April 26, 2023
There are innumerable good cartridges. Instead, let’s talk calibers. Bullet diameters. Most North American hunters pursue medium-sized animals. “Deer-sized game” includes all deer, plus pronghorns, sheep, goats, up to caribou and feral hogs. Black bears, elk and muskoxen are larger and tougher. Moose are much larger. Provided we keep big bears and bison out of the discussion, we could probably trim the list to just four sensible calibers: 6.5mm (.264-inch); .270 (6.8mm, .277-inch); 7mm (.284-inch); and .30-caliber (.308-inch).
Within each are at multiple cartridges using various cases. Case dimensions and load pressures dictate the propellent charge. These, in turn, dictate bullet weight and velocity, which is where the power comes from. So, while 6.5mm (.264) is of smaller diameter than .308, a 6.5mm cartridge isn’t necessarily less powerful than a .30. Bullet weight may be the same. 140-grain bullets are offered in all four calibers, but a 140-grain bullet from a 6.5 PRC, .270 Win. or 7mm Rem. Mag. will be faster and deliver more energy than a 140-grain bullet from a .30-30. Because the cases are larger and carry a larger propellant charge.
WEIGHT, SHAPE AND FRONTAL AREA
Even with just four caliber choices, these complicate comparisons. Velocity in feet per second (fps) and kinetic energy in foot-pounds (ft-lbs). Neither tell the whole story. Because of pressure, a heavier bullet cannot be pushed as fast as a lighter bullet. Since the formula that derives ft-lbs uses velocity squared, a lighter, faster bullet may produce greater energy than a heavier, slower bullet. However, a heavier bullet will generally penetrate more deeply on game.
Sectional Density (SD) is an index of a bullet’s weight in relation to diameter. All bullets of the same weight and caliber have the same SD, so all 150-grain .308 bullets have SD of .226, and all 180-grain .30-caliber bullets have SD of .271. SD suggests a bullet’s penetrating capability. A 150-grain 7mm has SD of .266, and a 175-grain 7mm has SD of .310. If of similar construction, these 7mm bullets will penetrate more deeply than .308 bullets of same weight.
Bullet aerodynamics are critical to a projectile’s flight characteristics. The index is Ballistic Coefficient (BC), based on comparison against a “standard projectile.” Most common is “G1 BC,” also a three-digit decimal. Hornady’s 7mm 175-grain Interlock round-nose has G1 BC of .285; their 7mm 175-grain ELD-X has G1 BC of .675. A higher BC indicates the bullet will shed velocity more slowly and resist wind better.
As with the tortoise and the hare, the race isn’t always to the swift. Downrange, a slower bullet with higher BC may overtake a faster bullet with less aerodynamic shape. Current interest in long-range shooting has created a whole class of new low-drag bullets with off-the-charts BCs. When I was young, a BC in the .400s was high. Today, BCs deep into the .500s are common, and some exceed .700.
Not everybody shoots at long range. Some high-BC bullets are designed for great performance on game, some are not. Whether paper or steel, targets don’t care about expansion and penetration. Hunters require terminal performance. The larger the animal, the better performance we need. Regardless of caliber or cartridge, hunters should use bullets designed for hunting, and avoid target bullets, which often means compromising on aerodynamics.
Bullets of larger diameter transfer more energy on impact and create larger wound channels. Thus, hunters should also consider frontal area. At similar velocity, a 150-grain .308 hits harder than a 150-grain .284, although the 7mm bullet should penetrate better.
How much more frontal area makes a noticeable difference? That’s a tough one. With bullets of similar weight and construction, at similar velocity, I believe a .277 bullet hits harder than a .264. I’m less sure that a 7mm (.284) hits visibly harder than a .270 with just .007-inch difference. Step up to .30 (.308), and there’s a difference. I’m not saying .30 caliber is the way to go, but hunters should not ignore bullet diameter—depending on what they are hunting.
For deer hunters, the world is your oyster. Pick a cartridge you admire, in any of our four calibers, choose a good bullet with the characteristics you like, and go bag your buck. Hunters who wish to reach farther, and sometimes hunt game larger than deer, might dig a bit deeper.
Largely thanks to the 6.5 Creedmoor, the 6.5mm is wildly popular today, with faster 6.5s like the PRC coming on strong. The earliest 6.5mms were developed in the `1890s for military use, with fast rifling twists to stabilize long, round-nose bullets. Add modern aerodynamics and the 6.5s offer amazing downrange performance. However, action length often limits our 6.5mms to bullets of around 140 grains. Great for deer, but on the light side for larger game.
The .277 came later with 1925’s .270 Win. For nearly a century a 1:10 twist was traditional, stabilizing bullets up to 150 grains, but not much more. The .270s are awesome for deer-sized game, and a good 150-grain .277 bullet is adequate for elk, though not generous. Because of that 1:10 twist, heavier bullets have not been possible. Now we have new .270 cartridges (27 Nosler and 6.8 Western), specified with faster twists, bringing bullets up to 175 grains and enabling higher BCs. These give the .270 greater range, and an edge on larger game. However, heavier bullets add recoil, and will not produce accuracy in traditional .270s. Start over or re-barrel.
The 7mm also started with 1890s military experiments, using long, heavy bullets, with fast rifling twists. Most 7mm rifles/cartridges can use bullets up to 175 grains. As much as I love my .270s, a 150-grain .270 cannot compete with a 175-grain 7mm on larger game or in long-range aerodynamics. New 7mm cartridges like 28 Nosler and 7mm PRC call for faster rifling twists, with low-drag bullets up to 195 grains. However, the 1:9 twist common in many existing 7mm rifles may not stabilize these longer bullets. Not a problem for most hunters. On game larger than deer, a 175-grain 7mm is a big bullet.
1892’s .30-40 Krag was America’s first .30 caliber, designed for a 220-grain round-nose, barreled with 1:10 twist. Since then, most .30 calibers have used 1:10 twist able to stabilize bullets from 150 to 220 grains. (The .30-30 is an exception, because of lever-action cartridge length limits.) For larger game, a versatile .30 caliber is thus the clear winner, because of heaviest bullets at largest diameter. With heavier bullets at speed, .30s also produce the most recoil, and not everybody needs that much gun.
The .30 caliber hasn’t escaped modernity. Super-high-BC .308 bullets now run up to 250 grains, but you probably can’t shoot them in Grandpa’s .30-06. Extreme-range shooters are using faster-twist barrels and accepting brutal recoil. Our new rifling twist conundrum works both ways: Accuracy is unlikely with a 150-grain deer bullet in a .300 PRC barreled for the new extra-heavy bullets.
Note: A .30 caliber isn’t necessarily the hardest kicker. Recoil, like energy, depends on weight and velocity. If velocity and gun weight are similar, a 148-grain bullet from a 6.5 PRC will kick about the same as a 150-grainer from any .270, 7mm or .30 caliber. If you choose a larger caliber, you aren’t obligated to shoot the heaviest bullets.
Which caliber for you? I have much-loved rifles in all four diameters, but few of us need all of ‘em. Think about your shooting and hunting. For most of us, with modern bullets and rifles/cartridges that shoot them, the .270s and 7mms are the most versatile options. As to which is best, I’m not sure. If I ever decide I’ll let you know.