Shoot better! BC & MV by Ron Spomer

Before you come to blows with your buddies over who has the “Best” rifle, you need to understand two acronyms: bee-cee and em-vee, more commonly seen as BC and MV. These stand for Ballistic Coefficient and Muzzle Velocity. They and they alone determine which rifle/cartridge/bullet combination will yield the best trajectory. They also contribute significantly to POWER, more accurately referred to as terminal kinetic energy.

Over the last century hunters and shooters have argued endlessly over which cartridges were best, meaning which ones shot the flattest, hit the hardest and killed game better than the others. Back in the mid 20th century one of the most common debates raged between advocates of the 270 Winchester and 30-06 Springfield. That, like most cartridge debates, created more heat than light because it’s not about the cartridge as much as the bullet.

Fired from a 6.5 Creedmoor at 2,700 fps, Hornady’s .264, 143-grain ELD-X bullet with a Ballistic Coefficient of .623 is so efficient that it can eventually overcome the initial energy advantage of this 180 grain bullet fired from a 300 Win. Mag. at 3,000 fps.

A cartridge, you see, is a package deal. You get your powder charge, your ignition source (primer) and your projectile (bullet) all gathered in a neat, durable, nearly waterproof brass container. The length, diameter, taper, shoulder angle, rim and neck of this cartridge case (shell) are largely immaterial to ballistic performance. It’s the quantity of powder and weight and shape of the bullet that matter. The idea that a slightly different container shape is going to result in significantly more accuracy, velocity or downrange performance is wishful thinking at best, self-delusion at worse.

So, directive #1 is this: stop obsessing over the cartridge. You might think the 309 Whamslaughter is the shooting gods’ gift to humanity, but nobody else (especially bucks and bulls) cares. It’s the bullet and only the bullet that impresses them. Any cartridge/rifle/sight combination that delivers the right bullet to the right place with sufficient energy to terminate the life force is the right set up. Some do it more easily than others.

Directive #2 is this: start obsessing over the bullet, particularly the bullet’s Ballistic Coefficient and Muzzle Velocity. Those two things have much more impact than do case size and shape.

Why Ballistic Coefficient and Muzzle Velocity Matter

Ballistic Coefficient reflects a projectile’s ability to resist air drag. It is expressed as a number like .250 or .379, .545, .618, etc. The higher the number, the more efficient the bullet. BC is a product of the bullet’s specific gravity (weight) and diameter (caliber) and shape (form factor.) The higher the weight and smaller the caliber and sleeker the shape, the higher the BC number. And the higher the BC number, the farther the bullet will fly before dropping. The higher the BC, the less the bullet will drift in a crosswind. And the higher the Ballistic Coefficient number, the more energy the bullet will carry downrange.

MV is a much more familiar acronym. Hunter’s have been lusting after muzzle velocity ever since the advent of smokeless powder around 1890. What smokeless powder does is provide much, much, much more chemical energy than blackpowder can. As smokeless powder burns (very rapidly) it converts to a gas which requires way more space than the solid did. To get this space, the gas pushes against its confinement, breaking through at the weakest point. In a rifle barrel, ideally, this weakest point is the bullet blocking the bore. Gas pressures as high as 65,000 psi (depending on the cartridge) drive bullets from muzzles at speeds as high as 4,200 fps. When the obstruction (bullet) clears the bore, the rapidly expanding, high-pressure gases suddenly break free into the atmosphere with a huge POP that can hit 160 dB of ear-ringing sound.

The higher the gas pressures and lighter the bullet, the faster the Muzzle Velocity. The instant the bullet springs free of the barrel will be its peak velocity. From there it’s literally all downhill. Why? Because of air drag.

Inconsequential as air might seem, it is a major factor limiting how far and accurately we can shoot. Gravity pulls all bullets down at a rate of 32 feet per second per second, meaning the fall is accelerating. That wouldn’t matter much if velocity remained constant. But air drag slows bullets considerably, reducing velocity by half after about 1,000 yards. That reduces how far the projectile can fly before it falls below the target. A 178-grain Hornady ELD-X .308 bullet (BC .545) shot at sea level from a 300 Win. Mag. at 3,000 fps and zeroed at 250 yards will slow to 1,531 fps at 1,000 yards and fall 313.15 inches in 1.41 seconds. But guess what? A 115-grain .243 Berger bullet (BC .545) fired from a 243 Winchester at 3,000 fps at the same target will slow to 1,531 fps and fall the same 313.15 inches. Both bullets will also deflect 70 inches in a 10 mph right angle wind.

