the independent, dependent, and controlled and mainly all the uncontrollable variables would deem an experiment pretty much futile
I hope this helps answer your question while also raising many more.