Some shooters believe there’s something magical about 6.5mm bullets. As proof, they ask us to examine .25 caliber, the bore size closest to 6.5mm. The highest ballistic coefficient for any .25 caliber bullet is under .500, yet several 6.5mm bullets have BCs over .600. See? Magic.
The E.R. Shaw MK-VII action is basically a Savage bolt-action, including the AccuTrigger, but with the front of the action threaded so that a barrel can be conventionally fitted.
Well, no, not really. Despite what True Believers suggest, the reason for the ballistic efficiency of 6.5mm bullets is not mystical, like the Mona Lisa’s smile, but simple physics. The explanation goes back to the beginnings of smokeless powder in the last two decades of the 19th century, when armies around the world equipped themselves with new cartridges. The prevailing notion was that a military bullet should be long, heavy, and blunt-nosed, in order to penetrate well. This was the basic form of bullets used in black powder rifles, and this thinking carried over into the newfangled smokeless rounds.
These could be of much smaller bore diameter than the black powder rounds, and the overall cartridge could be much smaller as well. Both were advantages, since more rounds could be transported by individual soldiers, pack mules, wagons, and trains.
Military rifles from several countries were chambered for smokeless 6.5mm cartridges, most with round-nosed bullets around 160 grains in weight at a muzzle velocity of 2,200 to 2,500 fps. In order to stabilize such a long bullet, at relatively modest velocity, the rifling twist had to be about one turn in 8 inches.
Then, early in the 20th century, sharp-pointed bullets were developed by the German military for the 8x57 Mauser. These “spitzer” bullets were copied by other countries, and it was found that 6.5mm cartridges and rifles could stabilize very long, ballistically efficient spitzers. Since all the countries that used 6.5mm cartridges also used the metric system, they usually settled on a 9-gram bullet. This is just about 139 grains, the metric equivalent of the 140-grain spitzer cited as being magic by 6.5mm enthusiasts.
This bullet proved to be very efficient at longer ranges, but this wasn’t because of any inherent magic in 6.5mm bullets. Most cartridges designed for sporting (not military) could use slower rifling twists, because commercial cartridge designers emphasized bullet velocity rather than bullet weight. An extreme example is the .250-3000 Savage, introduced around 1912, the first cartridge to achieve the muzzle velocity of 3,000 fps. The lone original factory load featured an 87-grain spitzer, which required only a 1:14" rifling twist.
Later .25 caliber rifles (and even some .250-3000s) used a 1:10" twist, but if .25 caliber rifles had started out in life with 1:8" twists, they could have used heavier bullets. A .257 spitzer weighing 133 grains would be the ballistic equivalent of a 140-grain 6.5mm, but a 133-grain spitzer won’t stabilize in the standard 1:10" twist of most .25 caliber rifles. The most ballistically efficient .25 caliber spitzers weigh around 115 grains.
Despite the “magic” of their faster rifling twists, 6.5mm cartridges never have been tremendously popular in North America. Mexico used the metric system, but chose the 7x57 as their first smokeless military cartridge. Canada was connected to Great Britain, so ended up with the .303 Enfield. The U.S. stayed firmly .30, with the .30-40 “Krag” and .308 Winchester acting as historical bookends to a fairly popular cartridge originally called the “Ball Cartridge, caliber .30, Model of 1906.”
Perhaps the first American 6.5mm cartridge, the .256 Newton, would have become popular if the Newton rifle companies hadn’t failed, because of World War I. But I still doubt the .256 would have made the grade, since the mighty Winchester Repeating Arms Company already had plans for a variation on the .30-06.
Before the war Winchester started developing a new cartridge for a bolt-action rifle they intended to offer. This design used a case much like the .30-06’s and a 0.288" bullet, a diameter favored by British firms. The project was put on hold during the war, when Winchester ended up producing military rifles for both the U.S. and Great Britain. After the war Winchester decided to change the bullet diameter to 0.277" and the .270 Winchester Center Fire (as it was originally called in 1925) pretty much pre-empted the 6.5mm market in North America, because its original factory load was a 130-grain bullet at over 3,100 fps. In most hunters’ minds this beat the heck out of a 160-grain roundnose at less than 2,500 fps.
Since then there have been quite a few 6.5mm cartridges introduced here, but none have proven as popular as the .270 Winchester. The 6.5 that’s come closest is the 6.5x55 “Swedish Mauser,” a military cartridge actually codesigned by Sweden and Norway for use in Model 96 Mauser and Krag-Jorgensen rifles. Cheap mil-surp 96 Mausers are the primary reason for the 6.5x55’s popularity, though it is an excellent cartridge and has been chambered in some commercial American hunting rifles.