Reloading Press: 7.62x51mm NATO (.308 Winchester)

The Reloading Press is an at-least-weekly feature here on Gaming Ballistic for 2016. Each week it looks at some interesting real-world cartridges and presents them with hopefully-useful information in GURPS Format.

7.62x51mm NATO

The 7.62x51mm cartridge occupies an interesting place in the US military history. After the tremendous success of the .30-06 semi-automatic weapon in WW2 with the M1 Garand rifle, the army was looking for a cartridge more suitable than the 63mm-long .30-06 for semi-automatic fire as well as potentially automatic fire. The Brits were interested in the .276 Pederson and .280 British, both of which were designed with a mind towards full-auto controllability. The USA, in a doctrinal choice which must seem baffling given where we are today, insisted in a .30 caliber cartridge to preserve the effectiveness of single shots out to 2,000 yards (!). 

The 7.62x51mm was made possible by improved manufacturing processes, with the cartridge nearly a half-inch shorter than the .30-06, but loaded to a slightly lower velocity.

Either the British or the USA rounds would have been acceptable to Canada at the time – they took the “Eh, whatever the US does, eh?” approach, and so NATO went with the .308 . . . which proved to be just as uncontrollable from shoulder-fired infantry weapons as might have been imagined. That didn’t prevent it from being highly effective in the machinegun role, with the M60 and later M240. The temporary wound channel of the .308 bullet can be the size of a volleyball in the right conditions, and the permanent cavity is very large as well (and devastatingly so if the round happens to be prone to fragmentation). Users of the projectile will swear by its ability to put a foe down; they’re not wrong.

The cartridge continues to see service in various forms, including as a sniper platform, as well as in theaters such as Afghanistan, where accurate long-ranged fire really is a mission requirement. There are a large number of variants of the projectile, and this article will try and cover the most military important. 


Ballistic’s Calculator Inputs

Basic inputs for the calculator are as follows, selecting 610mm for the (test) barrel length. The NATO-standard M80 ball round was selected as the basic package.

INPUT
150gr NATO 7.62x51mm
Chamber Pressure 50000 psi
Barrel bore 7.62 mm
Case Length 51 mm
Chamber Bore 11.53 mm
Barrel length 609.6 mm
Bullet Mass 147 grains
Aspect Ratio 3.65 L/Bore
Burn length 23.6 mm
Projectile Caliber 7.62 mm
Total Accelerated Mass 147 grains
Expansion Ratio 2.2 expansion
Projectile Load 1
Output Stats

This is the archetypical 7d rifle for GURPS, and the output for the 24″ test barrel is so near to exactly 7d calculated penetration that it’s almost amusing. Of course, for handiness and weight, barrels go quite shorter than this in the real world, with a 16″ barrel being common enough that designing a purpose-built cartridge that is .30 caliber, achieves the same test muzzle velocity, and does so out of a 16″ barrel makes a lot of sense (note that the 6.8x43mm SPC seems to have done exactly that, though with a .27″ instead of a .30″ projectile). 

In any case:

Notes
  • The velocity is at the muzzle. The velocity with a 24″ test barrel is tuned to match real-world data at 2750fps, the standard military load issued in bulk. 
  • There is no pi to pi- range listed for this cartridge, because the projectile will always do at least pi damage. There’s a good case to be made here that this round might do pi+ under some circumstances, and even pi++ in others (see Alternate Loads, below).
  • This round is one of the grounding points (along with the 9mm and .50BMG) in my ballistics model, so this one really should (and does) match the published penetration values.
At any useful barrel length this will equal the performance of the 5.56x45mm cartridge that replaced it in terms of terminal performance. The ammo will always be heavier, of course, and each magazine is going to carry 20 rounds for military issue rather than the 30 of the .223/5.56. Unless you’re in the full-auto business with a dedicated weapon, though, the high recoil (and GURPS Rcl) of the round will keep you in the three semi-auto shots business.

Take note that the test barrel is very, very long for a modern combat arm (but not for cold-war era rifles). Anything that will be issued in a role that’s not point shooting (snipers, designated marksmen) is likely going to have a barrel in the 16-20″ range these days, which means you’ll be dealing 6d+1 to 7d-1 instead of the usual 7d with the standard M80 FMJ.

That, of course, is far from the only option, and PCs should be choosy if they’re really desirous of the 7.62 (or if they’re being forced to use it because it’s standard issue). 

