Never use .308 in a rifle chambered for 7.62x51mm.

It's relatively common knowledge that .308 Winchester shouldn't be fired in a rifle designed for 7.62x51mm NATO. In this case, common knowledge is correct; however, the reason why it's correct is not what most people think.

It's Not the Pressure

The most commonly-cited explanation asserts that .308 Winchester operates at higher pressure than 7.62x51mm. SAAMI specifies a maximum average pressure for .308 Winchester of 62,000 PSI, but the oft-cited pressure of 7.62.51mm is 50,000 PSI--almost 25% less. If this is true, then it would logically follow that shooting an overpressure cartridge is a bad idea.

But that conclusion raises some questions. Winchester designed and provided initial manufacturing of the T-65 cartridge that was adopted as 7.62x51mm NATO, then began commercial production of the cartridge under the .308 Winchester name. Why would they boost pressure by an astonishing 25% for the civilian market? A zippier cartridge would probably sell better, but producing two near-identical cartridges where one operates at the other's proof-round pressures is logistically dubious.

There's also the issue of velocities. If .308 Winchester is loaded 25% hotter than 7.62x51mm NATO, it should exhibit notably superior ballistics. 7.62x51mm M80 ball ammo with a 150-grain bullet is loaded to a nominal velocity of 2,750 FPS at 78 feet from the muzzle. Operating on the assumption that .308 Winchester is loaded to 12,000 PSI more pressure, we should see noticeably superior ballistics from commercial .308 offerings:

Cartridge	Bullet Weight (grains)	Barrel Length (inches)	Velocity (FPS)	Velocity Increase Relative to M80 Ball
M80 Ball	146	22	2,750 (at 26 yards)	0%
Hornady TAP	155	24	2,797	1.7%
Winchester Super-X	150	Unknown	2,820	2.5%
Hornady InterLock	150	24	2,820	2.5%

The commercial offerings use longer barrels and measure closer to the muzzle, but show less than 3% improvement over the 7.62x51mm cartridge even with this measurement advantage and a supposed 25% higher chamber pressure. That's just not ballistically realistic. One could always argue that the commercial cartridges are underloaded to prevent disaster if used in military arms, but why would a cartridge company intentionally cripple the ballistics of a hunting round intended to be fired from a bolt-action rifle? And why would every single cartridge manufacturer not only download their rounds, but end up downloading almost exactly the same amount?

The problem with all these arguments, though, is that they're supposition. The data on M80 ball is printed in black and white in TM-43-0001-27, Army Ammunition Data Sheets for Small Caliber Ammunition, which flat-out says that 7.62x51mm NATO has a maximum pressure of 50,000 PSI. This would seem like the final word on the subject, but I believe TM-43-0001-27 is mixing up methods of measuring pressure.

Until sometime in the 1960s, the common method of testing cartridge pressure involved fitting copper plugs into pistons in a test barrel and measuring how much pressure crushed the plug. The measurements produced this way are expressed in Copper Units of Pressure, or CUP. It was common practice to use Pounds per Square Inch interchangeably with CUP--until piezoelectric transducers started to be used instead of the copper plugs. Piezo pressure measurements are much more accurate than copper crushers, and showed that there is no real relationship between CUP and PSI (beyond the fact that they both get higher as pressure goes up). Sometimes, they're the same. Most of the time, CUP is lower than PSI. As a rule, the behavior of copper under pressure is so quirky that there's no reliable way of translating CUP to PSI mathematically; you just have to measure both ways.

Here's a copy of the ANSI/SAAMI's "Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Sporting Ammunition for the Use of Commercial Manufacturers." .308 Winchester is listed with a maximum pressure of 62,000 PSI, which SAAMI cites as equivalent to 52,000 CUP...which is very close to the cited "PSI" figure in the TM. If you're still unconvinced that PSI are being substituted for CUP in the manual, look up the pressures listed for .30-06, .45 ACP, and .30 Carbine and compare them to SAAMI PSI and CUP values. Actually, don't bother--I'll do it for you.

Cartridge	TM "PSI"	SAAMI CUP	SAAMI PSI
7.62x51mm/.308	50,000	52,000	62,000
.30-06	50,000	50,000	60,000
5.56x45mm/.223	55,000	52,000	55,000
.30 Carbine	40,000	40,000	40,000

The "PSI" value cited in the TM for 7.62x51mm is very close to the CUP value published by SAAMI for .308, suggesting that .308 is perhaps a mere 2,000 CUP hotter than 7.62x51mm. What's more convincing, though, is the fact that .30-06 is listed at 50,000 PSI in the Technical Manual, which matches perfectly with its pressure in CUP according to SAAMI. The TM lists 5.56x45mm as having a pressure of 55,000 PSI which, given the time it was introduced, could be either PSI or CUP, given how closely its numbers match with SAAMI figures for .223. .30 Carbine is one of those examples of a situation where CUP and PSI match up and render the difference irrelevant.

What these numbers suggest to me is that .308 Winchester and 7.62x51mm NATO operate at nearly identical pressures, and the belief that they differ by 10,000 PSI stems from mislabeling. Taken in conjunction with the ballistics of commercial ammunition compared to military ammo, it makes a fairly convincing circumstantial evidence. I'm convinced, at least.

But if you're still not convinced where pressure is concerned, big deal. There's a far better reason not to use .308 Winchester in a 7.62x51mm chamber:

It's the Headspace

Safe chamber headspace for the .308 Winchester cartidge is between 1.6300 and 1.6340 inches. The equivalent minimum and maximum values for 7.62x51mm NATO chambers are 1.6355 and 1.6405 inches. Here are those numbers in chart form:

	.308 Winchester	7.62x51mm NATO
GO	1.6300	1.6355
NO-GO	1.6340	1.6405
FIELD REJECT	1.6380	1.6455

If a firearms is chambered to the 7.62 NATO spec, it is likely to be unsafe to fire .308 Winchester for the same reasons that it would be unsafe to shoot a rifle that it could swallow a field reject gauge with room to rattle: Excessive headspace allows cartridge brass to flow too far forward under pressure, potentially stretching at the web to the point that the case ruptures, causing an explosion.

7.62x51mm chambers get away with being so loose because 7.62 ammo is made thicker at the base than .308. The extra brass provides enough material to prevent ruptures, and is the reason why most reloading manuals advise downloading by about 10% when using military brass. Conversely, thinner .308 brass provides more case room, but less leeway in chamber dimensions.

However, there is a 0.0025 inch overlap between the 7.62x51mm minimum headspace and the .308 field reject headspace. Theoretically, if one's headspace were to fall in this region it would be safe to shoot .308. Realistically, closing on a NO-GO gauge indicates that use of the firearm should be limited only to necessary circumstances until its headspace is fixed. Closing on a field gauge, in a military context, represents the point at which pulling the trigger is more dangerous to one's self than the enemy.

On a more practical note, it's impossible to know whether any given chamber is cut for .308 or 7.62x51mm without headspacing it, and I wouldn't make assumptions based on how it's labeled or advertised: Even some gun sellers seem a bit unclear on the difference. I've seen guns advertised as being chambered in "7.62x51mm NATO (.308 Winchester)" which is the same as saying, "it's safe to fire (unsafe)."

Takeaway

Whether you believe it's headspace or pressure that distinguishes .308 Winchester and 7.62x51mm NATO cartridges, you can never go wrong by avoiding the use of commercial .308 ammo in a 7.62x51mm rifle.

That's not to say that 7.62 is automatically safe in a .308, though...