The Difference Between .308 Winchester and 7.62x51mm NATO
When I acquired a Springfield Armory M1A, I was eager to put ammunition in it and then shoot the ammunition at targets downrange; this is, after all, what guns are for. However, this eagerness was tempered by nagging, ill-formed doubts about what kind of ammunition to feed this fine piece of applied ballistics: .308 Winchester or 7.62x51mm NATO? Rifles in .308 are capable of hurling a bullet through 24 inches of oak from two football fields away, so it behooves one to be sure he's got the right kind of ammo chambered before gently nuzzling his face against the rear of the receiver. For that reason, I decided to find out the difference (if any) between .308 Winchester and 7.62x51mm NATO ammunition.
Finding an answer turned out to be no problem at all; in fact, I found three completely different answers within the first thirty minutes of internet searches. Truly, there is a bounty of information online! Unfortunately, a great deal of this information is contradictory, much of the rest is incomplete, and next to none of it is backed up by anything resembling an official or reliable source. As much as I love trusting random people on the internet, one has to draw the line somewhere.
To cut a long and boring story short, persistant research finally produced the answers I sought. More importantly, these answers came with enough documentation to convince me that they are correct. In the interest of saving others the effort, I've summarized the results here.
By way of disclaimer, all data in this article are provided merely to help you verify for yourself which ammo is safe to use in your gun. No assertions are made or implied as to its correctness or your safety as a result of following these guidelines, so if you have any doubts you should probably consult someone who carries the appropriate type of liability insurance. I'm not a ballistician, mechanical engineer, or metallurgist; in fact, I don't even have any interesting letters after my name. My only degree is a Bachelor of Science, which is abbreviated "BS". Treat all information printed here accordingly.
With that out of the way, here are the rules I personally follow when it comes to 7.62x51mm NATO and .308 Winchester:
- Never use .308 Winchester in a rifle chambered for 7.62x51mm NATO.
- Check the length of all 7.62x51mm cartridges before using them in a .308 Winchester chamber.
Thanks for visiting, and be safe.
[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:
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.
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 than pressure 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:
If a firearms is chambered to the 7.62 NATO spec, it is absolutely unsafe to fire .308 Winchester for the same reasons that you wouldn't shoot a rifle that was shot so loose that it could swallow a NO-GO 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.
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)."
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...
[Check the length of all 7.62x51mm cartridges before using them in a .308 Winchester chamber.]
If you're read all the way through the admonitions about not using .308 Winchester in a 7.62x51mm chamber, then you know that 7.62 chambers are dangerously long compared to what's allowed for .308 Winchester. Interestingly, the cartridge dimensions themselves for 7.62x51mm are identical to .308 Winchester. All that extra space is just to improve reliability of feeding and prevent headspace issues in firearms that love to slam the case into the chamber with enormous force; e.g. machineguns.
The reason I check the case length of military surplus ammo is because that extra headspace is forgiving to cartridges that might be slightly oversized. Ammo that might never have an issue for a military using 7.62x51mm arms could prevent a bolt from going into battery on a .308 chamber. In a bolt-action rifle, this could cause headspace problems or overpressure if the bolt is forced closed. In a semi-auto, it could cause a slamfire.
I've yet to find a 7.62x51mm cartridge that fails to fit a .308 case length gauge, and I've checked every single one that crosses my path. However, there are anecdotal accounts on the web that over-long rounds have been encountered. Case length gauges are inexpensive and can be used to check hundreds of rounds while watching a movie. Given that it only takes one out-of-spec case to destroy a gun and/or parts of the shooter's body, it seems like a reasonable investment to me.
You can buy a case length headspace gauge here.
This is an impressive amount of research with exceptional arguments about pressure and headspace. I agree that headspace can be a big problem when firing .308 in a 7.62 chamber, and that one should measure and be sure of chamber dimensions before attempting to fire .308 in a 7.62 chamber and vice versa.
However, I believe one important item has been overlooked in this study. As we know, the M14 was developed to succeed the M1 Garand, and the M1 shares with the M14 or civilian version M1A the distinction that ammo loaded to higher commercial pressure standards should not be shot through the M1. This holds true whether the M1 is chambered in its original .30-06 caliber or has been rechambered for 7.62 NATO. Because of its great balistics, Winchester began producing the .308 Win. cartridge for bolt action hunting rounds and because these strong action rifles could handle the increased pressure of high velocity rounds, the pressure was naturally boosted for the commercial .308 Win. cartridge.
Why is this so?
It's because the higher pressure of standard high velocity .30-06 cartridges (in the case of the M1) and the higher pressure of standard high velocity .308 cartridges are know to damage these gas operated rifles' operating rods. We have to remember that the T-65 was developed solely for proper functioning in the M14 and its maximum rated pressure was arrived at in part to prevent operating rod damage which at the best case could cause accuracy problems and in the worst case cause inoperability. For handloaders of the M1, Hornady's reloading manual recognizes this by listing load information for this rifle in a special "M1 Garand" chapter. For the M14/M1A, the Hornady also lists reduced pressure load data in a special chapter titled ".308 Service Rifle". Owner's of M1 Garand's chambered in .308 or 7.62 NATO should only use the ".308 Service Rifle" load data.
The only commercial .308 ammo I've previously fired through my national match M1A and my national match M1 chambered for 7.62 NATO is the lower pressure rated American Eagle. While it has functioned fine, the fire formed cases are so enlarged that I had to use a set of RCBS small base dies to resize them. Obviously brass life was very short for these reloads.
Bottom line is to measure your 7.62 NATO chamber with a cartridge OAL gage like the one produced by Hornady (formerly sold under the Stoney Creek brand name) before firing .308 ammo through it. Another way to gage the differences between chambers is to measure and compare the differences between 7.62 NATO brass that has been fire formed through a 7.62 NATO chambered M1A and some .308 Win. brass that has been fire formed through a typical bolt action rifle (e.g. Win. M70, Rem. 700, etc.) and compare and contrast the resulting case size differences. Invariably you're going to find that the fire formed tolerances on 7.62 NATO brass will be greater than that of the .308. Another method to measure and compare these rounds is to take before and after measurements with an OAL cartridge and case length gage after firing a 7.62 NATO and a .308 cartridge through your 7.62 NATO chambered M1A (loaded, of course, to lower M1A appropriate pressure).
Nevertheless, the bottom line here is to never fire high velocity commercial .308 cartridges through your 7.62 NATO chambered M1A's or M1's, unless of course you don't mind having to replace the weapons' damaged operating rod.
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