Alternate Loads


There are a bewildering number of alternative loads for this cartridge. I sorted through the Wiki page and picked out some important ones from the military pile, will make a note of civilian expanding rounds, and also throw down with a SLAP (saboted light armor piercing) projectile which is just plain nasty.

What follows is effectively a heavily trimmed selection from the 7.62x51mm Wiki page.

M198 (United States): 7.62×51mm NATO duplex round with two 84-grain (5.4 g) bullets. The first hits at 2750 fps for 5d, and it’s a hair’s breadth under the 0.75 wound channel mod to get it a full pi. The back projectile tends to be slower at 2200fps (4d pi).  I’d give both pi at less than about 50yds, and pi- at more than that.  1/2D is 275yds. This was more or less a failed experiment.


M276 (United States): 7.62×51mm NATO so-called “Dim Tracer” for use with night vision devices. Stats basically the same as M80 (150gr bullet).

M852 (United States): 168-grain (10.9 g) 7.62×51mm NATO Hollow-Point Boat-Tail cartridge, specifically designed for use in National Match competitions. It replaced the M118SB as the standard Match round. The bullet was very accurate at around 300 meters (competition match ranges) but suffered at longer ranges. 1/2D increases to 650yds, wound channel gets a tiny bump to 0.85. Velocity is a bit lower (2,550fps), so out of a 24″ barrel it will do 24.2 points of damage – call it 7d-1, though an oddly specified 6d+3 pi would be good too. Muzzle energy 3,290 J. Consider this mass-production match ammo.
M993 (United States): 126.6 grains (8.2 g) 7.62×51mm NATO armor-piercing round, black cartridge tip. This projectile uses a tungsten core surrounded by an aluminum cup and copper jacket. The 126.6gr total projectile weight slows down a bit faster (1/2D 450yds), but does 7d (2) pi out of the muzzle, delivering about 3,400J at that point. My understanding is these are very expensive.

MK 316 MOD 0 (United States): A 175-grain (11.3 g) round specifically designed for long-range sniping. With a 1/2D of roughly 700yds and 7d+1 from the muzzle, this ammo supposedly (from the right platform) will maintain 1 MoA out to 1,000 yds. Definitely match ammo – supposedly about the best quality you can get to minimize dispersion given a variety of in-the-field conditions.

T762TNB1 MK319 MOD 0 (United States): 7.62×51mm NATO Enhance Behind barrier performance Enhance Function & casualty and muzzle flash requirements in short barrel carbines, 130 grains (8.4 g). The bullet pushes about 2925fps from a 16″ barrel (call id 5d+7; the barrier blind property is what gives it the odd penetration value) using a combination of slightly elevated pressure and longer burn. The cost is designed to be close to the M80, or at least as close as possible, but you probably can’t take that to the bank. 1/2D 45oyds from 16″ barrel, and 3,350J muzzle energy. The base of the bullet is solid copper and designed to be what pushes through the barrier . . . not sure if this will drop the wound from pi to pi-; it’s quite possible.

DM111, Weichkern, (Germany): 147-grain (9.5 g) 7.62×51mm NATO ball cartridge, cupronickel-coated steel jacket. German equivalent to U.S. M80 round. In service with the German military. Known for severe fragmentation in human tissue due to its thin jacket, particularly around the cannelure. This one is interesting, because while the use of the M193 and M855 5.56x45mm projectiles (esp the M193) were decried in some circles as being cruel for their designed-in fragmentation-based wounding profile, this particular round did exactly that, but much more so. While all of the .308-caliber projectiles skirt the border of pi+ due to the sideways orientation they assume within the target, this one easily earns it, if not earning pi++.
Hollow-point or soft-point ammo: There are innumerable versions of this, and from a GURPS perspective they are supposed to take the projectile from pi to pi+. The real bullets will certainly do this, if not more so, as calculated wound channels easily hit 2.0 to 2.5 with expansion ratios of 2.0 to 2.5 easily and commonly obtained. By the book, you’d do 7d (0.5) pi+. By my calculations, you’re looking at more like 5d+1 pi++. There are so many variants of this type of hunting projectile that picking one is a bit presumptuous. 
M948 SLAP: Easily the most awesome thing fired out of this chambering. Designated the M948, it features a discarding plastic sabot (far right) wrapped around a 62-gr 5.56mm tungsten projectile. When all is said and done, though, you get 7d (2) pi- out of this, and the tendency for the sabot to come apart at the muzzle brake led some people to note that you should never shoot this ammo out of any firearm, ever. Was replaced by the M993 in service for some odd reason having to do with not having the troops barrels explode in their faces. 1/2D 400yds.
Platforms

Thompson Center Encore

While some have created pistols out of .308-caliber cartridges (the Thompson models coming immediately to mind), by and large you’re looking at rifles here.

