How do 6-groove barrels compare to 12-groove barrels?
by B.B. Pelletier
I don't know where this question came from, but it's been weighing on my mind for many days. I thought it was in a comment to this blog, but it could just as easily have been a topic on one of the airgun forums. I want to address it because it gets down to the fundamentals of the shooting sports.
More power - Tim Allen got it right!
Tim Allen's famous monolog in which he pokes fun at the male lust for more power had a profound message. As he skillfully points out, more power is not always the answer - or even a good idea! Take the Boss Hog motorcycle, for instance. It's a motorcycle built to accept a large-displacement Chevy V8 engine that develops over 500 horsepower! It's the very parody of a motorcycle! It isn't faster than other bikes, it doesn't accelerate quicker and it certainly doesn't handle as well as a hundred other conventional motorcycles. Yet, I want one. To be astride such a beast is to have the biggest, baddest ride in town - as long as Jay Leno doesn't ride up on his motorcycle, which has a 1,500-horsepower helicopter turbine engine!
It's purple and $32,000 in the standard version. Totally useless. Yet, I want one!
Consider the absurdity of the question
If more was always better, why don't sports cars have six wheels instead of four? Why aren't handgun calibers made larger every year? Uh, oh! I forgot. They are! Enter the S&W .500 Magnum revolver - breakfast of complete fools who can't wait for the world to know it. A revolver caliber so powerful that the average used one has fired less than one box of ammo - usually just five rounds (one cylinder full). Nevertheless, this gun sells very well. Good for S&W - not so good for those who have no idea what they're getting into until the hammer drops the first time! More is not ALWAYS better.
Joking aside, there are lots of things that are just as useless. We have advertising to thank for that. Advertising tells us so often that "MORE IS BETTER" that we've come to believe it. Which takes us to today's topic: Are more rifling grooves better (more accurate) than less?
In rifle barrels, the number of lands and grooves MAKES ABSOLUTELY NO DIFFERENCE, as far as accuracy is concerned. That's assuming that the minimum number of lands and grooves is adequate to stabilize the bullet/pellet. In World War II, the U.S. Army proved that two lands and grooves were enough to stabilize a 150-grain bullet in a .30/06 rifle. They accepted two-groove barrels in their Springfield 03A3 rifles because they could be produced faster than four-groove barrels (the previous standard), and faster production during a war is good. The decision-makers admitted that these rifle barrels were not absolutely the most accurate they could produce, but the four-groove barrels they replaced were no more accurate, so who cared?
What makes a barrel accurate?
The most important factors for a barrel to be accurate, according to Dr. F.W. Mann, who did 38 years of research on the subject, are straightness, uniformity of the bore and rifling, twist rate matched to bullet length and velocity (not as much for pellet guns, because we use high-drag diabolos instead of bullets), a choked muzzle and a uniform forcing cone with graduated rifling (called a leade) at the end of the chamber. Dr. Mann used barrels from several makers, but he did his best work with barrels handmade by legendary barrelmaker Harry M.Pope. Once he figured things out, Pope always used eight shallow lands in a lefthand twist. He often used a gain twist (one that gets faster) but not always. He always choked his muzzles by a half-thousandth. Many of his rifles were meant to be loaded at the muzzle, which would seem to negate the choke, but the heavy smack delivered by the explosion of the black powder charge bumped the bullet, squashing it at the base to fit the bore tightly.
Dr. Mann wrote his findings in his book, The Bullet's Flight From Powder to Target. This book is a classic and is still considered to be the best and most fundamental reference book on rifle accuracy, even though it was initially published in 1909, after 38 years of experimentation. Today's best gun writers have this book in their libraries to fully understand the complex, yet unchanging principles that govern the ballistic projectile we call a bullet.
Dr. Mann And Harry Pope both knew that the uniformity of the base of the bullet was extremely important to accuracy. That's why Pope rifling was as shallow as it was and why he used eight lands and no more. He found that each land put a burr on the base of the bullet that interacted with the gasses exiting the muzzle at high speed. This destabilized the bullet by a small but measurable degree. Dr. Mann provided the measurement. That's why the most accurate Pope barrels were muzzleloaders, even though they used cartridges loaded from the breech. By loading the bullet from the muzzle, the small burr would be on the front of the bullet, where Dr. Mann demonstrated almost no amount of mutilation had any effect on accuracy. You could say that Harry Pope perfected the microgrooved barrel, but Marlin was the company that coined the term.
The bottom line is this: the number of lands and grooves has no effect on the accuracy of a rifle barrel. Microgroove rifling can be beneficial, but only if all the other important factors are also correct.