Measuring Ballistic Coefficients of Benchrest Bullets
Recently I ran some tests of ballistic coefficients of various types of benchrest bullets. The ballistic coefficient is a value given a bullet that describes its ability to resist the effects of drag caused by the air. The higher the value, the greater this ability is.
The bullets I tested were in 22 caliber, 6mm and 30 caliber. They were all flat base bullets.
To measure ballistic coefficient, the velocity of a bullet must be measured at two points a known distance apart. This velocity change, combined with a measurement of the density of the air at the time of testing, are then run through a computer program which will calculate the ballistic coefficient (corrected to standard conditions).
To measure the two velocities, two chronographs are needed. At the muzzle I used an Oehler model 33 with Skyscreen II’s, and at a distance of 231 feet between screen pair centers I set up an Oehler model 35P with Skyscreen III’s. The latter has a built-in printer and works very well for down-range chronographing.
To measure the air density, the barometric pressure, (uncorrected for altitude) must be measured, along with the air temperature and the relative humidity. To measure the barometric pressure and temperature, I used an electronic “Ultimeter”, manufactured by Peet Bros. Company of Ocean, New Jersey. I used a standard household hygrometer to measure the humidity.
To calculate the actual ballistic coefficient, I used two separate but similar computer programs. The two programs agreed with each other very closely in calculating ballistic coefficient. They were both run on an IBM-compatible PC. One of them was a program available from one of the advertisers in NBRSA News: ProWare, Inc. of Portland, OR. The other was written by Bill Davis and described in the March 1989 issue of the AMERICAN RIFLEMAN. Anyone interested in running similar tests should read this article.
The 22 caliber bullet that I checked was a 52 grain HP bullet that was made in Rorschach dies. It was shot from a 21″, 14 twist, 22 Hex barrel. The average ballistic coefficient for ten shots was .219.
I tested several 6mm bullets. They included a short jacket 62 grain, a short jacket 64.3 grain, a 68 grain, and a 74.5 grain all made in Rorschach dies and a 68 grain Euber. All the bullets except the 68 grain Rorschach were shot from a 23″, 13 twist, 6PPC. The 68 grain Rorschach bullet was fired from a 24″, 14 twist, 6PPC. The ballistic coefficients were as follows: the 62 grain had a BC of .238; the 64.3 grain was .247; the 68 grain Rorschach was .269; the 68 grain Euber was .264; and the 74.5 grain was .279. All the ballistic coefficients listed are an average for ten calculations.
The 30 caliber bullet that I tested was a Chism 150 grain. It was fired from a 22″, 15 twist, 308 Winchester barrel. Its ballistic coefficient was measured as .467 for eight shots.
As can be seen from these tests, an increase in bullet weight will increase the ballistic coefficient for a given shape and length. Whether or not that increased ballistic coefficient will give the shooter any advantage in the wind depends on the velocity he can accurately drive the heavier bullet. For example if out of a 6PPC we can drive a 62 grain bullet, with its BC of .238 at 3300 fps and a 74.5 grain bullet, with a BC of .279 at 2900, the wind drift in a 10 MPH wind is the same. That is, at 100 yards wind drift is about an inch and an eighth, at 200 yards four and three quarters inches, and at 300 yards about eleven and a half inches.
The bullets I used are typical of flat base bullets from many makers of our benchrest bullets. Probably any bullet made in a Rorschach die will compare very closely to the ballistic coefficients that I measured if it has a minimum point opening. I believe that Rorschach uses the same series of laps for all of his dies so the basic bullet shape will be the same.
Knowing the ballistic coefficient of bullets is essential in predicting the down-range performance of our bullets. Remaining velocity, remaining energy, bullet drop, and wind drift are all dependant on the initial bullet velocity and its ballistic coefficient.
|22 CALIBER||52 GRAIN RORSCHACH|
|6MM||62 GRAIN RORSCHACH|
|6MM||64.3 GRAIN RORSCHACH|
|6MM||68 GRAIN RORSCHACH|
|6MM||68 GRAIN EUBER|
|6MM||74.5 GRAIN RORSCHACH|
|30 CALIBER||150 GRAIN CHISM|