Compared: The .270 Weatherby Magnum and 7mm Weatherby Magnum
By Chuck Hawks
You've wanted a Weatherby Mark V rifle and now, at last, you're ready take the plunge. You want it chambered for an all-around cartridge, but you think the long Weatherby magnums, the .300, .340, etc., generate more recoil that you wish to deal with. That means a standard (.30-06) length Weatherby Magnum cartridge and in all-around cartridges, the choice inevitably comes down to either the .270 Weatherby Magnum or the 7mm Weatherby Magnum. Both cartridges use standard length belted magnum cases with .530" rim and belt diameters, .512" head diameter and a 2.549" case length. Future Weatherby owners, this comparison is for you.
The .270 Weatherby Magnum
The .270 Weatherby Magnum is the original Weatherby Magnum, dating back to 1943. It was the first cartridge introduced by Roy Weatherby and many users would say it is still the best. It is the lightest kicking Weatherby caliber with all-around (CXP2 and CXP3) game getting capability. It is a standard (.30-06) length cartridge based on a shortened, blown-out and necked-down .300 H&H belted case given a Weatherby double radius shoulder. The bore diameter is .270" and the bullet diameter is .277", the same as the .270 Winchester.
Reloaders can use any standard .277" diameter bullet, generally ranging from 90-160 grains. The eight Weatherby factory loads currently offered come with bullets weighing 100, 130, 140 and 150 grains. Federal Premium offers a single .270 Weatherby factory load using a 130 grain Trophy Bonded Tip bullet at 3200 fps. Hornady, Remington and Winchester do not offer .270 Wby. Mag. factory loads.
Weatherby .270 Magnum rifles come with a 26" barrel and Weatherby factory load ballistics are derived in a 26" test barrel. The SAAMI maximum average pressure (MAP) for the .270 Weatherby is 62,500 piezo psi.
The 7mm Weatherby Magnum
The 7mm Weatherby Magnum was also designed to work through standard length actions. The cartridge is based on the .270 WM case necked-up to accept standard 7mm (.284" diameter) bullets. Like the .270 Weatherby, it is a flat shooting, all-around cartridge suitable for hunting CXP2 and CXP3 game.
Interestingly, the 7mm Weatherby was a poor seller until the introduction of the similar 7mm Remington Magnum, an over night success that became the world's best selling magnum cartridge. The success of Remington's Big 7 made shooters and hunters take a second look at the 7mm Weatherby Magnum, which offers slightly higher performance than the Remington offering in the same size package. The sales of the 7mm Weatherby picked-up.
Weatherby offers eight factory loads for their 7mm, loaded with bullets weighing 120, 139, 140, 150, 154, 160 and 175 grains. Federal Premium offers a single load in the caliber, loaded with a 160 grain Trophy Bonded Tip bullet. Reloaders have a wide variety of bullet styles and weights from which to choose, ranging from about 100-175 grains.
Weatherby 7mm Magnum rifles come with a 26" barrel and Weatherby factory load ballistics are derived in a 26" test barrel. The SAAMI maximum average pressure (MAP) for the .270 Weatherby is 65,000 piezo psi.
The .270 and 7mm Weatherby Magnums are long range, all-around cartridges. For the purposes of this comparison, we will stick with representative Weatherby factory loads. Both of these cartridges are probably at their best and most versatile with relatively heavy for caliber bullets weighing about 150 grains. In .270, Weatherby offers a factory load using a 150 grain Hornady InterLock Spire Point bullet. In 7mm, Weatherby offers a factory load using the very similar 154 grain Hornady InterLock Spire Point bullet, so these are the factory loads we will compare.
We will compare these two cartridges in velocity, kinetic energy, trajectory, sectional density (SD), bullet frontal area, killing power and recoil. The two cartridges are available in the same Weatherby rifles and ammunition is equally plentiful, so availability of rifles and ammunition are not factors. We will conclude with a summation of this comparison and a few comments about the suitability of these calibers for their intended purpose of hunting Class 2 and Class 3 big game.
Velocity is the most important component of energy. It also decreases bullet flight time and hence flattens trajectory. Many Weatherby owners believe that high velocity per se contributes to killing power, but this has been difficult to prove scientifically. Here are the velocities from the muzzle (MV) to 400 yards in feet per second of our selected loads.
The 7mm starts out with a 15 fps velocity advantage at the muzzle, but at 100 yards the two bullets are moving at the same speed and from there on the .270 has a very slender advantage. At 400 yards the .270 bullet is moving 37 fps faster. In practical terms, neither cartridge has a significant velocity advantage.
Kinetic energy is the measure of a bullet's ability to do work. The "work" in this case is expanding and penetrating deep into a game animal to destroy the maximum amount of tissue and kill quickly. Energy is an important factor in cartridge performance and killing power and a good indication of the power of similar rifle cartridges. Here are the energy figures in foot pounds for our comparison loads from the muzzle (ME) to 400 yards.
