Compared: The .270 Weatherby Mag. and 7mm Remington Mag.
By Chuck Hawks
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 nine Weatherby factory loads currently offered come with bullets weighing from 100-150 grains.
The .257 WM and 7mm WM are based on the .270 WM case necked-up and down and this trio has always been among Weatherby's best sellers, along with the big .300 WM. 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 Remington Magnum was one of the major cartridge success stories of the 20th Century. Introduced in 1962 in the then new Model 700 rifle, it is based on a .264 Win. Mag. case simply necked-up to accept .284" bullets and, like the .270 Wby. Magnum, it is designed to work through standard length actions. The Model 700 rifle and the 7mm Rem. Mag. were an instant success and the cartridge remains the best selling of all magnums.
The 7mm bullet diameter is .284" and Remington's Big Seven can use all standard 7mm bullets. The 10 Remington factory loads currently available are supplied with bullets ranging from 140-160 grains. Reloaders have easy access to bullets weighing from 100-175 grains. All major ammunition manufacturers offer factory loaded 7mm Rem. Magnum ammunition and it has become a world wide big game cartridge.
Like the .270 Weatherby, it is a flat shooting, all-around cartridge suitable for hunting CXP2 and CXP3 game. 7mm Remington Magnum rifles are normally supplied with 24" barrels and factory load ballistics are taken in a 24" test barrel. The SAAMI MAP is 61,000 piezo psi.
A notable difference between these two cartridges is the availability of rifles and factory loaded ammunition. Most rifle manufacturers' chamber for the 7mm Rem. Mag. and Federal, Hornady, Nosler, Remington and Winchester all offer factory loaded cartridges. Only Weatherby among the major rifle manufacturers chambers for the .270 WM and among the major U.S. ammunition manufacturers, only Federal, Nosler and Weatherby offer cartridges.
For the purposes of this comparison, we are going to stick with representative factory loads. These big case cartridges are probably at their best and most versatile with bullets weighing around 150-154 grains and those are the bullet weights we will compare in this article. Weatherby offers factory loads using the 150 grain Hornady Spire Point InterLock bullet (BC .462) for their .270 and Hornady Custom factory loads are available with the 154 grain Spire Point InterLock (BC .433) in 7mm Remington Magnum. Because these bullets are almost identical in form and capability, these are the factory loads we will use for comparison. We will compare these factory loads in velocity, kinetic energy, trajectory, sectional density (SD), cross-sectional area, killing power and recoil.
Velocity is the most important component of energy. It also decreases bullet flight time and hence flattens trajectory. Many experienced hunters feel 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 .270 WM starts out with a 210 fps velocity advantage at the muzzle and finishes with a 222 fps advantage at 400 yards. Clearly it is the faster cartridge and the advantage is increased by the Weatherby rifle's two inch longer barrel.
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. However, the .270's higher velocity gives it the advantage in kinetic energy.
The 7mm Remington 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 computed for the maximum point blank range of each cartridge/load (+/- 3") and assume an optical sight mounted 1.5" over bore and standard atmospheric conditions. Bullet rise and fall in relation to the line of sight are given from 100 to 400 yards.
The .270 WM has a MPBR of 318 yards when zeroed at 270 yards. The 7mm Rem. Mag. has a MPBR of 297 yards when zeroed at 252 yards. Both can take deer and similar size game with a dead-on hold at 300 yards, but the .270 wins the trajectory comparison.
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 .260 SD required of small bore calibers optimum 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 Rem. Mag. wins the cross-sectional area comparison, although the difference is not pronounced
There are various ways to estimate killing power, all of which are approximations and none of which are entirely accurate. Bullet placement is the most important factor in a cartridge's effectiveness on game and it is a function of the shooter's skill and judgment, not the cartridge itself. The construction and performance of the bullet is also very important, which is one of the reasons why we have chosen to compare bullets of the same design (Hornady SP InterLock).
An attempt to include at least some of the factors relevant to killing power (primarily impact velocity and bullet weight) is the Optimum Game Weight (OGW) formula developed by Edward A. Matunas and published in the Lyman 47th Reloading Manual. Matunas assumed that bullet design and placement are adequate for the task at hand. The numbers below indicate the size of animal (live weight in pounds) for which each load is presumably optimum at ranges from the muzzle to 400 yards. (For more on OGW, see the "Expanded Optimum Game Weight Table" on the Tables, Charts, and Lists Page.)
The .270 WM has an OGW advantage from the muzzle to 400 yards. At 400 yards it still has a 101 pound advantage. If a bull elk weighs 600 pounds, the .270 WM is optimum at 300 yards and the 7mm RM is optimum at 200 yards. The .270 WM clearly wins the OGW comparison.
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.
Shooting a bullet of similar weight at lower velocity and requiring a little less powder gives the 7mm RM a moderate advantage in lower recoil energy and a slight advantage in lower recoil velocity. A difference of 1.7 ft. lbs may not sound like much, but it can be felt by most shooters. As expected, the 7mm/154 kicks less than the.270/150 load, winning the recoil comparison.
Summary and Conclusion
The 7mm Remington Magnum is the most popular and widely distributed of all magnum cartridges in terms of both rifles and ammunition. It also kicks a little less than the .270 Weatherby Magnum and its bullet is moderately bigger in cross-sectional area.
The .270 Weatherby Magnum bests the 7mm Remington Magnum in velocity, energy, trajectory, sectional density and killing power. Both are fine all-around big game hunting cartridges with long and successful records hunting around the world, but if you can stand its extra recoil, the .270 WM has to be considered the winner of this comparison.
Copyright 2012 by Chuck Hawks. All rights reserved.