A 6.5mm General Purpose Cartridge versus the 7.62×51 NATO
In one sense, a general purpose cartridge (GPC) is simply a rifle cartridge that is useful for many purposes, such as hunting a wide range of game animals, self-defense, police work, and military use. You might consider the .308 Win. to be one such cartridge. In a more restricted sense, a military GPC is a rifle cartridge that can perform a wide range of the tasks expected of a military firearm. And in the current circumstances in the U.S., a military GPC should be able to replace both the 5.56 NATO and the 7.62×51 NATO cartridges for many, though perhaps not all tasks.
Anthony G Williams is the chief proponent of a military GPC. This post will review his criteria for that type of cartridge, with my commentary. I’ll also suggest a specific set of criteria for a GPC, modified from Williams’ proposals. I should note that Williams does not present one particular cartridge as the best GPC. Rather, he discusses the various possibilities and the necessary criteria that a GPC should meet.
Williams’ work can be found on his home page, in his military firearms discussion forum, in his books and articles, and, on the topic of a GPC, especially in this article: The Case for a General-Purpose Rifle and Machine Gun Cartridge (GPC). He also provides a prominent like to an article by another author: Towards a 600 m lightweight General Purpose Cartridge [PDF file] by Emeric Daniau.
The views in each of these articles will be considered in this post. The first article, by Williams, begins with a discussion of problems with 5.56 ammunition: it often fails to yaw, the range is limited to about 500 meters, the round is too light to stop an attacker with one hit. These problems with the 5.56 have become clear through our troops’ experiences in Iraq and Afghanistan, where longer range engagement of enemy combatants is common.
As a result, the military has turned to a wider use of 7.62×51 weapons. This ammunition has a much longer range and the bullets are of sufficient power to stop an attacker with one hit. However, the recoil is excessive. Williams says:
“Heavy recoil in a rifle makes it more difficult to train recruits, reduces accuracy, slows down rapid semi-automatic fire and makes fully automatic fire virtually uncontrollable.”
I would add that if we ever have a war which requires a draft, the draftee is not a professional soldier. He (or she) may be poorly motivated and may have relatively little training. You can’t sent draftees to war with a 7.62 weapon, especially one that is fully automatic.
Williams suggests 4 requirements for an effective military general purpose cartridge:
1. Effective incapacitation of enemy combatants with a center mass hit, even with a short barreled weapon
2. Effective after passing through various common barriers
3. Equal or exceed ballistics of the 7.62 round with a long barrel
4. “Finally, the weapons and their ammunition should have the lightest weight and lowest recoil consistent with the first three requirements.”
Williams sums up the benefits of a GPC that could replace the 5.56 and 7.62:
“If a single GPC could be achieved, the benefits resulting from all of the weapons within the section being suited to use at all combat ranges and being able to share ammunition, plus the simplification of weapon and ammunition acquisition, logistics and training would, I suggest, make this the most attractive option.”
My comments on these criteria:
The greatest benefit of a GPC would be the replacement of the 5.56 with a much more capable round. Replacing the 7.62×51 would be advantageous, but not the first priority. So if we field a GPC, we should first simply replace the 5.56 with the new cartridge/rifle combination. Later the 7.62×51 can be gradually replaced, if it is clear from experience in the field that the GPC will be an effective replacement.
A return to the use of lead bullets is also necessary for a successful GPC. The constraints imposed on ballistics by a ban on lead ammunition makes it impossible to replace both the 5.56 and the 7.62 with one more effective cartridge.
Green (lead free) bullets are lighter than the same length/shape lead bullets. Bullet weight is an important factor in terminal performance. In addition, sectional density — the weight of the bullet relative to its cross-sectional area — is a strong determinant of penetration and effectiveness. Green bullets (copper/steel) have a lower sectional density than a similarly designed copper/lead bullet.
Suppose we fight a war and the enemy is using lead bullets, while we are confined to copper/steel rounds. That restriction puts our soldiers at a substantial disadvantage. It may be true that the latest 5.56 “green” bullet (M855A1) is better than the older lead round. But I would chalk that up to the poor design of its predecessor. When you look at the ballistic coefficients of bullets, the green bullets have a lower BC due to the lower density of the metals. So I suggest that a GPC would need lead bullets.
The latest 7.62 “green” bullet is the M80A1 (lead free) which is said to be 130 grains with a minimum energy of 2915 joules (2150 ft lbs), implying a minimum velocity of 2730 fps. But the military has said that the new M80A1 round has a higher velocity than the 147 grain M80 (2750 fps and 2470 ft lbs). It is highly unlikely that the muzzle energy for the new round is the same, since the M80A1 is less dense and longer; it intrudes too far into the case, lowering powder capacity. But if the M80A1 has a velocity as high as 2800, the energy would be 2264 ft lbs.
