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No, Virginia, .308 Win. and 7.62mm NATO Are Not IdenticalAt distressingly frequent intervals, someone can always be be counted on to pop up on an Internet Forum somewhere and ask Is the .308 Win round different than 7.62x51 NATO?. There follows a diluge (sic) of responses explaining with different degrees of success, this well-documented issue. So let this serve as a "mini-FAQ" on the subject. The .308 Winchester and the 7.62mm NATO (nee T-65) cartridges are not the same, nor should they be considered interchangeable despite apparently identical external dimensions… the chamber drawings are in fact different.But as Clint McKee and Walter Kuleck of Fulton Armory note on their "award-winning" website: They are the same, 'cause nobody makes 7.62mm (NATO) ammo that isn't to the .308 "headspace" dimension spec. So 7.62mm ammo fits nicely into .308 chambers, as a rule. While the 7.62mm NATO cartridge has a maximum chamber pressure of approximately 50,000 pounds per square inch (psi), in the SAAMI book the .308 Winchester has a MAP (maximum average product) pressure of approximately 62,000 psi* (each by conformal transducer measurements, and therefore comparable). This is not to say that all .308 Winchester loads will develop such pressures, merely that they would be within manufacturing tolerances if they did so. Firing .308 Winchester ammunition in a firearm specifically chambered for the 7.62mm NATO risks damage to the firearm and injury to the shooter.* - This translates to approximately 52,000 cup (Copper Units of Pressure). Chamber Headspace Gauges .308 WinchesterGO: 1.630"NOGO: 1.634"FIELD REJECT: 1.638" 7.62 x 51mm NATOGO: 1.635"FIELD REJECT: 1.6455" Chamber Pressures .308 WinchesterMAP: 62,000 psiMPSM: 66,000 psiMinimum Proof Pressure: 83,000 psiMaximum Proof Pressure: 89,000 psi 7.62 x 51mm NATOMaximum: 50,000 psiProof pressure: 67,500 psi Sources: .308 Winchester data from ANSI/SAAMI document Z299.4-1992, Pressure and Velocity, Centerfire Rifle Sporting Ammunition7.62 x 51mm NATO headspace data from Jerry Kuhnhausen's M1/M1A shop manual.Pressure data from TM 43-001-27: Army Ammunition Data Sheets Small Caliber Ammunition According to Ken at Clymer Tools, noted maker of headspace gauges, the problem isn't the round itself, it's the headspacing.A 7.62 NATO Go gauge is .003-inch longer than a .308 Winchester Go gauge. The 7.62 NATO NoGo is also longer, to the tune of .004-inch. It's entirely possible to chamber and have an accident with a .308 Winchester round in a rifle that would be safe for 7.62 X 51mm. A chamber in 7.62 that could barely close on a 7.62 NoGo could swallow a .308 Field gauge. Add to this the fact that .308 Winchester brass, being of commercial manufacture, is much thinner than that of the 7.62 NATO, and expands alot more, could possibly lead to casehead separation.And just when we thought that we had this 7.62mm NATO stuff down pretty pat, along comes Adam Firestone at Cruffler.com with his taste for the arcane, who makes a compelling brief that much of what many thought they "knew," was all wrong! An excerpt:Many shooters are aware of the differences between the dimensionally similar 7.62mm NATO cartridge and the .308 Winchester. What most are not aware of is that all cartridges called "7.62mm NATO" are not created equal, and that there is significant variation, both dimensionally and ballistically, between 7.62mm NATO cartridges as manufactured by different countries, and even between such cartridges as manufactured by different arsenals within the same country. As a result, the terms "NATO spec" or "NATO standard," which imply that all "NATO" cartridges are the same or to indicate the fitness of given 7.62x51mm ammunition for a specific use, are misleading.The Standard that Never Was: Debunking the Myth of NATO Standard Ammunition is an eye-opener… and iconoclastic! Once again, we need to challenge our perceptions.