The Truth About Ballistic Efficiency
July 15, 2016
Ballisticians and rifle cranks love the word " ballistic efficiency," and it's become a common part of our vernacular. We love a specific rifle cartridge because it's so efficient. We purchase a rifle chambered for a new round because it's more efficient than what we were hunting with last year.
The term has become a sales pitch for cartridges in much the same way that the term "reliability" sells new cars, but how well do we understand the concept of ballistic efficiency — and how does it relate to energy, trajectory, pressure and a host of other interconnected factors that come into play every time we punch a primer?
Ballistic efficiency is a tricky subject, and when labeling a cartridge as "efficient," we must remember that there are a number of factors that can affect overall cartridge efficiency. Most shooters know certain cartridges work better with certain powders. This seems simple enough, but some shooters give credit to a cartridge for the work of the propellant.
When ammunition companies switched from black powder to smokeless propellants, the velocity and energy produced by the .30-30 Win. jumped dramatically. Was the .30-30 suddenly a more potent cartridge? You bet it was, but switching powders accomplished this rise in velocity and energy.
Likewise, Hornady's Superformance ammunition is considerably faster than most traditional factory ammo, but that's a result of Hornady's use of a proprietary powder. They didn't change the .30-06; they just gave it a boost.
This may seem like basic information, but discussions on ballistic efficiency can sometimes turn into a bad sketch comedy where misconceptions and misinformation leave everyone confused and discombobulated.
First, and this is important, ballistic efficiency is relative to propellant. What works in one round may not work for another. That's important to understand. Forty grains of IMR 4350 doesn't burn at the same rate that 40 grains of Reloder 25 does, so it's important to keep things relative.
Like with any good science experiment, we have to keep the variables the same. It won't do us any good to compare the .243 Win.'s ability to push an 80-grain bullet at 3,000 fps in front of 46 grains of IMR 7828 with the .300 Wby. Mag.'s capacity to launch a 150-grain bullet at 3,363 fps with 86 grains of Reloder 22. That'd be like saying a Rolls Royce jet engine is more efficient than a Briggs and Stratton because it can fly a jet to Switzerland. Power alone doesn't equate to efficiency.
We must also take into account variables such as burn rate. Load a hot, long-cased magnum with a powder that burns too fast and you're never going to see the results you could achieve with a slower-burning propellant. Propellant choice is variable, and good handloaders understand that they have to match the cartridge with the right powder to effectively compare ballistic efficiency.
The one variable that remains constant is the design of the case.
The standardized case for a .30-06 has retained exactly the same dimensions for the last hundred-odd years, and the primary factor behind how "efficient" a cartridge is boils down to one thing: case architecture.
To understand ballistic efficiency, you truly must have a handle on how rifles cases are designed and how changes to a design will alter things such as burn rate and compression. Interior dimensions, the slope of the shoulder, the overall length and other factors play into overall powder column burn. Case design and efficiency go hand-in-hand — period.
It's generally assumed that, with all things being equal, a larger case capacity reduces overall efficiency. Take a look at the accompanying chart where I have matched several 7mm loads and their ballistic efficiency with each load in feet per second per grain of powder — one way to measure efficiency. Case capacity gets larger as you go down the list. Consequently, velocity goes up (as does energy) but efficiency drops in every case.
Generally speaking, larger cases allow for more powder to be stuffed in the load, but overall efficiency goes down. The 7mm WSM and the 7mm Rem. Mag. are a particularly interesting comparison, because while they are very close in overall case capacity (the 7mm WSM holds 83.0 grains of water, the 7mm Rem. Mag. holds 83.2), the 7mm WSM offers a 1 percent increase in efficiency. The similarity in overall case capacity indicates that it is the overall design of the Winchester's case that leads to efficiency.
I ran these comparisons with a couple different powders, and while as I mentioned it's true you could tweak the efficiency of a particular cartridge by matching it up with a particular powder, on an apple-to-apple basis the cartridges with smaller volumes are more efficient.
Winchester's line of Short Magnums (and Super Short Magnums, which have faded away) was designed with efficiency in mind. I contacted Mike Stock, centerfire product manager at Winchester Ammunition, and asked him how the case design of the WSMs achieves this increase.
According to Stock, the WSM's case diameter and short overall length result in a more efficient overall burn of the powder, meaning that more powder is burned more quickly. This means that there is very little powder that is left unburned, and this allows the WSM cartridges to achieve maximum velocity in shorter barrels.
"Improved airflow efficiency and a large diameter powder column make these cartridges more efficient," Stock said. "A short, fat powder column is so near the primer that you are getting more burn before the bullet leaves the case."
But Stock said a major factor in the efficiency of current cartridges such as the WSMs is at least also due in part to having more powders from which to choose.
"When the .30-06 was designed, there were only a few powder options," he said. "Today, there are 30 or 40 more, which allows engineers to match new cartridges with the proper propellant."
According to Stock, magnum cartridges "belch a lot of unburnt powder" in short barrels. Modern, more efficient loads allow for maximum velocity from shorter tubes, all while staying within pressure limits.
From a real-world standpoint, that means hunters can carry a rifle that is lighter and has a shorter barrel and can expect the same results they could only achieve with heavier guns with long pipes only a few years ago. Better efficiency also means cartridges can almost match the trajectory of older magnums with less muzzle blast, recoil and powder.
"You can change powders, bullets and barrel length, but there's nothing you can do to change case volume on a particular cartridge," Stock said. "Modern cartridges are designed around modern powders."
