January 04, 2011
By Scott E. Mayer
Which shoots more accurately, a flat-base bullet or a boattail bullet? The answer, and the reason, will surprise you.
By Scott E. Mayer
Flat-base bullets are inherently more accurate than boattail bullets because it is almost impossible to put boattails on perfectly. Even so, over long range, a boattail's ballistic benefits outweigh any manufacturing flaws.
Check with any Highpower shooter worth a shot, and he or she will most likely tell you to use boattail bullets for the best accuracy. The 1,000 Benchrest crowd will probably tell you the same. But ask the "short-range" Benchrest shooters — -the ones who literally compare the size of one-hole groups to see who wins — -and you're more likely to be sent after a box of custom-made flat-base bullets. Why the difference? Both long- and short-range shooters are looking for the smallest groups, the best ballistics, the greatest accuracy, and the highest possible scores. What works for one should work for the other, right? Not necessarily.
Flat-base bullets are fairly simple to manufacture compared to boattail bullets. So long as you have high-quality, consistent jackets, consistent cores with no voids, square bases, quality bullet-forming dies, and excellent quality control and cleanliness, there is no reason most manufacturers can't make decent flat-based bullets. Custom bullet makers even take flat-based bullets to accuracy levels hard for most shooters to imagine. There's less that can go wrong in manufacturing a flat-base bullet with the main points being that the bullet is round and the base is square to the bearing surface, or bourrelet. Because they can be manufactured to higher tolerances and less manufacturing variables, flat-base bullets are inherently more accurate than boattail bullets. In the field instead of on a target range, flat-base bullets tend to hold onto their cores much better than boattails with less instance of "bananafication," or the squirting out of the lead core from the copper jacket upon penetrating. Core separations have lead to improvements including Hornady's Interlock, various bonded bullets and manufacturers such as Nosler coming up with "Solid Base" boattails that offer the benefits of boattails, but with the cavity inside the bullet jacket square in the bottom like a flat-base instead of tapered inside the boattail.
Regarding terminal ballistics, Nosler makes its boattail bullets behave like flat base bullets by making "solid base" boattails. That design has the lead core bottoming out in a flat-base pocket above the boattail.
If there's a drawback to flat-based bullets, it's that their center of gravity is more toward their rear, which doesn't help stability. But for a given caliber and bullet weight, flat-base bullets can be made shorter than boattail bullets and are thus stabilized in a slower twist. Another drawback is that as flat-base bullets pass through the air, the air flowing over the bullet's surface wants to slam back together at the base. That creates quite a bit of turbulence resulting in base drag. So long as a bullet is going supersonic, or faster than the speed of sound, which is about 1080 fps at sea level, the greatest amount of drag is on the nose of the bullet as it compresses the air faster than the air can get out of the way. At high velocity, the base drag is not as significant a percentage of the overall drag. But once a bullet drops below the speed of sound, into the transonic and subsonic velocity range, base drag becomes the dominant drag component.
Compared to flat-base bullets, it's more difficult to manufacture accurate boattail bullets. Bullets are formed in dies, and the boattail will always be off by 1/2 the amount of clearance between the punch and the die. Tolerances have to be extremely tight. In a manufacturing environment, it's simply not possible to put boattails on perfectly square or perfectly straight, but the better manufacturers sure come close. And as a bullet gets larger in diameter, the tolerance error between die and punch becomes less a percentage of the overall bullet and less of a factor in accuracy. That's why large boattail bullets, such as Hornady's .50-caliber A-Max are some of the most accurate long-range bullets in the world.
It takes eleven steps to form the Hornady .50-caliber A-Max bullet. It is one of the most accurate boattail bullets on the market. Because of its size, manufacturing tolerances are a smaller percentage of the overall bullet, and affect accuracy less.
Boattails increase the ballistic coefficient of bullets, which helps them overcome air resistance and wind deflection. The shape also helps the air flow transition over the heel of the bullet and reduces the base diameter resulting in less base drag. Many shooters use that as their argument for shooting boattails over flat-bases, while others look at how often they take a shot at something so far away that boattails seem to matter and dismiss them as unnecessary. It's true that the tail really proves its worth at longer ranges and lower velocities, but it's wrong to dismiss them because of that.
For one thing, boattail bullets are simply easier to load when handloading. They naturally align and right themselves as they're guided up into the bullet seating die and gently ease their way into even tight case mouths with reduced chances of crumpling the case. For a given bullet diameter and weight, the bourrelet on a boattail bullet is less, usually resulting in lower pressure for a given powder charge, and the center of gravity is moved farther forward helping with bullet stability.
Boattail bullets are simply easier to load when handloading. They naturally align and right themselves as they're guided up into the bullet seating die and gently ease their way into even tight case mouths with reduced chances of crumpling the case.
It's true that the difference in the amount of drop between flat-base and boattail bullets won't amount to much until well past the range at which most of us shoot, but the boattail's better ballistic coefficient makes errors in wind deflection correction and range estimation more forgiving. Whatever loss of accuracy there is from the boattail's inherent manufacturing flaws are more than offset by their ability to overcome adverse or unknown shooting cond
itions.Presently, there are efforts being made in the Benchrest shooting community to make boattail bullets that behave like flat-bases at all ranges, and flat-base bullets that behave like boattails at long range. Those efforts include rebated boattail designs that have an abruptly reduced bullet diameter before the boattail is formed, and FBVLD, or flat-based very low drag bullets that have an extremely long nose, or ogive (pronounced o-jive), for minimum nose drag to compensate for the greater base drag. Berger Bullets is also working on a new method of forming boattails so that they are more concentric.
Claims of the rebated boattail include greater reduction in base drag for subsonic bullets, and noticeable reduction in base drag for supersonic bullets. Another claim is that any shot dispersion caused by the muzzle blast is reduced and barrel life is increased because there is less gas cutting.
There are efforts to make flat-base bullets with the ballistics of boattails, and boattails with accuracy of flat-bases. Efforts include the Berger Very Low Drag, or VLD (l.) and the Berger Low Drag, or LD (r.) bullets.
Claims for the FBVLD include not only the improved manufacturing tolerances, but also a higher stability factor for a given bullet weight and rifling rate of twist. With a higher stability factor, a FBVLD is more forgiving if the air is cold and dense, and the twist rate marginal to begin with.
So what should you shoot, flat-base or boattail bullets? Personally, I think that regardless of whether you're shooting game or punching paper, you should try several brands of boattail and flat-base hunting or target bullets, respectively, and then shoot whichever one is most accurate in your gun at the range and in the conditions you intend to shoot. Whichever you choose, most Benchrest shooters who shoot at relatively close range will stick with the flat-base bullets until someone starts winning with a particular type of boattail, and the Highpower shooters and 1,000 yard Benchrest shooters will probably stick with boattails for their long-range benefits.