The Case for a Lightweight Medium Machine Gun

John Hanka
November 15, 2004

Soldiers universally report a great love for the M240 medium machine gun. They love its reliability. They love to hear the sound of heavy 7.62 NATO slugs ripping downrange, taking chunks out of cars, bricks, trees, and the enemy they conceal. But that firepower comes at a price: that baby is a heavy sucker! What if we could give our troops the same firepower for 71% of the weight? I propose replacing the 7.62 M240 with something like a M249 LMG (currently in 5.56 NATO) upchambered to the 6.5 Grendel topped with a 144gr Lapua FMJBT.

Consider the following scenario: Suppose an M240 machine-gun crew goes into combat with 2,000 rounds. In 7.62 M80, their ammo weighs about 110 lbs (one round of 147gr 7.62 M80 weighs 386 grains, and, of course, one pound equals 7000 grains). In 6.5 Grendel, their ammo weighs about 82 lbs (one round of 144gr 6.5 Grendel weighs 286 grains). This is a difference in total ammo load of 28 lbs.

Now, an M240B medium machine gun weighs 27.6 pounds, and an M249 SAW light machine gun weighs 15.6 pounds. What I propose is to rechamber the lighter M249 to the 6.5 Grendel, since the 6.5 Grendel is intended for M16/M4 form-factor rifles and the cartridge has the same OAL as 5.56. By doing so, we save 12 lbs on the gun itself versus the M240.

The bottom line is that our M240 crew and their helpers hump a total of 138 lbs and our 6.5G M249 crew hauls 98 lbs, a weight savings of 40 lbs, or 71%. With the 6.5 Grendel, they can either save the weight and call it a day or, if they really miss those extra 40 lbs, they can carry another 980 rounds of 6.5 Grendel.

But someone might say, “Who cares? We need a heavy cartridge like 7.62 M80 for those situations when 5.56 just don’t cut it. Already a ‘double-tap’ with 5.56 has become standard practice. How could a smaller cartridge ever replace the range and punch of 7.62 M80?” Well, consider that with a 300 yd zero the 6.5 Grendel goes neck and neck with both 175gr M118LR and 147gr M80 out to 500 yards and then begins to edge both in trajectory and wind drift. At 500 yards, the 144gr 6.5 Grendel has more punch, or energy, than 147gr 7.62 M80 (963 ft lbs versus 917). See 6.5 Grendel ballistics chart here. The bottom line is that a heavy 6.5 Grendel loading in 144gr FMJBT basically equals or exceeds the external ballistics of the M240’s 7.62 M80, and that with a weight savings of 40 lbs for our M240 crew.

But someone else might say, “What about terminal ballistics? It’s one thing for a bullet to fly through the air all pretty-like, but what about when it hits the target and has to get down to business? You can’t get around the fact that a 7.62mm caliber bullet simply makes a bigger hole than a 6.5mm bullet.” It’s true that, not considering any fragmenting or tumbling, a 7.62mm bullet hole is obviously 1.12mm bigger than a 6.5mm bullet hole. However, if a bullet yaws and then fragments, the size of the bullet hole only matters for the first few inches or so of penetration “neck.” Subsequent to fragmentation, caliber is moot and the total mass of the bullet from which the fragments generate is what matters.

We’ve also got to realize that, in the real world, the effect of any small-arms caliber on enemy combatants is variable. We’ve got plenty of anecdotes about motivated enemies hit with multiple 7.62, and even .50 cal, rounds who continued to fight on. One can never underestimate the power of will and adrenaline when discussing terminal effects.

But since we can’t measure those psychological factors in the lab, the best we can do is try and measure physical damage. And physical damage is physical damage! A bullet that does more physical damage to vital structures, that gives more total volume of permanent wound channel (with a bit of temporary cavity shock effect thrown in for good measure) is a bullet that is more effective than another. Does a 6.5 144gr FMJBT create more physical damage than a 147gr M80? Sorry, but I don’t know at this writing. Limited terminal ballistics testing of the 144gr 6.5 Grendel in calibrated gelatin has occured, but I am privy only to sketchy results. However, 7.62 M80 terminal characteristics in ballistic gelatin are quite well known. At any rate, I can’t say with absolute authority at this time, but I’m quite certain that since a 144gr 6.5 Grendel has nearly as much lead mass as 147gr 7.62 M80, it’s going to do at least as much damage on target. If you can do it with 7.62 M80, you can do it with 6.5 Grendel, and that with equal or better external ballistics and a weight savings of 40 lbs.

We not only have to consider terminal ballistics on unprotected opponents, but on intermediate barriers they may take cover behind. The more cover you can deny an enemy, the more casualties you can inflict. If we can’t hit ’em, we can’t hurt ’em! How does the 6.5 Grendel do with regard to penetration? I can’t yet prove it, but I suspect the 6.5 Grendel might have even better penetration than M80, owing to its higher sectional density. At a demonstration in May 2004 at Blackwater training facility, the 144gr 6.5 Grendel penetrated 1.575" of the latest armored glass. This indicates that it can, at the very least, hold its own with M80 against targets wearing body armor or taking cover behind trees, walls, cars, etc. The bottom line is that an M249 MG in 6.5 Grendel can bust through walls as well as, and probably better than, an M240 in 7.62 M80 — and this at a weight savings of 40 lbs.

Given all this, it’s at least worth considering that the soul of an M240 could be contained in the body of an upchambered 6.5 Grendel M249. My thesis here needs tough testing to bear it out, of course, but it’s looking pretty darn solid from here. The 6.5 Grendel has the potential to give medium machine gun crews the same, if not better, trajectory and wind drift and recoil (all adding up to increased hit probability), as well as terminal ballistics than 7.62 M80, but for 71% of the weight. The military is constantly trying to save troops weight. That’s good, but, often, they have to compromise some capability to do so. By “Grendelizing” our medium machine gun, we don’t lose a darn thing except the chore of humping excess pounds!