The 7mm Remington Magnum
Date: Aug 02 2007
It is probably almost impossible for anybody younger than 40 to understand the way the 7mm Remington Magnum took over the big game hunting world in the 1960s. When introduced in 1962 it was supposedly the supreme all-around cartridge: as hard-hitting and flat-shooting as most .300 magnums, yet it only kicked about like a .30-06.
Part of the reason for all the excitement was the original factory ballistics: a 150-grain bullet at 3,260 fps and a 175 at 3,020. These did match up pretty well with the 180-grain load of the .300 H&H, then listed at 2,920 fps, and even when the .300 Winchester Magnum was introduced a year later it didn't beat the 7mm Remington by much: a 150 at 3,400 and a 180 at 3,070 fps.
Any student of today's factory ballistics can see that all those loads have been reduced – and the 7mm Remington Magnum's most of all. Today's standard factory 150- and 175-grain muzzle velocities are 3,110 and 2,860 fps.
Partly today's standard factory loads are slower because they're chronographed in 24-inch instead of 26-inch barrels (standard practice in 1962, even when 26-inch barrels weren't available on many factory rifles). But another reason, especially applicable to the 7mm Remington Magnum, is that when the shooting industry made the switch from the copper-crusher pressure gauge to the more accurate piezo-electric gauge, it was found that the 7mm Remington Magnum often showed relatively wide swings in pressure of 10,000 psi or more. (The common .30-06, on the other hand, will normally show maximum pressure variations of around 5,000 psi with the same load.) Nobody knows exactly why this happens with the 7mm Remington, but it's been widely known among laboratory ballisticians for decades.
Now, the maximum pressure for any SAAMI cartridge is two parted. There's the maximum average pressure and the maximum pressure allowable for any individual round. This is the reason the maximum average SAAMI recommended pressure for the 7mm Remington (61,000) is lower than for most other magnums. The .300 Winchester Magnum, for instance, has a maximum recommended pressure of 64,000 psi, and the .300 WSM, 65,000.
Now, these pressure swings are not something you can see, whether by examining the primers of fired cases or any other home method or pressure guessing. And they are not dangerous – as long as you use recent loading data. Older loading data worked up with copper crushers should not be trusted. Also, the 7mm Remington Magnum is a worldwide cartridge, and throat dimensions can vary depending on the manufacturer. So use only the most up-to-date data, and work up carefully, especially if your 7mm Remington Magnum was made outside the U.S. If your velocities greatly exceed that of factory ammunition, back off.