A rimless bottlenecked rifle cartridge, the T65 was the result of a years-long test program, one that found the T65 to demonstrate power equal to that of the classic .30-06. It could fire 147 grain rounds at 2,750 FPS.

This cartridge laid the groundwork for what the 7.62 NATO would eventually become and is widely understood to be the first iteration of the cartridge. It had a 47mm case and carried steel jacket lead core 150 grain Flat Base bullets.

It was this cartridge that found favor among the military for emergency operations. But, as always, the commercial market was a cash cow begging to graze and Winchester saw the opportunity to make big bucks off a civilian model.

In no time at all (1952, to be exact), they had introduced the .308 Winchester. Here’s where the argument can be made that they aren’t different since the .308 Win was specifically designed to be a civilian variant of the 7.62x51mm NATO, even hitting the market two years before the NATO adoption of the 7.62.

Today, .308 Win ammo is the most popular short-action, big-game hunting cartridge around, leaving its twin in the dust. It is an effective caliber for everything from big game hunting to self-defense.

Having said that, let’s delve into some of the ways in which these two compare and contrast. I trust you’ll be surprised at what we find.

Under Pressure

This is where things get a touch confusing. The maximum pressure numbers for these two calibers are typically shown to be around 63,000 PSI for the .308 Win and just 50,000 PSI for the 7.62. That may seem like a significant difference and grounds for people to think it an ill-advised move to fire a higher pressure .308 cartridge in a rifle that’s fitted for the 7.62 NATO. But hold your horses there, fella. It really isn’t that simple.

The truth is that that so-called 50,000 PSI is not accurate, at least not in terms of pounds per square inch. 50,000 is more accurate if you’re talkin’ copper units of pressure or CUP which is a whole other animal. CUP is an inferior method of measuring pressure which depends on looking at how tiny an amount of copper disks compress when you fire your weapon. The approximate difference between CUP and PSI works out to about 8,000 which I think we can agree is a giant leap in terms of rendering a precise figure.

To put it another way, the max pressure for the 7.62 NATO is somewhere closer to 58,000, placing it closer to the PSI of the .308 Win than one might be led to believe. The 4,000 difference is hardly a difference at all, at least not one that would disrupt the trajectory of a shot aimed at a sluggish boar.

Getting into the Thick of It

The case thickness of the .308 Winchester versus the 7.62 NATO is a sticky wicket. Frankly, it’s a pain in the butt to try and compare the the case thickness of any two rounds, but I’ll do my best here to break it down for ya.

Summarily, military brass almost always have thicker case walls than commercial ones. Selecting a few samples from my bucket of brass, I measured the spent rounds and this is what I came up with:

• Lake City 7.62x51mm NATO = 183.5 grains
• Hornady .308     = 169.6 grains
• Winchester .308 = 163.3 grains

That’s a pretty big difference, wouldn’t you say? The fact of the matter is, thicker walls in conjunction with similar exterior dimensions equates to less powder capacity and a lower top end. This means lower pressure and less velocity.

The thicker brass of the 7.62 will figure into the following.

The Right Headspace

This is where the real big difference comes into play. Military rifles designed for the 7.62 NATO generally have longer chambers. In things like machine guns or assault rifles, there needs to be room for reliable feeding and ejection.

The answer is more headspace, specifically longer chamber headspace. If you’ve never heard of headspace before, essentially it’s the distance from the bolt face to the point in the chamber where forward motion of the cartridge case is stopped. When chamber headspace is too small, the bolt won’t close properly or may need extra force to shut.

So where do these two bad boys stack up when it comes to headspace? The .308 Win headspace is between 1.630 and 1.6340 inches while the 7.62 NATO is between 1.6355 and 1.6405 inches.

The published numbers place the difference within six-thousandths of an inch, but surplus 7.62 rifles can be anywhere from 10 to 15 thousandths longer than a commercial .308 Win. That may not seem like a big deal, but firing thinner commercial brass in a long-chambered rifle can cause the brass to stretch and even potentially cause a possibly dangerous case rupture.

With thicker military brass, this isn’t a concern because the military rifle is designed for it. With thicker brass, you know it can handle that excess stretching in a longer chamber throat.

Establishing your headspace shouldn’t be a problem. You can easily order a set of .308 Winchester Go/No-Go headspace gauges online. Once you’ve removed ejectors or extractors to get rid of all sources of tension, you can use the gauges to check chamber size.

Avoiding Any Risk

The simplest way to prevent any harm from coming to your or your rifle is to go “back to basics.” In other words, if you wanna be safe, go the conservative route and only fire 7.62x51mm ammo in rifles that are chambered for 7.62 NATO. The same can be said of the .308 Win.

But if you want to take a walk on the wild side, you might use the 7.62 NATO in a .308 Winchester rifle. While dangerously high pressure may occur, it’s not all that likely. In fact, it’s a fairly rare occurrence.

Although many modern 7.62 chambers are okay because they’re generally cut closer to .308 dimensions due to the .308’s long commercial status, it is best to stick to what you have where headspace is concerned. Don’t push it if you don’t have to.

If you’re working with an AR-15, you want to make sure you’re using the right ammo for your chamber. .308 Win is a viable caliber for this kind of rifle. It’ll give you the range and penetration you’re looking for. AR rifles are also chambered for 7.62x39mm. While they’re typically able to be adapted to a wide range of calibers, it’s fairly uncommon to hear AR and 7.62 NATO in the same sentence.

Another important thing to consider is ammo storage. Where you keep your ammo is important because gunpowder is a hazardous material that could harm your children, your pets or even you yourself. I like to divide my ammo up and keep it in a temperature-controlled gun safe with a gun safe dehumidifier so that it doesn’t have a chance to corrode.

In the end, it all comes down to what kind of person you are—a responsible gun owner or a risk taker. Personally, I always advise my friends and fellow gun enthusiasts to take all proper safety precautions when they’re using these cartridges or any others.

They might think I sound like a broken record, but it’s for their own good and I like to think they’ll thank me down the road. There’s no good reason to compromise your rifle or your well-being.

Conclusion

Choosing the right ammo is always difficult, but I hope that this article has helped you to see how these two cartridges are similar and why either of these calibers may be right for you. Good luck, God speed and happy hunting.