Update: This military source claims a velocity of 3050 fps, but that value is undoubtedly from the M240 machine gun, with a barrel length of 24.8″. Using a Powley computer and some reasonable assumptions, it looks like 2800 fps from a 14.5″ barrel in the M4A1.
Based on a small photo of the M80A1 next to the M80 (1.12″), the former appears to be about 10% longer, making it 1.23″.
What might the BC be for this bullet? Well, the M80 has a G7 of .200 and the M80A1 is a lighter bullet, so it could be less than that figure. By comparison, the Berger .270 caliber 130 grain (lead) VLD has a G7 of .231. There is no possibility that the M80A1, as a fatter and less dense round, gets close to that value. It would overly-generous to suppose the G7 BC of the M80A1 to be even .210. But let’s do some math to find a reasonable estimate of the value.
We don’t know the BC, but we know the caliber, length, and weight. That should be sufficient to estimate the G7 BC, given a set of BCs from bullets of the same caliber and length. BC is directly proportional to weight, given the same basic shape, the same caliber, and the same length.
The 155 gr HBC .308 bullet has a G7 of .236 and a length of 1.237 (per JBM Ballistics). Dividing 130 by 155 gives us a reduction in BC to 83.87% for a G7 of .198.
The 168 gr Hornady HPBT .308 bullet has a length of 1.232 and a G7 of .222. The same math gives us a G7 of .172 for the M80A1. But let’s do a few more bullets and then average the results.
The 167 gr Lapua Scenar has a length of 1.236 and a G7 of .216. This gives us a G7 of .168.
The 150 grain Nosler Ballistic Tip has a length of 1.228 and a G7 of .203. The result is a G7 of .176 for the M80A1.
The 155.5 grain Berger “fullbore” has a length of 1.250 and a G7 of .237. The same math gives us a G7 of .198.
Based on the above considerations, I would put the M80A1 ballistic coefficient (G7) at between .168 and .198, with a likely value (from the average BC) of .1824 — and that is not very good.
Given the values of 130 grains, 2800 fps, and a high-end estimate of the G7 BC as .200, the trajectory of the M80A1 is as follows:
range (m) / velocity (fps) / energy (ft lbs)
725 meters 1254 fps and 454 ft lbs
850 meters 1071 fps and 331 ft lbs
900 meters 1035 fps and 309 ft lbs
1000 meters 984 fps and 279 ft lbs
If a GPC is to replace the 7.62×51, it should exceed the capabilities of the M80 and M80A1 rounds. The M80 is a 147-grain copper/lead bullet with a (claimed) G7 BC of .200.
705 meters 1255 fps and 514 ft lbs
850 meters 1055 fps and 364 ft lbs
900 meters 1024 fps and 342 ft lbs
1000 meters 975 fps and 310 ft lbs
Neither round is very effective at long ranges.
Ideally, at range, you want the bullet to be above transonic (>Mach 1.1), because many bullets are unstable in the Mach 0.9 to 1.1 velocity range. For ICAO conditions (used as a standard for comparing bullet trajectories), 1250 fps is just above Mach 1.1 (which is about 1230 fps).
As for bullet energy at range, 400 ft lbs is on the low end of lethality as it is a typical muzzle energy for a 9mm pistol round. Certainly, a bullet can be effective at lower levels of terminal energy, but not very effective. So I would set the maximum effective range for any GPC cartridge at no less than 1250 fps and no less than 400 ft lbs. A 115 grain bullet has about 400 ft lbs of energy at 1250 fps. So for heavier bullets, the velocity will be the limiting factor. For lighter bullets, the energy will be the limiting factor.
Bullets in the range of 120 to 150 grains are needed for a military GPC. Lighter bullets (90 to 115 gr) can outperform the 5.56 round, but they cannot maintain enough energy for long range shots to equal or exceed 7.62 ballistics.
In his article “Towards a 600 m lightweight General Purpose Cartridge”, Emeric Daniau considers a number of good options for a GPC. One suggestion is a case based on the .30 Remington, necked down to 6.5mm, similar to my suggestion for the 6.5 ICS:
“For example, a 51 mm long case with a body diameter of 10.7 mm (essentially the full-length .30 Remington case necked down to 6.5 mm, case capacity of 2.9 cm3), firing the “long” 7.1 g bullet at a calculated MV of 835 m/s….”
