7.62 vs. .308
Interested in starting a bar fight? Go ahead and walk into any bar in Chandler, AZ (full of gun gurus), and ask the guys about the differences between .308 Winchester and 7.62x51mm NATO ammo. The simple and practical explanation is this – They are different; They aren’t different; They actually might be different.
Oddly enough, these statements are actually all true. To avoid firearm nuances of things like piezo transducers, we’ll cover the differences in a very, very simple way.
The History of 7.62 NATO vs. .308 Winchester
During the 40’s and early 50’s, military-level scientists (rocket surgeons) wanted to improve the efficiency of the M1 Garand rifle. The cartridge of the said rifle is .30-06 and remains the earliest iteration of the 7.62×51 NATO ammunition. The commercial market was much more nimble, however, and the .308 Winchester was launched in 1952. It wasn’t until 1957 that the military (US Govt) had a cartridge and rifle hitting the barracks with similar specifications.
The Difference in Pressure
Some confusion comes into picture here. The max pressure value that’s thrown around for 7.62 ammo is 50,000 psi. As for .308, the max pressure is astoundingly 62,000 psi. Firstly, it appears to be a large difference between the two values – so much that gun enthusiasts deem it unsafe to fire a “higher pressure” .308 cartridge in a rifle built specifically for 7.62x51mm NATO ammo. Although commercial .308 ammo has a higher pressure than 7.62x51mm NATO, the brass and chambers are the values to understand there. “The devil is in the details” here, meaning that if I were to put 50,000 psi as a legitimate label for one of these ammunitions, it actually would be incorrect. See, that large 50,000 number is actually an accurate representation of copper units of pressure or CUP. The actual psi of the NATO ammo could possibly be 58,000 – with CUP to psi conversions. This new 58,000 is much closer to the .308’s, max pressure of 62,000.
“As both loads are routinely proof tested at far higher levels, this 4,000 isn’t a difference that’s going to alter the trajectory of Michael Moore’s daily Krispy Kreme run.” – Ammoland.com
The Difference in Thickness
Measuring the exact thickness of a cartridge is a huge pain, especially when your Dremel tool cuts any cartridge right in half… Let’s go ahead and illustrate the difference:
The average weight of a .308 is somewhere between 163.3 and 169.6 grains, whereas the average weight of a 7.62 shot is 183.5 grains. That’s a significant difference! The minimum difference is 13.9 grains, and the max is 20.2 grains. What a large stigma! Thicker walls combined with similar exterior dimensions means less powder capacity and lower “top end”. Lower pressure and velocity, for the thicker cartridges. The thicker 7.62 cases is significant, due to something called ‘headspace’, which we’ll talk about below.
The Difference in Headspace
“Last but not least we get to the real difference. Military rifles for 7.62x51mm NATO can, and usually do, have longer chambers. In things like machine guns powered by ammo made all over the world, there’s got to be some slack for reliable feeding and operation with all that violence going on during the feeding and ejection process. The solution is to make the chamber headspace a bit longer. If you’re not familiar with headspace, think of it as the distance from the bolt face to the point in the chamber that stops forward motion of the cartridge case. If chamber headspace is too long for a cartridge, it will float back and forth in the chamber. If headspace is too small, the bolt will not close properly or will require excessive force to close.” – Ammoland.com
Just how much is the difference in the headspace for these two cartridges? Good question, the .308 chamber headspace is between 1.630 and 1.6340 inches, whereas the 7.62x51mm NATo is between 1.6355 and 1.6405 inches. While the numbers show a rough six-thousands of an inch difference, it’s not unusual for the headspace in a surplus 7.62 rifle to be 10 or even 15 thousandths longer than a commercial .308 round.