If you haven’t served in the military, you probably won’t have been exposed to the 7.62×51 Nato cartridge. A survey conducted in 2021 showed that thirty-two percent of US citizens own guns, of which sixty-eight percent own rifles.
Now mention .308 Winchester, and virtually everyone will have heard of this caliber. Few will know that the ammo is astoundingly similar to the military-designed 7.62×51 Nato cartridge.
You can shoot 7.62×51 ammo in a .308 rifle but not the other way round due to the pressure differences. The main difference between the 7.62×51 Nato and the .308 Win is that the .308 Win operates at higher pressures, is loaded using thin-walled cases, and is slightly shorter than the Nato equivalent.
The 7.62×51 military-developed cartridge is virtually identical to the .308 Winchester, but there are some essential yet subtle differences between the two cartridges. Let’s review the differences between these two great chamberings.
- Why Are The 7.62×51 and .308 Winchester So Similar?
- Differences Between 7.62×51 and .308 Winchester
- Why Shouldn’t You Fire A .308 Win in a 7.62×51 Nato Gun?
- How To Tell 7.62×51 and .308 Win Apart
Why Are The 7.62×51 and .308 Winchester So Similar?
The 7.62×51 Nato cartridge was adopted by the US and later Nato forces in 1954 after extensive testing and trials. A ballistically identical replacement had to be found for the 30-06 chambered M1 Garand.
The 30-06 had proven too long to function reliably in the then-new, more compact Springfield M14 rifle of the day. The stunning result is the cartridge that we know today as a 7.62×51 Nato.
In the meantime, Winchester played a brilliant card by introducing the civilian equivalent of the 7.62×51 in 1962, two years before the official adoption and Nato designation of the cartridge as the 7.62×51 Nato. Winchester called their version the .308 Winchester.
Differences Between 7.62×51 and .308 Winchester
The below table gives a breakdown of the primary differences between the two cartridges
|Chamber Pressure (PSI)
|Average bullet head weight (Grain)
|Bullet Head Options
|Ball, Tracer, Armour piercing
|Jacketed Expanding, Monolithic
|Average bullet velocity
|Headspace Range (Inch)
|1.6355 to 1.6405
|1.6300 to 1.6340
|Bullets safe to swap and shoot
|No (7.62 Nato only)
|Yes (.308 Win & Nato 7.62)
The 7.62×51 Nato cartridge is a military development, while the .308 Winchester is the civilian equivalent with a few minor differences. The intention by Winchester Repeating Arms Company, based in New Haven, Connecticut, was to duplicate the cartridge.
Unfortunately, the military lengthened the headspace ever so slightly in their final design stages. By then, the .308 Win had been released, and there was no turning back.
Cartridge Case Difference
Visually the cartridge cases of the 7.62×51 Nato and the .308 Winchester look identical. However, careful measuring will reveal that the Nato case has a slightly longer headspace of 1.6405 inches versus the slightly shorter .308 Winchester that measures a max of 1.6340 inches.
The 7.62×52 Nato has thicker cases than the .308 Winchester cartridge case wall thickness.
Rifle Chamber Length Difference
As a result of the extended headspace of the 7.62×51 Nato case, the chambers of rifles firing this round also need longer chambers. As an average, the Nato chambers exceed the SAAMI specs of the .308 Winchester by 0.006 to 0.010 inches.
Operating Pressure Differences
The maximum chamber pressure for the 7.62×51 Nato rifles is 50,000 PSI, whereas the .308 Winchester is 62,000 max. This is about a twenty percent difference in operating pressure which is significant.
The Nato cartridge was made for semi-automatic and automatic rifles. A balance between reliable functionality and keeping the recoil down during automatic firing would have been a significant consideration in keeping the operating pressure down. Excessive bullet velocity was also unnecessary as most combat situations occur at fairly close range.
The civilian .308 Winchester cartridge is used almost exclusively for hunting and target shooting. It is used in strongly built bolt action rifles that are well suited to the higher pressure cartridges.
The 7.62×51 Nato operates at a lower pressure than the .308 Win, the headspacing is longer on the Nato case, and the chambers of Nato rifles are longer than .308 Winchester rifle chambers.
The Danger Of Firing A .308 Win in a 7.62×51 Nato Chamber
The cartridge case is a crucial component without which modern weapons can not operate. The case, of course, contains the propellant (gun powder), houses the primer that ignites the propellant, and holds the bullet head firmly in place.
The cartridge case also performs the task of sealing the inside of the rifle’s chamber and prevents gasses and resulting pressure from escaping in any direction other than along the barrel.
The internal profile of all weapons chambers is shaped precisely to match the external profile of its cartridge casing, providing support to the case walls while the bolt head supports the case head during firing. As you know, the extreme pressure build-up within the chamber causes the bullet head to be driven out of the barrel and on its way to the target.
Why Shouldn’t You Fire A .308 Win in a 7.62×51 Nato Gun?
The chamber and the headspace of 7.2×51 Nato weapons are slightly longer than those of the .308 Winchester chambered rifles. The .308 Winchester round will easily chamber in the Nato weapon, but that doesn’t mean it is safe to shoot.