My proposed cartridge is similar to his suggestion, with about the same case capacity (~45 grains). His 6.5mm GPC has a case capacity of 2.9 ml (44.75 grains). However, the 6.5 ICS has a longer body, longer shoulder, and shorter neck than the .30 Remington. The 6.5 ICS is 2.10″ in case length, a little longer than the .30 Remington. I’ve also revised the case dimensions to have less taper in the body.
Williams, Daniau, and other authors are concerned with the overall weight of the ammo carried by each soldier and each platoon, and they seek to keep that weight as low as possible. I think that a more effective round necessitates more powder, a larger case, and a heavier bullet. So we have to make the trade-off of accepting heavier cartridges than the 5.56 (a little lighter than the 7.62).
The 7.1 g bullet preferred by Daniau is 108 grains (Lapua Scenar) at 2740 fps. But the G7 BC as measured by Bryan Litz is only .225, not the .25 that Daniau uses. The former BC is less effective at long ranges than the 6.5 ICS, due to the lighter lower BC bullet and the smaller case capacity:
800 meters 1237.9 fps and 367.4 ft lbs [Litz G7]
800 meters 1267.9 fps and 385.4 ft lbs [Lapua coefficient of drag from radar]
For a 16-inch barrel at 2400 fps, the 6.5 ICS with a 142 gr SMK should provide the following trajectory (using Litz G7 of .301)
16-inch barrel 6.5 ICS (142 gr SMK)
800 meters 1310 fps and 541 ft lbs
14.5″ barrel 6.5 ICS (142 gr SMK)
800 meters 1273 fps and 511 ft lbs
The 142 grain SMK high BC bullet can outshoot the M80 and probably the M80A1. The higher BC and higher weight, as compared to the 108 grain round, provides longer range and more energy at range.
I should add that Williams and Daniau each consider a number of other options for a GPC.
Chambering the Wrong Cartridge
One important consideration, not often discussed, is the possibility of chambering of the wrong cartridge in any particular firearm. Presently, a 7.62 will not chamber in a 5.56 AR, and the 5.56 round and magazine are so different from the 7.62 that no one would accidentally chamber a 5.56 in a 7.62 AR. But what would happen if the military switched to a wider 6mm or 6.5mm caliber cartridge that would fit in a 5.56 AR? The result could be catastrophic.
Here is an example of this type of problem with the 300 BLK. “A single 300 BLK cartridge somehow found its way into a box of loose .223 cartridges, and was subsequently loaded into a 30-round PMAG, which was then handed to an AR-15 shooter….” Long story, short: Kaboom.
For 7.62 ARs, the problem is reversed. In the event of a switch to a GPC of a narrower caliber, accidental chambering of a 7.62 round in a firearm with the narrower barrel would also fail catastrophically.
So any successful GPC must be unable or unlikely to be chambered in the 5.56 AR and the 7.62 AR, and vice versa. This practically rules out any GPC with the same case dimensions as either currently fielded caliber. We can’t replace the 7.62×51 with the 260 Remington or the 7mm-08 or anything similar.
My suggestion for a military GPC is the 6.5 ICS, which has a case significantly narrower in diameter than the 7.62, and broader than the 5.56. It could not be chambered in the 5.56 due to length and diameter. A 6.5 ICS would fit loosely into a 7.62 chamber, which should be noticeable. And even if the round were fired, the narrower bullet would not jam in the wider barrel. And then the reverse situation could not occur: a 7.62×51 cartridge (.473″) will not fit in any ICS chamber, due to the narrower dimensions of the ICS (.421″)
Ballistics Compared to the 7.62×51
The ideal GPC will have 7.62×51 ballistics or better. But, given the criterion of lower recoil, this requires a high BC and then either a lower bullet weight or lower velocity.
The 6.5 ICS fares well in comparison with the 7.62 round by utilizing high BC bullets from 123 to 142 grains. An effective 142 grain bullet can replace 147 and 155 grain 7.62 rounds. Williams favors the 6.5mm bullet at about 108 grains, but I’m skeptical of the effectiveness of that bullet weight. It would certainly outperform the 5.56, but not the 7.62×51. At extended range, you’ve lost a great deal of velocity, and so you need the higher bullet weight. I prefer the 142 grain SMK (G7 BC .301), but the 123 grain SMK is also a viable option. A 16-inch barrel is sufficient for the 142 grain round; the 123 grain needs a barrel of 18-inches to perform similarly.
The 6.5 ICS with a 142 grain high BC bullet is a good fit as a replacement for many of the roles of the 7.62×51. This ICS round, from a 20-inch barrel, has better ballistics than the 175 grain 7.62 round from a 24-inch barrel. The bullet weight is sufficient to replace the 147 and 155 grain 7.62 options. The BC is high enough to give the round a range of about 900 meters, depending on barrel length (i.e. muzzle velocity).