These realizations have led to a change in overall cartridge design during recent years, and if you examine new cartridge designs (the WSMs, Ruger's Compact Magnums, the 6.5 Creedmoor and others), there has been a shift in cartridge architecture toward wider, straight-walled cases with sharper shoulders and minimal body taper. That's no accident. It is a direct effort toward getting more out of shorter, more efficient cartridges.
The current upswing in long-range shooting and hunting (regardless of what you think of the latter) means more rifle owners are stretching the limits of their guns, and according to Stock, efficiency should be a concern to long-range shooters.
At extreme ranges, bullet design is very important, and bullets with a high ballistic coefficient buck the wind better and lose velocity more slowly than bullets with low BCs. But serious long-range shooters need to understand why efficiency is critical because the endeavor requires that you are getting the most out of every grain of powder you're burning.
According to Dave Emary, chief ballistician at Hornady, understanding how case design and propellants affect a cartridge is critical to selecting the right cartridge. Newer powders offer hunters and shooters options when developing loads, for instance, and improving efficiency should be a consideration for any reloader.
"Basically, ballistic efficiency is getting the most push you can for the money," Emary said. He says that over-bored cartridges (high powder capacity with a small bore diameter) tend to produce more recoil, more muzzle blast and require longer barrels and slower-burning powders to work effectively.
"More efficient cartridges are typically more accurate because there is less bullet tip-off when the projectile exits the muzzle, and they are more shootable because there is less recoil and muzzle blast. In addition, high velocity bullets reduce barrel life."
For comparison, he points to the .308 Win. versus the .30-06 Springfield. With bullets up to about 165 grains, the .308 delivers similar velocities with five to seven grains less powder.
The advantage of a larger case is that it works better with heavier bullets. Over-bored cartridges shooting lighter bullets are inefficient, and he pointed to the example of the .264 Win. Mag. versus the 6.5 Creedmoor. The larger .264 Win. Mag. requires a long barrel and careful powder selection and produces higher muzzle energy.
In addition, lighter calibers are more shootable and are less likely to spew unburnt (read: wasted) powder from the muzzle.
According to Nosler reloading data, the 6.5 Creedmoor will drive a 140-grain .264-inch bullet at 2,672 fps when using a max load of W760 powder. The .264 Win. Mag., using the same bullet and 54.5 grains of the same powder (also a max load), achieves a muzzle velocity of 2,942 fps. The 6.5 Creedmoor is averaging just under 66 fps per grain of powder while the .264 Win. Mag. is making just under 54 fps with a grain of powder. The .264 Win. Mag. load requires 25 percent more powder to achieve a nine percent increase in velocity over the Creedmoor, and the .264 Win. Mag. requires a longer, heavier barrel to do so.
"Everybody thinks about speed, but that's not the only consideration," Emary said. "A .300 Win. Mag. firing a 150-grain bullet is an extremely inefficient cartridge and is less shootable than smaller rounds."
Understanding ballistic efficiency and cartridge design is important for hunters. In any hunting situation, you are faced with multiple variables that, when combined, result in the success or failure of a particular shot on game — given that the bullet is correctly placed.
Many of these we know and understand; bullet design, shot angle, recoil and muzzle blast (and our ability to handle these without flinching), velocity and even gun weight are critical factors when you are selecting the right gun/load combination for your next hunt. Ballistic efficiency plays a role as well.
For most big game hunting at moderate ranges, the ballistic efficiency of a cartridge plays a greater role in the result of the hunt than you might initially assume. The 7mm Rem. Mag. is a great deer cartridge, but you aren't getting as much bang for your buck as you might think.The cartridge's larger case capacity means that it is capable of firing a projectile much faster than a smaller, more efficient cartridge such as the 7mm-08 Rem.
But that extra power comes at a price, and because it is more efficient, the smaller 7mm-08 has a trajectory curve that is very close to the magnum out to 200 yards, and it can be built on smaller, lighter rifles that kick less and cost less to shoot.
Mountain hunters should be particularly interested in ballistic efficiency, too. When you are climbing and hiking at high elevation, every ounce you carry matters, so you'd better be sure that you aren't carrying any wasted weight. Smaller, more efficient rounds like the 7mm WSM allow you to achieve virtually the same trajectory you would glean from a 7mm Rem. Mag. or Wby. Mag. with a longer case, but they will work on short-action rifles with shorter barrels.
Even dangerous game rifles are benefitting from the new age of more efficient cartridges. Take the .375 Ruger, which was designed by Hornady. Its shorter, fatter case is more efficient than the long, sloping case of the .375 H&H and can be built on rifles with a standard .30-06-length action. A shorter action means a shorter bolt stroke, and shorter bolt stroke means faster follow-up shots.
Ruger and Hornady accomplished this by creating a cartridge that had the same base diameter as the .375 H&H (.532 inch) but had a wider, straighter powder column and a sharper shoulder that increased capacity and efficiency. The result was a short .375 that was more efficient and more powerful than the original.
That's not to say that cartridges like the 7mm Rem. Mag. or the .375 H&H are obsolete. In fact, they're both great choices. Ballistic efficiency perhaps isn't the most important consideration when selecting a cartridge, but it is an essential element of cartridge design that is worth bearing in mind when you select a rifle.
If you can accomplish what you need with less powder, less recoil and a lighter rifle, isn't that an option worth considering?