Ballistic Coefficients of 6mm Boattail Bullets
By: Daniel Lilja
After I conducted the tests measuring the ballistic coefficients of several flat base bullets in 22, 6mm and 30 caliber, published in an earlier edition of NBRSA NEWS, I began to wonder how boat tail bullets would compare.
I have shot many boat tail bullets in hunting and long-range rifles, but I had never shot a boat tail bullet in a benchrest rifle. To remedy that problem, while at the Super Shoot this year I bought and borrowed four different styles of 6mm boat tails from several makers.
Because shooters often associate a higher ballistic coefficient as meaning a “better” bullet, I decided not to list the name of the bullet maker but to just describe the bullet’s physical shape.
A high ballistic coefficient does not necessarily mean that one bullet is better than another. There is far more than just ballistic coefficient that enters into a bullet’s accuracy potential. For example, take a look at the values for the boat tail bullets in this article and compare them with the ballistic coefficient of the 22 caliber flat base bullet. The 22 caliber is about .22 but I know there have been many matches where I have been beaten shooting my 6PPC by a fine-shooting 22.
These tests were run using the same methods as the earlier experiments with the flat base bullets. The only exception to this was the screens on the muzzle chronograph, a model 33 Oehler, were changed. I used Skyscreen III’s in this test, instead of the Skyscreen II’s used before. The downrange chronograph was the same, a model 35P Oehler with Skyscreen III’s. Air density was measured with the same instruments. I used a standard hygrometer to measure relative humidity and an Ultimeter to measure temperature and uncorrected barometric pressure. The velocity and air density data was fed into Bill Davis’ ballistic coefficient program in an IBM compatible computer. As a control, I again measured the ballistic coefficient of the 68 grain, flat base, 6mm Rorschach bullet that was used in the previous experiments.
In the first test the ballistic coefficient was measured at .269. In this control test the value was measured at .261. This amounts to a difference of about three percent. Although it would have been nice to have had both tests come out exactly the same, such was not the case. In thinking this over, my conclusion is the factor that can make the largest change in ballistic coefficient is an error in one of the velocity readings and probably this was the cause.
Prior to running the tests on the boat tail bullets, I set up two pairs of Skyscreen III’s on one bar with the same midpoint. With both chronographs operating, it was interesting to note the velocities of both. Often the velocities were exactly the same. At times, though, the two would be different by as much as 15 fps. The skyscreens were fitted with their glint shields and this definitely helped. A cloudy day is also better for chronographing even with the shields. In the previous tests of the flat base bullets, I had run this same test of both chronographs and found them to be very close. Whatever the cause for the difference in the two values of the 68 grain flat base bullets, I feel that it is related to chronographing error.
Measurements of air density are important, but not as critical as velocity errors. Relative humidity has the least effect of any of the measurements. A total change in humidity from 1 percent to 100 percent changes the ballistic coefficient by only about 1 percent.
It should be kept in mind, though, in making comparisons of the boat tail bullets to the flat base bullets, that there was a slight difference in the tests of the control bullet, favoring the flat base bullets with a higher ballistic coefficient. As before, the values listed are an average for ten shots.
As I said above all four of the bullets that I tested were 6mm boat tails. At least three of these bullets are available from our makers of bench rest bullets. One of the styles is not generally available in any great quantity; the maker prefers not to sell bullets.
The first bullet I tested weighed 68.8 grains, had a .070″ diameter meplat, or nose diameter, a boat tail .080″ long, was .865″ in overall length, and had a 7 caliber tangent ogive. The ballistic coefficient of this bullet was .271.
The next bullet was a 66 grain bullet with a .070″ diameter meplat, a .090″ long boat tail, .865″ overall length, and had an eight caliber tangent ogive. This bullet had a ballistic coefficient of 279″.
The third bullet weighed 76 grains. It had a meplat of .035″ in diameter. This very small nose was made by bumping the bullet after it had been pointed in another die and further closing the opening until it was shut. The diameter of .035″ is essentially the jacket wall thickness doubled. The only smaller point on a bullet that I have seen was on some experimental bronze bullets that were turned on a CNC lathe. Those bullets were so sharp it would hurt to drop one on your hand from just a few inches. This third bullet was .915″ long, has a boat tail .100″ in length and is a 12 caliber secant ogive. The ballistic coefficient of this bullet is .366. This is the bullet mentioned that is not readily available.
The last bullet to be tested weighed 67.3 grains, had a meplat diameter of .050″, was .850″ long, had a boat tail .060″ long and was a 9 caliber tangent ogive. The ballistic coefficient was measured at .326.
In comparing these bullets to the flat base version, the dimensions of the 68 and 74.5 grain Rorschach bullets are .850″ in length with a nose diameter of .070″. The 62 and 64.3 grain bullets were .770″ long with the same meplat. All the Rorschach bullets had a 7 caliber tangent ogive. The Euber flat base bullet is .835″ long, has a .070″ diameter nose and appears to have a 7 or 8 caliber ogive.
In drawing a few conclusions from both of these tests, I found that for a given shape and length of bullet, the ballistic coefficient goes up with an increase in weight, as it also does with the addition of a boat tail. A smaller nose diameter and a sharper profile will also increase the coefficient. A secant ogive can be up to twice the radius of a tangent ogive and this further decreases drag, increasing the ballistic coefficient. Some of these changes, such as adding a boat tail, decrease stability. Other changes, such as increasing weight without increasing length, increase stability. All of the bullets in this test were fired in a 13 twist 6PPC barrel and were found to be very accurate.
Accuracy is, after all, the prime requirement of any bench rest bullet. From a bench rest shooter’s point of view, an increase or decrease in ballistic coefficient is only important if it improves accuracy. At 100 yards, I doubt that we would notice much difference in accuracy of any of the bullets tested with a load and barrel that shoots them well. At two and three hundred yards, however, when the wind is moving, a high ballistic coefficient bullet has a definite advantage. How great the advantage is what experimenting and testing and competition are all about.