308 vs 7.62 NATO, When Is It Safe To Switch Rounds?

308 vs 7.62 NATO, When Is It Safe To Switch Rounds?

.308 vs 7.62 NATO: These two common cartridges may seem the same, but they’re not identical. While there are quite a few different .30 caliber rounds referred to as “.308” or “7.62,” this article will focus specifically on the .308 Winchester and 7.62x51mm NATO cartridges.

.308 vs 7.62, Notable Differences

Pressure

According to the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI), it is not considered “unsafe” to fire either round in weapons chambered for either .308 Winchester or 7.62x51mm NATO. However, .308 Winchester rounds are rated for a slightly higher pressure than 7.62x51mm NATO. The Bureau of Alcohol, Tobacco, Firearms and Explosives advises all shooters to check the barrel stamp of their firearm and only firing the rounds advised by the weapon’s manufacturer.

Case Thickness

Generally 7.62mm NATO brass is thicker than its commercial counter part. With the same exterior dimensions, this means there’s more room for powder in the .308 Win brass. As a result, commercial rounds can be up to 12,000 psi hotter than NATO. Therefore, general consensus dictates it’s perfectly safe to fire 7.62 NATO in a .308 Win chamber, but the reverse is inadvisable.

Full-auto weapons tend to prefer thicker brass, and the 7.62mm NATO is built to stretch in a wide range of chambers.

Chamber Size and Headspace

While the exterior dimensions of both cartridges are the same, the dimensions of their chambers are not. Winchester headspace is around 1.630″ while the 7.62mm NATO’s headspace is closer to 1.6405″. Overall, there can be up to a 0.013″ difference between the two with military chambers measuring longer than commercial.

Commercial .308 Winchester brass can stretch by 0.008″ or more during discharge. In a larger chamber, the thin walled brass can actually burst. Thicker military brass can stretch further and fill the extra space found in a military chamber. While it is possible in some scenarios to fire a commercial round through a 7.62 NATO chamber cut closer to .308 Win dimensions, firing a commercial round at max pressure in a chamber with a large headspace can be disastrous.

(image source; Stephen Redgwell/303british.com)

History of the .308 Winchester

.308 Winchester (image source; Wikipedia)

The .308 Winchester was developed in 1952 as a hunting round for Winchester’s new Model 70 and Model 88 rifles. Based on the .300 Savage, the .308 Winchester was a massive success and quickly became one of the most popular cartridges for big game hunting around the globe.

.308 Win Features

  • Caliber: 0.308″ (7.62mm)
  • Bullet Weight: 100 – 240 gr (usually 150-168 gr)
  • Neck Diameter: 0.3433″ (8.72mm)
  • Shoulder Diameter: 0.4539″ (11.53mm)
  • Base Diameter: 0.4709″ (11.96mm)
  • Rim Diameter: 0.4728″ (12.01mm)
  • Case length: 2.015″ (51.18mm)
  • Max Pressure (SAAMI): 62,000 psi
  • Max Pressure (CIP): 60,191 psi
  • Velocity (150 gr): 2,820 fps
  • Energy (150 gr): 2,648 ft-lbf

In North America, the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) rules the max pressure for a .308 Win to be 62,000 psi (430 MPa). In other areas of the world regulated by the Commission Internationale Permanente (CIP), .308 Win cartridges are ruled to handle a max pressure of 60,191 psi (415 MPa), but the CIP requires all rifle cartridges to be proofed at 125% max pressure, which is 75,275 psi (519 MPa).

.308 Win

.308 Winchester Dimensions (image source; Wikipedia)

History of the 7.62x51mm NATO

7.62x51mm NATO (image source; Wikipedia)

The 7.62x51mm NATO cartridge was developed in 1954, just two years after the .308 Winchester. It was the first small arms cartridge standardized by NATO. The round is often confused with the Russian 7.62x54mmR and 7.62x39mm cartridges, all of which carry the same caliber.

The new cartridge was used by the M14 rifle, which would go on to replace the M1 Garand as the standard service weapon of the United States Military in the 1960s. The 7.62x51mm NATO round is used by both infantry and on mounted weapons.

The ballistics of the 7.62x51mm NATO round are similar to the .30-06 Springfield M1906 service round. Advancements in propellant technology allow the 7.62x51mm NATO to perform on par with the .30-06, but from a much smaller cartridge.

The U.S. Army designates the 7.62x51mm NATO as M80 ball 7.62mm. An improved version of this round known as the M80A1 uses a copper jacketed bullet with an exposed steel penetrator. The M80A1 rounds use 114.5 gr less lead than traditional M80 ball ammunition.

7.62x51mm NATO Features

  • Caliber: 0.308″ (7.62mm)
  • Bullet Weight: 147 gr (standard), 175 gr (sniping round)
  • Neck Diameter: 0.345″ (8.8mm)
  • Shoulder Diameter: 0.454″ (11.5mm)
  • Base Diameter: 0.470″ (11.9mm)
  • Rim Diameter: 0.473″ (12.0mm)
  • Case length: 2.015″ (51.18mm)
  • Max Pressure (NATO): 60,191 psi
  • Velocity (147 gr): 2,733 fps
  • Energy (147 gr): 2,437 ft-lbf
  • Velocity (175 gr): 2,580 fps
  • Energy (175 gr): 2,586 ft-lbf

7.62x51mm NATO Ballistic Data

 

7.62x51mm NATO (image source; Wikipedia)

Overall, if you’ve got a weapon with .308 Win stamped on the barrel, you’re good to go. If it’s chambered for 7.62 NATO, you should steer clear of any .308 Win.