In the world of long range shooting, 6.5mm cartridges are fantastic. They offer lower recoil and superior ballistics to almost any service-rifle-sized cartridge in existence. Is this because there is something magical about 6.5mm (.264″) diameter bullets? No. The reason is simple.
Over 100 years ago, when cartridges such as 6.5×55 Swedish and 6.5×52 Mannlicher-Carcano came into existence, they were from the very beginning loaded with long (with respect to diameter) bullets. Ballisticians and researchers from this time period had realized the potential for such a cartridge, and 6.5mm bores were soon found in the hands of soldiers in various countries around the world. These long bullets offered a high ballistic coefficient, enabling the bullets to fly through the air with less wind drift and drop than other cartridges of the era. Because they were lighter than the projectiles used in .30 caliber service rifles, the 6.5s offered lower recoil given the maximum safe chamber pressures and resulting velocities of the era, too.
These long bullets required fast twist rates (say, 1 in 8″ or so) to stabilize, and so as the years passed and commercial manufacturers looked at making rifles in popular military calibers, they too produced rifles with fast twist rates to stabilize the common ammunition types. These included military surplus ammunition and commercial loads based on the military ballistics.
At the same time that 6.5 rifles were being manufactured with fast twist rates, the seeds of what would become traditional twist rates for other cartridges were being planted. Commercial rifles in other calibers – such as .30 cal rifles which dominated the world’s service rifle production for most of the 20th century, if only due to simplified logistical support – have relatively slower twist rates and most cannot stabilize bullets which compare to the long 140gr 6.5s. Those that can must launch these long and heavy .30 caliber bullets at high velocities in order to compete at long range. Only when you get to .300 Winchester Magnum does the .30 cal shoot as flat as most 6.5s, and only with the heaviest bullets, resulting in greatly increased recoil.
As a result, history has provided the 6.5 diameter cartridges with a solid manufacturing and ballistics background for the purpose of maintaining effectiveness at long range. The older 6.5 cartridges have mostly fallen to the wayside, with the notable exception of 6.5×55 Swedish, the most powerful of the old 6.5s, and one of the longest. Modern technology and increased chamber pressures have enabled ballisticians to match the power of 6.5×55 Swedish in a shorter cartridge, such as .260 Remington, which is essentially a necked down .308, and .260 Creedmoor, which is very comparable to .260 Remington with a few advantages relating to brass life and overall length.
Even shorter 6.5s have come out, though, such as 6.5 Grendel. Made to fit inside the AR-15 platform, the 6.5 Grendel has approximately 50% less case capacity than the .260 Remington, and thus lags significantly behind other 6.5mm cartridges in terms of long-range ballistics. As a result of this lack of power, 6.5 Grendel cannot propel the long 140gr bullets to significant velocities, even from 24″ barrels – 4″ longer than a standard M16, significantly reducing maneuverability. Lighter 123gr bullets are still effective from 6.5s, but have more wind drift. At, say, 1200 yards, where 6.5 Creedmoor and .260 Remington are still going strong, 6.5 Grendel projectiles are dropping rapidly. Interestingly enough, the 6.5 Grendel even offers inferior external ballistics to a .308 Winchester firing a 155gr Lapua Scenar bullet.
There are problems with the adaptation of the cartridge to the weapon, too. The 6.5 Grendel case has a much larger case head/rim (the same as 7.62×39) than 5.56, which most AR-15s are chambered in, and this weakens the bolt significantly. Given a harsh firing schedule – such as that which might be seen by a military unit – the 6.5 Grendel bolts would start to fail much sooner than the 5.56 bolts. In addition, a different magazine is required.
Does this mean that 6.5 Grendel is absolutely useless? No. Due to its similarities to 7.62×39, converting AK platform rifles to – or manufacturing new AK platform rifles in – 6.5 Grendel is relatively easy and does not offer the same drawbacks as the AR platform. Also, 6.5 Grendel is far superior to 7.62×39 in terms of external ballistics, and extends the effective range of the rifle by more than double.
For the most part, 6.5mm cartridges have a lot to offer over cartridges of comparable length. However, as stated in the beginning of this article, the diameter itself isn’t magic. A properly stabilized high BC bullet at a high velocity will perform well at incredible ranges. Most 6.5s can do this and offer relatively low recoil, too. Other 6.5s – such as 6.5 Grendel – fall short, but still have uses.
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