As bedbugbilly says... if it ain't broke, don't fix it! It doesn't matter why it works if it works. However, being a curious sort, I like to know why.
My opinion only is that if dimpling does improve accuracy it does it due to closer ball to bore fit. I can say that after launching hundreds of pounds of lead balls down range from my smoothbore shotguns that the closer the ball is to bore diameter the better accuracy is.
I can also say that accuracy much beyond 50 to 60 yards becomes a bit iffy with groups enlarging exponentially most likely due to balls picking up random spins from air drag creating the "curve ball" effect like a baseball pitched with a spin on it.
So, I decided to do a bit of research after reading your post. Certainly no scientific or conclusive but interesting. Dimpled golf balls are discussed and all golf balls have dimples I understand it to reduce drag so gain distance. That appears to be the case. Some interesting info here:
http://www.furthereducationlessontra...rodynamics.pdf
which indicates that a dimpled ball leaving the muzzle with no spin should have less drag than a smooth ball. That should be pretty easy to test so I'll dimple some of my 0.662" or 0.678" RB's then try them in my 12 ga. any significant difference in trajectory should be obvious. Since the balls are loading into shotcups the surface should be protected in the bore.
Taking the less drag into consideration, now I am wondering if that might translate into less chance of random spins or at least longer flight time before a spin may develop (fast enough spin to affect flight). Once a spin develops the ball will develop "lift" on the surface rotating with air flow so will deviate flight in that direction. Prior I had assumed that a rough surface would tend to induce more spin sooner due to that rough surface but if that rough surface does reduce drag then that would be a wrong assumption. Again, should be fairly easy to test for any significant differences.
Also, according to Scientific American a dimpled gold ball has about 1/2 the drag of a smooth golf ball so pretty significant improvement in BC!
Now the difference between golf ball dimples and file "dimples"... they are truly different! Just how that affects drag is another question and harder to test. It's easy to use files or wire bristles to rough up a ball but much harder to make an evenly spaced consistent golf ball like dimpling. However, I think one could assume similar effect if trajectory is flatter when a file roughened ball is used as that would imply reduced drag.
So, lots of words for a simple thing! I guess this will have to go into my "To Do" list since RB's are still my favourite shotgun projectile. I have to think that a patched round ball from a musket will behave pretty much the same as a round ball launched from a smoothbore cartridge shotgun so results should be comparable.
Longbow