January 04, 2011
By David Tubb
Why small-diameter bullets beat the big, fast magnums at long range.
By David Tubb
The DTAC 117 MJPT (metal jacket, plastic tip) 6mm bullet is an outstanding performer at 1,000 yards.
Many people know me as an across-the-course High Power shooter, but I've had a good deal of success in NRA High Power Rifle Long Range competition as well, and that has consumed a greater portion of my development, testing and training time in recent years.
Development and refinement of cartridges, bullets and means to attain educated information to forecast downrange ballistic behaviors are three areas of interest to the 1,000-yard shooter.
For years, people struggled with high-capacity magnum cartridges with the belief that higher and higher velocities were key to more control over downrange performance.
However, magnums such as .300 Winchester or the .30-338 have always been notorious for "elevation shots," the loss of points resulting from high or low impacts. The reasons for this are numerous, but a good overall assessment is that there is so much propellant in these cases that it's not possible to closely control ignition and burn consistency. I'm also convinced that the excessive recoil and vibrations induced by heavy loads play a part.
Barrel life is another factor, and many of the truly over-bore combinations won't deliver even 1,000 rounds of reliable accuracy before throat erosion takes its toll on the barrel. Sometimes this rapid erosion can affect scores even with a relatively new barrel.
Over time we began to reduce the caliber size in both long-range and across-the-course cartridges. The idea was to attain high velocities at lower recoil levels, and I have worked extensively with both 6.5mm and 6mm calibers. Of these I have settled on the 6mm. I am convinced the .243 caliber bullets demonstrate the best combination of velocity and accuracy.
Specifically, I began working with cartridges based on the .250 Savage case rather than the more common .308 Winchester parent cartridges such as .260 Remington and .243 Winchester.
Although the impetus for my development of what is now commercially known as 6XC was to engineer a round that was a better match for box magazine use across the course, the efficiency of this cartridge and its amazing accuracy level have made it my choice for 1,000-yard use as well. I used the cartridge in a TUBB 2000 to set an NRA High Power Long Range championship record of 1600, the first perfect score fired.
Previously, I had used the wildcat 6.5-.284 for long-range competition. It's a good cartridge, but its accuracy level is not quite on a par with the smaller round.
The true keys to long-range performance on target come from pure accuracy (small group sizes) and bullets designed to maintain high velocities downrange. Consistency is an advantage that no amount of velocity, or propellant, can overcome.
Running any of the new-breed 6mm bullets at increasingly higher and higher velocities has not proved to be any advantage in ultimate score. I am able to attain approximately 3,050 fps from a DTAC 117-grain bullet. This bullet has a ballistic coefficient of 0.600, confirmed by actual field tests. This approximates that of the 142-grain Sierra MatchKing 6.5mm many choose for 1,000-yard events.
As I mentioned, it's the consistency of elevation impacts that truly dictate how accurate a long-range round is. Machine-rest testing has shown me significantly reduced elevation groups, and I've confirmed that on the firing line at Camp Perry.
Efforts to ensure elevation consistency are a combination of attention given to details on the loading bench, such as weighing each propellant charge, and taking steps to improve the ballistic consistency of the bullet itself.
The author and his son stand in front of a 1,000-yard target fired prone with DTAC 115-grain 6mm bullets. Note the elevation consistency, which is key to good long-range scores.
It's easy to determine how consistent velocities are using a chronograph during testing, but it's not necessarily showing us what really matters, which is consistency of velocities at the target.
A load with a low standard deviation (consistent muzzle velocity readings) is certainly the first step in choosing competition ammo, but that will not tell you how consistent these velocities will be maintained farther downrange. Downrange chronographing can be a challenge, but it's well worthwhile when developing long-range ammunition.
Due primarily to the production process in making a hollowpoint match bullet, there will be inconsistencies in the meplat or extreme tip-end of the bullet. These inconsistencies are influential in having each bullet in a box exhibit consistent behaviors downrange, and that is because they'll have slightly different effective ballistic coefficients.
Essentially, a ballistic coefficient is a measure of how much speed a bullet loses; the higher the BC, the less speed it loses. If bullets are flying at different speeds, though, there will be greater elevation displacements on target.
A few years ago I developed a tool that uniforms the meplats of match bullets. Testing showed significant reductions in elevation group sizes. I strongly recommend this effort to any serious long-range shooter. No matter what the cartridge/bullet combination chosen, it's an easy way to improve on-target consistency.
Ultimately, however, my efforts in bullet design have shifted toward incorporating plastic inserts to seal the meplat. This technology can be so well controlled that uniforming is no longer necessary. I realize that I'm certainly not the first to have incorporated this format into long-range bullet design. The difference, or my goal, was to also incorporate increased accuracy potential and a best possible BC profile into those I have engineered.
Ultimately, small groups at long range come most readily from an efficient, highly accurate cartridge firing bullets that are equally consistent in their downrange performance. A high score at 1,000 yards comes more from precise and consistent target feedback than it ever will from just higher and higher velocities.