Some rifles have two lands and grooves and the number goes up, all the way to micro-groove barrels. Some rifles have polygonal rifling with no lands or grooves that one could see. How would this engraving of the bullet affect BC?
Poly rifling is just like normal, but has the hard edges "rounded". Hex type rifling as found in some of desert eagles don't have "normal" perceivable rifling.
Either way there are a literal gamut if tests showing anything and everything is good or bad. What most of it Congress down to is ease of cleaning. In the end what constitutes a "good" shooting barrel is more about matching bullet diameter against bore/groove dimensions.
All barrels have bore and groove dimensions that are followed by barrelmakers, whether they are polygonal rifling, conventional cut, hammer forged or any type of configuration from two lands to as many as one wants and shaped in any manner.
Typically the land height of a .224" bore is 0.065 mm or 0.0026" and the land height of a .50" bore is 0.115 mm or 0.0045".
Ignoring all the advantages of more/less lands, the shape of these lands and considering only how the number and shape affects BC and airflow over the surface of the bullet, do more or less lands positively or negatively influence BC or airflow?
Are there credible studies that have looked at this?
Logic suggests that it would be more of a rotational drag change... i.e. how much the air resistance slows the bullet spin. You could eventually end up with a bc change due to bullet wobble induced my marginal stability (and therefore degraded bc) though. I doubt the results could show more than a minor difference though at the ranges shoulder fired rifles are capable of.
The only other difference I can think of is the fact that a rather tight barrel will likely make the bullet obdurate at a slightly smaller diameter than a worn/loose barrel. The resulting difference in diameter would cause the tighter barrel to make the bullet grow compared to the looser barrel. That could change bc with a possibly lesser frontal area on the narrower bullet. Again, basically a fart in the breeze in most cases....
I know when my brother owned a 460 s&w he was fond of swaging (.458") 420gr hardcast to .452" for the pistol. Those bullets grew one eighth of an inch when swaged.
I've been able to measure a difference in rotational drag for different rifling types. The testing and results are published in 'Modern Advancements in Long Range Shooting'.
There is a benefit to retaining higher rpm's when the bullet goes below about 1500 fps because it will be more stable and not lose as much BC in transonic. I've heard that some riflings reduce drag more beyond rotational effects and I'll be testing that soon.