Since about 800 ft. lbs. on target with a suitable bullet is the accepted standard for humanely harvesting CXP2 game and 1200 ft. lbs. is a reasonable minimum for CXP3 game, it is clear that both cartridges carry plenty of punch. The 7mm has a relatively minor 127 ft. lb. advantage at the muzzle. The .270 slowly eats away at the 7mm advantage until, at 300 yards, the two are virtually equal (only 10 ft. lbs. apart). At 400 yards, the .270 has attained a tiny 9 ft. lb. advantage in energy. As we found with velocity, in energy the two cartridges are functionally tied.
The 7mm and .270 Weatherby Magnums are flat shooting cartridges. The flatter a bullet shoots the less the shooter needs to compensate for bullet drop and the better his or her shot placement is liable to be. The following trajectories (in inches) are taken from the Weatherby catalog and are based on a 300 yard zero, computed for a sight line 1.5" over the center of the bore. Bullet rise and fall in relation to the line of sight are given from 100 to 400 yards.
The trajectories of these two cartridges and loads are nearly identical. Only a difference of 0.1" separates then at 400 yards. The trajectory comparison is a tie.
Sectional density is defined as the ratio of a bullet's weight in pounds to the square of its diameter in inches. SD is important because the greater the SD, other factors being equal, the deeper a bullet's penetration. Penetration is an important factor in the length of the wound channel and the amount of tissue disrupted. Obviously, to kill quickly a bullet must have sufficient penetration to reach and disrupt the animal's vital organs. Here are the SD numbers for our .277" and .284" bullets.
Both are well above the .225 SD considered excellent for CXP2 game and the .250 SD required of small bore calibers for CXP3 game. The .277/150 grain bullet slightly shades the .284/154 grain bullet in this important specification. However, in the field they are both in the same class.
Greater cross-sectional area means that, other factors (such as the percentage of bullet expansion) being equal, the fatter bullet should create a wider wound cavity, damaging more tissue and hastening the animal's collapse. Here are the cross-sectional areas of our two bullets in square inches.
It should be obvious that a .284" diameter bullet has a greater cross-sectional area than a .277" diameter bullet. The 7mm Wby. Mag. wins the cross-sectional area comparison, although the difference is not pronounced. Both sectional density and cross-sectional area directly influence the extent of the wound cavity in a game animal. In terms of wound cavity volume, an advantage in SD tends to compensate for a disadvantage in cross-sectional area and vice-versa.
Killing power is the most difficult factor to quantify. Bullet placement is by far the most important factor in a cartridge's effectiveness on game and that is a function of the hunter's judgment and skill, not the cartridge used. The construction and performance of a hunting bullet is also very important and practically every type of hunting bullet is available in both .284" and .277" diameter. Kinetic energy is one indication of potential killing power, as are bullet frontal area and sectional density, but none of these tells the entire story.
There are many ways to estimate the killing power of big game cartridges. Some seem to correlate with observed results in the field and some are simply concocted to promote the author's point of view. One of the newer, and better, methods of estimating killing power is the "Hornady Index of Terminal Standards" (HITS), which includes factors such as bullet weight, sectional density, ballistic coefficient and impact velocity. HITS are normally calculated for a range of 100 yards, a typical distance for shooting large game. Here are the approximate HITS scores for our comparison loads.
A HITS rating of 901-1500 is considered adequate for large game, such as elk and red stag. Both cartridges are clearly adequate for killing large game at ranges in excess of 100 yards. The 7mm Mag's four grain advantage in bullet weight translates to an inconsequential six point davantage in HITS advantage. Again we have a functional tie, as no animal can live on the difference.
Recoil is always an important consideration, as anyone can shoot better with a cartridge that kicks less. Remember that bullet placement is the most important factor in killing power. Presumably, one of the reasons for choosing either of these cartridges over, say, a .300 Magnum is to minimize recoil. Here are the approximate recoil energy (in ft. lbs.) and recoil velocity (in fps) figures for our comparison loads, measured in 8.5 pound hunting rifles.
A difference of 2 ft. lbs. and 0.6 fps may not be dramatic, but it can definitely be felt by most shooters and, as recoil exceeds about 20 ft. lbs. it becomes increasingly distracting. In terms of lower recoil, the .270/150 load wins the recoil comparison.
Summary and Conclusion
The .270 Weatherby Magnum and 7mm Weatherby Magnum are too similar in velocity, energy, trajectory and killing power to choose a winner. The 7mm is somewhat superior in bullet cross sectional area and the .270 wins the SD comparison by a small margin. Only in the .270's lower recoil is there a clear difference and even that is not pronounced.
Both are fine all-around big game hunting cartridges with long and successful records hunting around the world. They are available in the same rifles, at the same price and have the same number of factory ammunition choices. Perhaps, mostly because of its lower recoil, the .270 is the slightly more logical choice. However, the real decision will probably hinge on whether you are a .270 fan or a 7mm fan. Certainly, no game animal can live on the difference.
Copyright 2013 by Chuck Hawks. All rights reserved.