There are several ways to manage the problems caused by the dependence of BC on velocity. One way is to use a G1 BC thatís averaged for the speed range youíre interested in. This will get you close, but what if the BC of the bullet is advertised for a speed range thatís different than what youíre interested in? Itís not easy to adjust the BC for different average velocities.
Another way to deal with the problem of a velocity dependent BC is to give the BC in several velocity Ďbandsí (Sierra bullets uses this approach to advertise the BCs of their bullets). This can be an accurate approach, but it leaves a lot of room for misinterpretation. For example, many shooters donít understand why there are different BCs and choose the wrong one. Furthermore, not all ballistics programs allow you to input multiple BCs.
In short; the use of the non-representative G1 standard (Figure 1) to define BC is responsible for the velocity dependence and associated problems with BCs.
A Better Standard For Long Range Bullets
If you look at the G1 standard projectile again in Figure 1, you might think; ďitís too bad there isnít a standard thatís more representative for modern long range bulletsĒ. In fact, there are several standard projectiles, all with different shapes, that are much more representative of modern long range bullets than the G1 standard.
The standard that bears the closest resemblance to most modern long range bullets is the G7 standard, shown in Figure 2.
Figure 1. The G7 standard projectile.
As you can see, the G7 standard projectile, with its long boat tail and pointed ogive bears a much stronger resemblance to a modern long range bullet than the G1 standard projectile. As a result, the BC of a modern long range bullet thatís referenced to the G7 standard is constant for all velocities! In other words, a trajectory thatís calculated with a ĎG7 BCí doesnít suffer from the same velocity dependence problems and inaccuracies as calculations that are made with a G1 BC.
Another benefit of using G7 BCís is that it allows a more fair comparison between bullets. For example, consider two .30 caliber 168 grain match bullets from different manufacturers. Even if both projectiles are identical in shape and weight, itís possible for them to have different advertised BCs if the BCs are calculated for different velocities. For instance, if one of the bulletís BC is calculated for a 3000 fps (muzzle velocity) and the other is calculated for an average velocity between 3000 fps and 1500 fps, then the BC thatís based only on muzzle velocity will be higher, but less relevant for long range shooting than the average BC.
In other words, the two bullets actually have the same BC, but the Ďsmoke and mirrorsí that results from the velocity dependence of G1 BC creates the illusion that one bullet is better than the other. If you considered the G7 BC of the two bullets, it would be the same for all speeds.
You may observe that not all bullets look more like the G7 standard, and thatís true. For the short, flat based, blunt nosed bullets, the G1 standard is actually more representative. For that reason, BCs for flat based bullets should continue to be referenced to the G1 standard. In other words, the G7 BC is better for boat tailed bullets, while G1 BCs are better for flat based bullets.
<Previous | Home | Next Page >