6.5mm, .270 and 7mm: The Golden Mean in Hunting Calibers
By Chuck Hawks
6.5mm, .270 and 7mm encompass what most of us at Guns and Shooting Online consider the ideal caliber range for hunting CXP game. Unlike the .24 and .25 calibers, these cartridges use serious big game hunting weight bullets, typically about 120 to 160 or 175 grains. They shoot flat enough to take game without "holding over." Unlike most of the larger calibers, standard (non-magnum) cartridges in these calibers typically generate relatively moderate recoil that allows most hunters to do their best shooting.
What you've got is a selection of flat shooting, mild kicking, standard cartridges that will kill CXP2 game with authority. In North America, that means animals such as feral hogs, pronghorn antelope, whitetail deer, blacktail deer, mule deer, goats, sheep and caribou. In Africa they serve nicely as plains game calibers. (The 7x57 is especially admired in southern Africa, where it has been in widespread use for well over 100 years.) In addition, they are sufficiently versatile to bring down CXP3 game (elk and even moose) with appropriate loads and careful shot placement.
The standard cartridges between 6.5mm and 7mm that are reasonably well known in North America include the 6.5x55mm SE, .260 Remington, 6.5mm-284 Norma, .270 Winchester, 7mm-08 Remington, 7x57mm Mauser and .280 Remington. The largest and most powerful of these are the 6.5-284, .270 and .280. The 6.5mm-284 won its spurs as a premier long range match cartridge. It is based on a necked-down .284 Winchester case, the capacity of which is almost identical to a .30-06 case. The famous 270 and .280 are based on necked-down .30-06 cases. The .260 and 7mm-08 are based on the short action .308 Winchester case, necked-down. The 6.5x55 and 7x57 are based on intermediate cases that are in-between the .308 and .30-06 in length.
The typical medium weight bullet for all of these calibers is around 140 grains, which should tell you something about their similarity. Operating at similar maximum average pressure (MAP), the .260, 6.5x55, 7mm-08 and 7x57 can launch 140 grain bullets at muzzle velocities (MV) between 2650-2850 fps, which means a +/- 3" maximum point blank range (MPBR) of 260-285 yards. That allows shots on big game animals to around 300 yards without holding over the animal's back, which is considerably farther than most hunters can reliably score fatal hits. (According to a recent SCDNR study, the percentage of missed or wounded deer climbs dramatically when hunters attempt shots at 150 or more yards.) The greater powder capacity of the 6.5mm-284,.270 and .280 allows them to launch 140 grain bullets at 2900-3000 fps, which results in a MPBR extending from 286 yards to almost 300 yards.
Here are typical trajectories, computed for a 1.5" sight height and (for purposes of comparison) a 250 yard zero:
In terms of killing power, the typically higher impact velocities of the 6.5mm-284, .270 and .280 may have somewhat more "shock" effect on smaller size animals at medium range and they hit harder at long range. Approximately the same killing power can be had, with less muzzle blast and recoil, from the 6.5x55, .260, 7x57 and 7mm-08 at shorter distances. (The high velocity cartridges hit with similar kinetic energy at 300 yards as the standard velocity numbers do at 200 yards.) Here is an energy comparison of the 7x57 Mauser at MV 2700 fps and .280 Remington at MV 2950 fps, using the Hornady 139 grain Spire Point bullet (BC .392):
There is surprisingly little difference in the ultimate killing capability of any of these calibers. Those of us on the Guns and Shooting Online staff own and use rifles in all of these calibers and can testify that no CXP2 animal can live on the difference between them. W.D.M Bell, the famous African ivory hunter, slaughtered hundreds of elephants with brain shots using the modest 6.5x54mm and 7x57mm cartridges with (respectively) 160 and 175 grain bullets. The 7x57mm Mauser, which he called the .275 Rigby, became his favorite elephant cartridge! (The 7x57 was also known as the .275 Rigby because Mauser supplied bolt actions to British rifle builder Rigby, who sold them under their name and cartridge designation.)
The actual bullet diameters are .264" (6.5mm), .277" (.270) and .284" (7mm). Comparing 140 grain bullets in these calibers, the .264 bullet has the highest sectional density (SD), while the .284 bullet has the biggest frontal area. The .277 bullet lies between these extremes. (For comparison, the highly effective 150 grain .308 bullet fired from cartridges such as the .30-30, .308 Win. and .30-06 has a SD of .226.) Here are the SD's for 140 grain bullets in our three calibers:
Here are the bullet cross-sectional areas for our three calibers:
If you were to test otherwise identical 140 grain bullets (non-expanding solids, for instance) in each caliber at the same impact velocity in the same test medium, the 6.5mm bullet should create the longest cavity, while the 7mm bullet should create the widest cavity. The .277 bullet should be in the middle in both areas. In the field, if the bullet does not exit the animal, the depth of the wound channel and its diameter seem to more or less balance in terms of the total amount of disrupted tissue. Your deer will not be able to tell the difference.
Due to its higher SD, the 6.5mm bullet should have the highest (best) ballistic coefficient and the 7mm bullet the lowest, given bullets of the same shape. For example, comparing Federal/Speer Trophy Bonded Bear Claw (TBBC) bullets, here are the BC's:
The TBBC is a Mag-Tip, flat base, spitzer bullet. Sharply pointed boat-tail bullets (Nosler Ballistic Tip, Swift Scirocco, Sierra GameKing, etc.) will have higher BC numbers. In practice, the difference in BC is not significant over typical big game hunting ranges.
Given these facts, you may have reached the conclusion that the differences between these calibers, when shooting similar bullets, are inconsequential. That is also the opinion reached by most of us on the Guns and Shooting Online staff. As hunting cartridges, the 6.5mm-284, .270 and .280 are as alike as peas in a pod. Ditto for the 6.5x55, .260, 7mm-08 and 7x57 when loaded at the same MAP. No matter which caliber you choose, you will have the confidence that comes from hunting with a cartridge that will not kick you out from under your hat and is respected around the world.
Copyright 2011 by Chuck Hawks. All rights reserved.