175 grain SMK bullet (7.62×51) at 2650 fps in a 24-inch barrel, with a G7 BC of .243:
Muzzle: 2650 fps, 2730 ft lbs
810 meters 1253.0 fps and 609.9 ft lbs
900 meters 1131.8 fps and 497.7 ft lbs
500 meter zero
900 meters -252.6″ drop and 104.2″ drift
For some limited roles, the 7.62 may be a better fit, due to the higher bullet weights. But the 6.5 ICS is easily more effective than the 147-grain ball ammunition most common in that caliber. It’s also a better replacement for the 5.56 than the 6.8 SPC or the 6.5 Grendel. The bullet weight is higher, the BC is better, and the range is longer.
142 grain SMK bullet at 2500 fps from a 20-inch barrel, with a G7 BC of .301:
Muzzle: 2500 fps, 1971 ft lbs
810 meters 1371.9 fps and 593.3 ft lbs
900 meters 1266.6 fps and 505.7 ft lbs
500 meter zero
900 meters -238.4″ drop and 84.1″ drift
The 6.5 ICS provides about the same energy at long ranges and a higher velocity, all from a barrel 4 inches shorter. The 6.5 ICS has 14 inches less drop and 20 inches less drift. There is little advantage to the 7.62×51, even when shooting the 175 grain round. Ballistics of the M80 and undoubtedly the M80A1 are worse than the 175 SMK.
Weight (ammo and gun)
The ICS is designed around the 7.62×51 COAL of 2.800″. The disadvantage to this choice is that the gun would be nearly as heavy as the 7.62 rifle. But the weights of recent models of the 7.62 (or .308) AR have been coming down. A few companies now offer 7.62 pattern rifles that weigh 8 lbs or less: DPMS GII (7.25 lbs), FN SCAR 17 (7.9 lbs), Smith and Wesson M&P10 (7.71 lbs). So rifle weight is becoming less of an issue.
Also, the lower recoil of the ICS might allow for some weight savings in the bolt carrier group and buffer. And the barrel can be 4-inches shorter without loss of performance. But the gun would still be heavier than an AR-15 type rifle.
One advantage to this use of the AR10 (7.62×51) platform: there is more room for high BC bullets with long noses. Another is that the cartridge can be a narrow diameter and still have sufficient powder capacity for good velocities; this design feature avoids the problems inherent to short fat cartridges (like the Grendel). And the narrow diameter also permits a higher number of rounds in each magazine.
The ICS makes a trade-off here, accepting a higher gun weight in exchange for numerous benefits. My opinion is that the longer COAL is essential to obtain a larger case capacity than any AR15 (5.56) rifle and to accommodate high BC bullets. The use of a narrow long case also provides greater efficiency than a short fat case.
The muzzle energy of the 6.5 ICS with a 142 grain bullet is several hundred ft lbs less than for the 7.62. And that value is a good indication of recoil.
7.62×51 with 175 grain SMK at 2650 fps: 2730 ft lbs
7.62×51 with 147 grain M80 at 2750 fps: 2470 ft lbs
6.5 ICS with 142 grain SMK at 2500 fps: 1970 ft lbs
The recoil for the 6.5 ICS, calculated using this website, should be around 10 ft lbs with a recoil velocity of 8 fps, which is quite mild. I used a gun weight of 9 lbs since the magazine and optics add weight to the bare rifle (powder 38 grains, bullet 142 grains, velocity 2500 fps).
The lower recoil is obtained by using less powder in a smaller case, and by keeping the bullet velocity to a moderate level. The 142 grain bullet has a high BC, which accounts for the better ballistics despite a lower velocity.
The lower velocity also provides a longer barrel life. High velocity rounds shorten barrel life. Recoil can be lowered further, if desired, by choosing the 123 grain bullet. But as a result, range and terminal energy will be reduced.
The 6.5 ICS outshoots the 7.62×51 at long ranges and is a vast improvement over the 5.56 at all ranges. At short to medium ranges, the 7.62 can exceed the 6.5 ICS in bullet weight, velocity, and terminal energy. But at those ranges, a 142 grain SMK is more than sufficient for a one-shot incapacitation of an enemy combatant.
As a candidate for a General Purpose Cartridge, the 6.5 ICS meets all requirements — except that the ammunition weight and rifle weights are higher than ideal. But every GPC must make the trade-off of greater ammunition weight for greater ammunition effectiveness. Without more powder, a heavier bullet, and a heavier case, you can’t obtain greater effectiveness. A platoon might carry fewer rounds, due to the greater effectiveness, or look for weight savings elsewhere and accept the trade-off of greater weight.