What does each caliber offer to a potential general purpose cartridge (GPC), especially one designed for military use? The larger calibers offer higher bullet weight. Those heavier bullets tend to have higher ballistic coefficients (BC), which reduces drift due to the wind and helps maintain velocity over long ranges. The smaller calibers offer higher velocity, and that velocity reduces bullet drift and drop. The ideal bullet would have high velocity, high weight, and a high BC, with resultant lower numbers for drift and drop as well as effectiveness at long ranges.
Such a cartridge and bullet exists, in the .300 Win Mag and .338 Lapua. But recoil is high, far too high for a general purpose rifle (GPR). And the rifle itself has a greater weight and barrel length than would be desirable for a GPR. So when we look for the right bullet and cartridge for general use, we need to make some trade-offs to lower recoil and allow for a lighter rifle with a shorter barrel.
Below is a chart of several common calibers, evaluated according to the distance at which the velocity falls to 1250 fps, the bullet energy falls to 400 ft lbs, a bullet drop of 8 feet (96 inches) is reached (using a 200 meter zero), and a wind drift of 4 feet (48 inches) is reached.
The muzzle velocity for each round was chosen based on a desired muzzle energy of 1800 ft lbs, which is intermediate between the 5.56 and 7.62 NATO rounds. BCs are Litz G7 values, except for one Lapua bullet whose trajectory is from Lapua’s Coefficient of Drag data. The JBM ballistics program was used to evaluate each bullet choice: http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
The highlighted bullets are the best choice in each of 4 calibers. The .277 caliber did not make the grade because the higher and lower calibers were each clearly superior. And the .308 rounds simply do not have enough muzzle velocity, given the limitation of 1800 ft lbs of muzzle energy.
The top .224 caliber bullet (5.56 mm) is the heavy-for-caliber 90 grain Sierra MatchKing, with a G7 BC of .257. Yet the bullet is still light enough to allow for a high velocity (3005 fps) within the 1800 ft lb muzzle energy limit (set somewhat arbitrarily for this comparison). The effective range of this round is 915 to 1035 meters, considering the terminal velocity and terminal energy.
This round does present some problems, though, as a candidate for a GPC. The bullet weight is too light to be a viable replacement for any of the 7.62×51 rounds. Terminal effectiveness is based on many factors, but bullet weight is important. A 90-grain pistol round is considered light for self-defense purposes. From a rifle at close range, the 90 grain bullet is sufficient. But it quickly loses effectiveness as range increases and velocity drops. At long ranges, a rifle round behaves more like a pistol round; it might not be traveling fast enough to fragment or tumble. So its effectiveness depends more on bullet weight than at close range.
Another issue is that any design changes to the bullet, such as adding a steel tip or using a lead-free design, quickly drops the BC to a less than optimal level. The minimum barrel twist needed for a 90 grain SMK is a rather tight 1:6.5 inches (according to Sierra). And the 5.56 NATO cartridge is not compatible with this bullet. It would require the COAL of the AR10 platform and a larger brass case, also, for sufficient velocity.
Another problem endemic to all the lighter calibers is that the high velocity needed to make the round effective increases the wear of the barrel, shortening its service life. Overall, the .224 caliber has too many disadvantages.
At the other end of the spectrum, the .284 caliber (7 mm) bullets tend to have the opposite problem. The bullets are heavier than is needed for terminal effectiveness. The lighter bullets in this caliber have poor BCs. The heavier bullets reduce velocity too much, or offer excessive recoil. And when a high BC bullet has low velocity, it loses much of its ballistic advantage. The wind drift affects it more, because it spends a longer time in the wind. And bullet drop is greater, despite the high BC.
For a cartridge with a higher acceptable recoil, the 7mm bullets, launched at high velocity, have excellent ballistics. But this caliber simply does not meet the criteria for a GPC.
.243 and .264 calibers
For a GPC, smaller calibers are excluded because the bullet weights are too low and velocities are too high, and larger calibers are excluded because the bullet weights are too high and velocities are too low. Within the middle calibers, 6mm and 6.5 mm, two bullets stand out, each heavy for caliber: the 105 grain Berger (.243) and the 142 grain SMK (.264).
The 105 Berger has less drift, less drop, and greater range above transonic velocities. But the 142 SMK has more terminal energy, due to the higher BC and higher bullet weight. The 142 has a similar trajectory to the 105, but it’s lower muzzle velocity should provide a longer barrel life. The heavier bullet also has an advantage in terminal effectiveness. A bullet weighing only 105 grains is too light for hunting medium game, and in my opinion too light for optimum effectiveness, at all useful ranges, against enemy combatants. A 142 grain round has sufficient weight to be effective even at its maximum range, and to be a viable replacement for the 130 and 147 grain 7.62×51 rounds used by the military.
Competitive long-range precision shooters prefer the 6mm and 6.5mm cartridges, for their excellent ballistic performance. Of the two, more top competitors use the lighter faster 6mm round. But these are target shooters. They have no concern for terminal effectiveness. And they do not mind if their barrel life is short, as long as they can win the match. So their preference for the 6mm round is based on a different set of criteria. When we consider the criteria for a military GPC, barrel life and terminal effectiveness favor the heavier 6.5mm round.
In this bullet comparison, the best bullet for use in a general purpose cartridge (GPC) would be the 6.5mm 142 grain SMK — or any bullet of similar weight and performance in 6.5mm. This round would also be preferable for self-defense, police work, and hunting over the lighter faster 6mm rounds.