In this post we are going to discuss a little about ballistics and cartridge performance. Specifically if you are planning to purchase a rifle or have one built, what kind of ballistics and cartridge performance can you expect given the different variables that influence those two factors. Things like barrel length, caliber, and the ammunition used can all influence the ballistics and cartridge performance of a particular rifle. If you put some thought and planning into what features are the most important you can better plan your purchase or build so that it performs to your expectations.
Ballistic Coefficient (BC)
Perhaps the most important factor that will determine the performance of a rifle and cartridge is the ballistic coefficient (BC) of the bullet fired from the rifle. When discussing ballistics and cartridge performance you will hear a lot of talk about the ‘BC’ of the projectile. People are referring to the ballistic coefficient of the bullet. The way ballistic coefficients work is the performance of the bullet is measured and rated with a score against a model. Some really smart people have come up with drag models that help measure and model how a bullet flies through the air with a computer. Depending on how efficient a bullet is during testing will determine it’s score against the model.
There are two main models people use when discussing ballistics and cartridge performance. G1 and G7. The G1 drag model has been around longer and is typically the model used by most manufacturers when rating the BC of the bullet the manufacturer produces. For example, the very common 30 caliber 175gr Sierra Boat Tail Hollowpoint projectile has a G1 value of 0.496 above 1800fps. It also has a G7 value of 0.243. What does all that mean? Those are the scores in regard to the G1 and G7 drag models. The higher the scores the better. The Hornady 178gr Hollow Point Boat Tail 30 caliber bullet has a G1 value of 0.530. This is a higher score than that of the Sierra Matchking.
The higher the score, the better the performance of the bullet. A bullet with a higher ballistic coefficient is a more efficient design. It will drift less over the same distance in the wind as a bullet that scores lower. It will also maintain its velocity and energy longer. That becomes significant with regards to effective range. As bullets begin to transition from supersonic to subsonic flight, this is called transonic flight, the bullet can become unstable while making the transition and that can have severely detrimental effects on accuracy. Generally, your accurate maximum range of fire is within the supersonic flight distance of the bullet.
Which model is better? My answer to that is whichever model gives you better dope. If you run a chart with a G1 value for your bullet and muzzle velocity, check it with live fire. If the adjustments that chart calls for line up more precisely with what you use in live fire than a G7 value, use the G1. Vice versa is also true. If the G7 value works better for you, use that. The textbook answer says the G7 drag profile is a better model of bullet flight than the G1 and it does not depend on velocity data to come up with accurate firing solutions. That said there are many people that have great success using G1 values. I’m not particularly sold on either, like I said, use whatever works better FOR YOU!
Barrel length gets a lot of attention when talking about ballistics and cartridge performance. As a rule of thumb, a longer barrel on the same gun of the same caliber will allow you to attain a higher muzzle velocity. In general, higher muzzle velocities tend to equate to better ballistic performance down range. However, there is a point of diminishing return. The faster the muzzle velocity the more pressure you need to attain it. Pressure is what wears out a barrel. So while a fast moving bullet performs well, the usable life of the barrel may be much shorter if you are pushing the bullet hard to achieve that extra performance.
Since we know longer barrels tend to yield higher muzzle velocities, it makes sense that a longer barrel would allow a shooter to get better velocity out of a given cartridge. This can be done without having to push the bullet as hard to get the speed and performance the shooter desires. This discussion starts to get complicated because all these variables are inter-related and tweaking one can affect others. For example, we mentioned that the G1 drag model is velocity dependent. Therefore by using a longer barrel, and achieving a higher muzzle velocity, we may affect the ballistic coefficient of the bullet. While 0.496 is an accurate G1 value for the 175gr Sierrra, if you manage to get the bullet moving above 2800fps, the G1 ballistic coefficient is now 0.505.
If it wasn’t obvious, what’s really important is not so much how long your barrel is, but what kind of muzzle velocity it lets you achieve. The barrel need only be long enough to attain a desirable muzzle velocity. You can even cheat the barrel length factor a bit by hand loading your own ammunition. If you load your own ammunition, as long as the pressure levels are safe, you can push a bullet faster out of the same barrel than when using factory ammunition. You can therefore theoretically run a shorter barrel and still get the same or better muzzle velocity as what factory ammunition produces in a longer barrel.
This is another large influence variable on ballistics and cartridge performance. Generally, when speaking about different bullet weights of the same caliber, and often bullets of different calibers, the smaller bullet has better ballistic performance. That is because it takes less effort, powder, and pressure to get that bullet moving faster. A fast moving bullet with an efficient design is hard to overpower and outperform. The one caveat here is that the lighter the bullet gets, the less energy it delivers on target. That is significant for the hunting crowd. It also matters when trying to spot hits on steel or to put bad guys in the ground.
You can also cheat the system a bit here. While well designed light bullets perform well. Generally the bigger and heavier the bullet gets, the higher the ballistic coefficient becomes. The 338 Lapua Magnum is a good example. While a 123gr bullet fired out of a 6.5 Creedmoor at 2900fps performs very well with a 0.547 ballistic coefficient, you can trump it. A 285gr bullet fired from a 338 Lapua Magnum at 2800fps will outperform the Creedmoor, even at the slower velocity, because the ballistic coefficient is 0.700. This is what I mean about all these variables being sort of inter-connected and influential.
Here’s another example. A 123gr Lapua Scenar fired from a 6.5 Creedmoor at 2900fps has a BC of 0.547. That’s a very high performance cartridge. What about a 105gr Berger Hybrid? It is a lighter bullet, but a more efficient design. It has the same 0.547 BC. Because the weight of the 105 Berger is lighter, you can fire it faster, commonly in excess of 3100fps, which we know from our discussion earlier will yield better performance. However, it will do so at the expense of energy delivered on target. So keep in mind the purpose of the rifle when deciding which bullet weight you plan to use.
Wrapping up Ballistics and Cartridge Performance
So of the variables we’ve discussed, which should you emphasize? Which are most important? I suggest you look at the bullet you plan to fire first. Pick something that has the highest BC available as that helps establish a firm groundwork for a rifle purchase or build. When you know what class of bullet you want to use, you can start to look at different calibers and what muzzle velocities and barrel lengths are typical of those calibers. The purpose behind the rifle is the most important variable. The trend in tactical rifle matches is towards high BC, light weight, bullets at high velocity. That translates to excellent performance in the wind and a flat shooting cartridge.
However, that may be completely in opposition to the needs of a hunter, who needs more energy on target at closer ranges. In those cases you might want to use heavier bullets and slightly slower velocities that will deliver more energy to the target. If a heavier bullet is used with an efficient design you may still make gains in terms of wind performance and energy retention with higher ballistic coefficient bullets and calibers. I know this can be kind of confusing at times since all these variables and terms are so inter-connected. If you have questions, leave them in the comments below and I’ll do my best to answer them!