Some notable platforms:

M14

The M14 was the first entry here from the USA, and was a very traditional-looking rifle. Wooden furniture and 22″ barrel. The thing weighs 9.2lbs empty, and about 11 lbs loaded, so you’re going to want to be strong. Highly jumpy in full-auto fire.

FN-FAL

The FN-FAL is the classic full-length battle rifle employed by pretty much every country but the USA that belonged to NATO. It sported a 21″ barrel and the classic 20-round magazine. Variants weighed between 9.5 and 13 lbs (that one featured a heavy barrel and 30-round magazines for use as an ersatz SAW.  Like the M14, it could beat you up pretty bad in full-auto fire, and some eliminated the feature. The rifle is over a meter long (43″).

H&K SR9

The G3 series was the Heckler and Koch platform for the 7.62x51mm NATO cartridge. It featured a 450mm barrel (17.7″) and was about 40.5″ long. A short-barreled version (G3K)  featured a 315mm barrel and 28″ length with the stock collapsed. Even the small ones were 9-10 lbs empty. Notable was the accurized and extremely expensive PSG1. Civilian shooters in the USA could try and procure the modified SR9 instead.

SCAR-H or SCAR 17

Three additional rifles that should be mentioned would be the SCAR-17, an up-chambered version of the SCAR rifle by FN (makers of the FAL). This gas-piston system is designed to be quite modular. The light version firing the 5.56x45mm cartridge did not find favor with the US Special Ops command, but the heavy version, the SCAR-H in 7.62x51mm did find some favor. The rifle comes in 13, 16, and 20″ barrel versions, and all versions are within a quarter-pound or so of 8lbs, empty.

Kel-Tec RFB

The final two are both bullpup-configured rifles, making up for the desire for a long barrel in a heavy-caliber rifle by putting the action behind the firing hand. The first is the Kel-Tec RFB, which appeared in roughly 2007. Gas piston, accepting FAL magazines, it comes in 18, 24, and 32″ barrel lengths . . . the last one capable of delivering enough velocity to hit 7d+2 at the muzzle, but dropping off into 7d+1 land soon enough that it’s not worth expanding the chart. The shorter-barreled versions are 8.1 and 8.7 lbs empty, while the “target” barrel length is 11.3 lbs. Overall length with a 32″ barrel is 40″ – as long as a full-sized G3 but with 15″ extra barrel. The carbine variant with barrel length equivalent to the G3 is only 26″ long. It’ll run you just shy of $2,000.

Desert Tech MDR with reflex sight and suppresor

The final entry is the Desert Tech MDR, which is either almost-available or just-available. A very, very expensive weapon when fully configured (and including a suppressor, which adds nearly $1500 to the cost, so YMMV). It features a 16″ barrel, 27″ overall length (more with suppressor, I think), and a $4,000 cost as pictured to the right! Caliber conversion kits are available, allowing 6.8SPC, .300 Blackout, and 7.62x39mm with a barrel change and mag insert replacement. Like the Kel-Tec, it’s a forward-ejecting system.

Parting Shot


The 7.62x51mm cartridge was more or less designed around “battle rifle” length specs. If you’re expecting a meter-long rifle that weighs the best part of 10 lbs, you are looking for an “efficient” version of the .30-06, which is where the specs originally came from.

However, as they say, that was then, and this is now. If you start looking at 16-18″ barrels, or even 24″ out of a bullpup to allow close-quarter fighting and handiness (though at the cost of sight radius . . . which a good optic can probably address for you), then you’re into the 6d to 6d+2 range – and maybe a slightly smaller caliber, or the same caliber but optimized for a shorter barrel would do? There are a billion wildcat cartridges out there, and either finding or creating one that would fit the bill in a 16-18″ barrel would be straightforward. Still, the .308 is here to stay for at least a while, and there are a lot of platforms and cartridges to choose from.

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