It is improbable that a rifle chambered for the 7.2×51 Nato chambering will spectacularly fail or blow apart when a .308 Winchester round is fired in it, even though the pressure is roughly twenty percent higher than the standard Nato ammo operating pressure.
Weapons that have been pressure tested are regularly exposed to double the recommended chamber pressure during testing, so they are generally very strong. Don’t try duplicating these pressures.
The danger when firing a .308 Winchester round from a 7.62×51 Nato chambered rifle comes from the head spacing differences. The pressure builds inside the cartridge case and the chamber during the firing sequence. As the pressure increases inside the cartridge, the case walls are pressed against the surrounding chamber walls.
During the expansion process, the bullet starts moving forward, entering the chamber mouth, which traps the cartridge neck and shoulder up against the front of the chamber. As the pressure continues to build, the main body of the case is pressed against the chamber walls leaving only the rearmost section of the casing, the case head to be forced back against the bolt head, stretching the case.
Cartridge cases are mostly made from brass. Brass is slightly malleable and can stretch, but only so much. The cartridge casing can tear or fail if the case stretches excessively during the firing sequence. In most instances, a phenomenon called case head separation occurs.
When the cartridge casing fails during the firing sequence, the mixture of extremely hot gasses, partially burned propellent, and debris is expelled through the tear in the cartridge casing.
The gasses that exit the cartridge are under extreme pressure and can only escape through the bleeder holes in the rifle bolt face. The gasses hopefully end up safely in the magazine from the bolt face and don’t harm the shooter.
Fortunately, firearm designers factor such failures into the design of their weapons, and in most instances, all ends well enough. Firearms that pose a high risk are those with pitted chambers and early examples of rifles marked 7.62×51 Nato. Either semi-auto or bolt action.
Case head separation is more likely to occur when thin-walled cases such as those found on .308 Winchester ammunition are fired in an oversized chamber. Not that there is any fault with a thin-walled case, it’s just that the ammunition is made for the use made for use in the correctly sized chamber.
Do not try to load another round if a ruptured case or case head separation happens to you. The odds are good that part of the case body, shoulder, and neck will still be lodged in the chamber. The possibility also exists that the bullet head could be stuck in the barrel, which is seriously dangerous if you shoot a follow-up shot. Don’t take a chance!
Firstly, check that the barrel is unobstructed using a ramrod fitted with a copper brush. Should your barrel be blocked, take the weapon to a gunsmith to remove the head. Trying to remove a stuck head yourself can cause irreparable damage to the rifling in the barrel. This is a job for a professional.
Should the bullet have passed through and the barrel is clear, the wire brush should dislodge the remains of the cartridge casing, even if it takes a few passes through the chamber with the brush.
Modern .308 Win chambered rifles can safely fire the 7.62×51 Nato round as the bore diameters are identical. The chambers are often sufficiently long to accommodate the longer but lower pressure Nato cartridge. Simply put, if your bolt closes safely on the Nato cartridge, you can fire it.
When changing ammo, always remember that the point of impact will vary from that of the ammunition previously used due to changes in bullet weight, velocity, and chamber pressures. Never assume that all .308 Win ammo, for example, shoots to the same point of aim in your rifle.
The best advice is to use the correct ammunition for the proper chamber to avoid issues.
How To Tell 7.62×51 and .308 Win Apart
The only sure-fire way of telling the ammunition apart between the 7.62×51 Nato and the .308 Winchester is to look at the case headstamp visually; the bullets all look pretty much the same even if you hold them up next to each other.
Unfortunately, it isn’t easy to differentiate the ammunition, or maybe it is. What we know is that the Nato cartridge was known as the T65. It seems that the military cartridge suppliers did their best to try and confuse the issue, although there must be a logical explanation for each headstamp. Some reasonably common head stamps included XM246E3, FAT1E3, and T65ER.
Since 1952 many countries across the world have produced ammunition for the military under contract. Each country used its headstamp containing different symbols, numbers, or letters. Research what these mean before deciding to fire any ammunition that looks like it could work in your weapon, and never shoot the ammo if you are unsure.
For the average recreational shooter, a sure-fire way of identifying the .308 Winchester cartridge is to look for the .308 Win headstamp used on all the commercial ammunition. Anything else resembling the .308 Win but not stamped as such requires further investigation.
Although the 7.62 x 51 ammunition and the commercially available .308 Winchester chambered ammo are virtually identical in terms of appearance.
Ammunition identified as 7.62×51 Nato is safe to fire in a .308 Winchester chambered weapon. The reverse is not always good to try. Chambers marked 7.62×51 Nato is marginally longer and has greater headspace than those marked .308 Winchester.
Be careful and don’t fire .308 Winchester ammunition in any chamber marked 7.62×51 Nato, as this may well lead to personal injury or damage to your weapon.
The Nato rifles chambered for the 7.62×51 cartridge have longer chambers than the .308 Winchester, which can cause the .308 Win case to split if fired in a Nato rifle. The .308 Winchester cartridge is loaded to higher pressures than the Nato version, promoting case head separation if fired in the wrong chamber.