300 Win. Mag., 178-gr. Hornady ELD-X

243 Win., 115-gr. Berger VLD Target

Let’s restate that: Both the big, 178-grain .308 bullet and the little 115-grain .243 bullet fly the exact same trajectories!

How Ballistic Coefficient and MV Equal Identical Performance Regardless of Cartridge or Caliber

Here’s a key to resolving much of our traditional cartridge arguments: Ballistic Coefficient and MV determine absolutely everything about ballistic performance except accuracy and downrange energy, and they contribute to those, too. We’ll see how later. For now, here’s the BIG REVELATION: As long as BC and MV are the same, the trajectory curve will be the same regardless the caliber or weight of the bullet.

This is a shock to most shooters, as it was to me when I first learned it. But it’s true. Put another way, an 80-grain .224 bullet with a BC of .457 at 3,300 fps will shoot as flat and drift as much as a 300-grain .375 bullet with a BC of .457 at 3,300 fps. There’s a huge different in mass between an 80-grain bullet and a 300-grain bullet, yet they fly the same flight path!

Read that previous paragraph again and think about it. This information can save you considerable expense in powder and bullets. What it means is you can shoot a lighter-recoiling rifle and get the same trajectory curve as bigger magnums. You just have to use projectiles with the same Ballistic Coefficient and Muzzle Velocity. You want to shoot steel gongs at 700 yards? 1,000 yards?  1,500 yards? A hot .224 or .243 centerfire could do it just as well as a big 300 or 338 magnum with a lot less recoil, a lot less powder, a lot less copper and lead, and a lot less cash outlay.

Because this 178-grain Hornady ELD-X has the same Ballistic Coefficient rating as the 115-grain Berger 6mm, they will both fly the same trajectory if fired at the same MV. The only difference will be retained energy. The heavier bullet wins that category.

Ballistic Coefficient and MV Don’t Provide Equal Energy

Here’s where BC and MV don’t win the day: Kinetic energy — the amount of “punch” carried by a bullet — is not equalized by Ballistic Coefficient and MV. A heavier bullet at a given MV will always leave the station carrying more energy than a lighter bullet. If BC is identical, the heavier bullet will retain this advantage. But if BC of the heavier bullet is significantly lower than that of the light one, the lighter one can eventually equal and even exceed the retained energy of the heavier one. This is how the little 6.5 Creedmoor with a high BC 143-grain bullet at 2,700 fps MV can out-punch many 180-grain bullets fired from a 300 Win. Mag. at 2,900 fps. Note in the ballistic charts below under the ENERGY category how the 6.5 retains more energy at 1,000 yards, even though it starts out with 1,282 f-p less.

6.5 Creedmoor, 143-gr. Hornady ELD-X bullet, .623 BC.

300 Win. Mag., 180-gr. bullet, .420 BC.

How to Apply Ballistic Coefficient and MV and Bullet Mass in the Field

So how do you strike the right balance between caliber, cartridge, BC, MV, bullet weight, recoil and performance? First, determine what you need to accomplish. Then figure out the terminal energy level you think you’ll need to do that. There are no hard and fast rules about game-killing punch. Some insist you need 1,500 f-p impact energy to cleanly take an elk, yet many, many elk are routinely dropped with 1,000 f-p and often less. Bow hunters take elk with less than 100 f-p energy. Unless you strike the central nervous system or get lucky with hydrodynamic shock, bullets (and arrows) kill via hemorrhaging. An expanding .224 bullet through the heart or even lungs can do that as effectively as or better than a non-expanding 50-caliber slug. Of course, more energy and a bigger slug always provide more insurance. The safest bet is to shoot the biggest, highest BC bullet at the highest MV you can handle accurately without flinching. But if a big magnum makes you flinch and miss, you’re better off with a lighter recoiling rifle throwing an efficient bullet equally fast. Accuracy first, then high BC and MV. Those are reasons why light recoiling cartridges like the 243 Winchester, 6mm Creedmoor, 6.5 Creedmoor and 260 Remington help many hunters shoot more accurately. And accuracy is job one.

Finally, if you’re just shooting long-range targets or small game, throw up your hands and shout Hallelujah!  You are free to work with light-recoiling, small calibers and light bullets with high BC ratings. They’ll shoot just as flat, just as far, drift just as little as the big rounds with the same Ballistic Coefficients and Muzzle Velocities. With less recoil, you’ll shoot better and enjoy it more.

Conclusion

Ballistic Coefficient and Muzzle Velocity determine projectiles’ trajectories. Maximize both to minimize drop and drift and maximize retained energy.

The author often practices with mild-recoiling 22 centerfires set up to match the trajectories of his larger-caliber big game rifles, saving wear and tear on his body and wallet. 

Posted by jomh