7.62 NATO vs .308 Winchester ammo: What’s the difference?
Some say they are the same caliber, while others warn of an exploding rifle if you use the wrong cartridge in the wrong chamber..308 Win Ammo | 7.62×51 NATO . Do a search for “.308” and you will find similar results, albeit with fewer references to 7.62. So, what gives? Are 7.62 NATO (a.k.a. 7.62x51mm) and .308 Winchester the same thing? Or are they different? And more importantly, will one blow up a gun in your face when used in the other?
7.62×51 ammo and 308 Win ammo are often used interchangeably, and it isn’t necessarily wrong. 308 / 7.62 are both standard cartridges, but 7.62×51 NATO is a military-designed round, so it’s slightly larger than the civilian equivalent. .308 Winchester and 7.62 NATO use the same-size bullet but have different case wall thicknesses and different max pressure ratings. .308 Winchester is rated for a slightly higher max pressure rating, so while best practice is to use the exact same ammo as the gun barrel is marked for, you can often use either .308 Win or 7.62 NATO in barrels marked for .308 Winchester. Barrels marked for 7.62 NATO should only use 7.62 NATO ammunition. If you ask which one is better, 308 v 7.62, all you can do is look at the numbers.
The military has used 7.62 NATO ammo since 1954. It’s been used to feed a variety of weapons including the standard service rifle. That is until the M14 was replaced with the M16, which is chambered for the smaller 5.56mm. Today, they primarily use it for sniper 7.62 bullet with a full metal jacket weighing 147 grains has a muzzle velocity of 2,750 feet-per-second with 2,468 foot-pounds of energy..308 Win Ammo | 7.62×51 NATO
While 7.62 NATO and .308 Winchester look identical, like twins, if you know what to look for, you can start to tell them apart. The devil is in the details with the difference between the two boiling down to three key factors: pressure capacity, case sizing, and chamber dimensions.
NOTE: Many shooting newbies may see the number combination “7.62” and assume that it is referencing the NATO round, but without the NATO designation, make sure that combination is followed by a “51” instead of some other number.
All 7.62×51 rounds are cool for use in the proper NATO weapon, but 7.62x39 is the original caliber of the infamous AK-47. The new(ish) .300 Blackout bears a similar (albeit rare) designation of 7.62×35, and the standard Russian sniper round is the 7.62x54R. Need help keeping it straight? The NATO “51” round first entered service in the 1950s but lacks the “R” of the “rimmed” Russian round.
The U.S. military’s 7.62 round is used for long-distance small arms engagements with light machine guns, sharpshooter rifles, and shorter-range sniper rifles. While the .308 Winchester is the civilian competitor and is frequently employed in hunting rifles and military replica firearms, such as the M14 (Springfield Armory M1A), AR-10/SR-25 (multiple civilian variants), and FN SCAR-H (SCAR 17)
Measuring the thickness of cartridge cases is kind of a pain, especially since I tend to mash them all up when trying to cut them in half with my Dremel tool. So, I took the shortcut to illustrate the difference. From my big bucket of .308 / 7.62 brass, I selected some representative samples of both commercial .308 Winchester and 7.62x51mm NATO brass and weighed them. I picked several of each and averaged the weights. I didn’t measure the cases because they’ve been fired, so that won’t tell me much other than the general size of the chamber from which they went bang..
Last but not least we get to the real difference. Military rifles for 7.62x51mm NATO can, and usually do, have longer chambers. In things like machine guns powered by ammo made all over the world, there’s got to be some slack for reliable feeding and operation with all that violence going on during the feeding and ejection process. The solution is to make the chamber headspace a bit longer. If you’re not familiar with headspace, think of it as the distance from the bolt face to the point in the chamber that stops the forward motion of the cartridge case. If the chamber headspace is too long for a cartridge, it will float back and forth in the chamber. If the headspace is too small, the bolt will not close properly or will require excess force to close.
How much different is the headspace? The .308 Winchester chamber headspace is between 1.630 and 1.6340 inches (SAAMI Info). The 7.62x51mm NATO is between 1.6355 and 1.6405 inches. While the published numbers show about six-thousandths of an inch difference, it’s not unusual for the headspace in a surplus 7.62 rifle to be 10 or even 15 thousandths longer than that of a commercial .308. That doesn’t sound like a big deal, right up to the point where you fire thinner commercial brass in a long-chambered rifle. The brass will stretch, possibly enough to contribute to a dangerous case rupture. Doing the same thing with thicker military brass is no big deal and the way the system was designed. Thicker brass can handle some extra stretching into a longer chamber throat, so it’s no big deal.