7.62 NATO or 308 Winchester? What’s the Difference?

Copyright 2001 – Stephen Redgwell

Do you have a military surplus rifle chambered in 7.62 x 51mm? If so, you probably bought it to get a “308 Winchester” at a really low price. Well, you messed up. It’s like looking at a rabbit and a hare. They’re close, but each is distinctive, and you have to understand what you’re looking at.

Early in my military career, I worked on the FNC1 rifle (7.62 NATO). For someone that had only handled civilian shotguns and rifles, this was going to be a real treat! It was only natural that curiosity would generate questions and comparisons about what I thought was the same cartridge – 308 Winchester and the 7.62x51mm NATO.


One of my personal rifles was a Midland 2100 chambered for 308 Winchester. It was made in England by a small company – Midland Rifle Co – but owned by Parker Hale. Since there was a lot of military 7.62x51mm available, I shot them from my 2100 from time to time. There was never a problem chambering or firing the military stuff. It was a pleasant diversion over the long (read boring) periods spent at the range for base small arms qualifications.

Around this time, I discovered that shooting reloaded cases fired from an FN was virtually impossible. They didn’t want to chamber. My frustration led me to ask an older armourer what was wrong. In a nutshell, he told me that they probably didn’t fit because they stretched. I was using a Lee Loader in 308 Winchester and didn’t know that they only resized the neck, leaving the rest of the case untouched. The shoulders were blown forward on initial firing, so the case was simply too long to fit my rifle’s chamber! Hmmm…

I tried the gauges from work in my own rifle and was surprised to discover that none of them would fit! The corporal brought a set in from home and explained the difference. Here’s what I discovered.

308 Winchester (SAAMI) Gauges

GO – 1.6300″
NOGO – 1.6340″
FIELD – 1.6380″
Shop (Military) Gauges

GO – 1.6350″
NOGO – 1.6405″
FIELD – 1.6455″

The gauge sets were different too! What gives? He told me that the dimensions of the two new and unfired cartridges were basically the same. The difference lay in two areas – chamber size and cartridge makeup.


Look at the picture below. The top chamber represents a military rifle, the bottom one a commercial sporter. With many military rifles, their chambers can be significantly longer than, say, a Remington 700. Note that the military chamber would fail a NO GO check, but pass a FIELD check using the proper shop (military) gauges.

There is a .013″ difference in chamber length however, between these two “Safe & Serviceable” rifles!

There will be distinct differences between chambers of military rifles based on:

Type – Ishapore, Garand, FN etc.
Amount of Use
Condition at Time of Sale

Just by looking at the dimensions, you can see that using SAAMI gauges is bound to reveal “problems”.


I said at the beginning that these two cartridges were basically the same, dimensionally. While they may share similar external measurements, there is one major difference that you can’t see – case wall thickness.

Military brass is thicker. It needs to be. It was made to function and stretch in a wide variety of firearm chambers. Don’t forget that the condition of spent brass is of no importance to the service. Like 99% of military brass, it won’t be reused.

Where does that leave you? Follow this rule and you won’t have any problems.


Why not? Right away you know that the chamber is measuring more than 1.6380″ (SAAMI FIELD) – possibly more. Commercial 308 ammunition may stretch by 0.008″ when fired! Possibly more. It may even rupture! You must get the headspace corrected to bring it in line with SAAMI specs. Hey, it’s for your own safety!

Based on this rule, find your situation below.


You shouldn’t have any problems.


There is a potential problem with brass integrity. Commercial cases are NOT as thick as their military cousins. There is less brass, so if you use them in an original, unmeasured military chamber, there may not be sufficient material to stretch and fill the chamber without rupturing. That’s bad!!


You know that commercial brass is thinner. An easy way to see this is to weigh a fired case of each type. You’ll see that the commercial case is lighter. There may not be enough brass left – after lengthening – for any decent case life. You will only aggravate the situation by full length resizing (it weakens the case by overworking the brass), loading at or near maximum or both.

If you own a milsurp bolt gun like an Ishapore 2A or 2A1, consider neck sizing the brass for better case life, fit and accuracy.


Military ammunition is loaded to maximum average pressure 50,000 CUP (approximately 58,000 PSI using the modern piezo transducer method of measurement). This standard is used to ensure better consistency round to round. It is proofed at 67,000PSI. The ammunition can then be used in a wide variety of firearms with no ill effects.

Look for the NATO design mark.

NATO Design Mark

Commercial ammunition has a SAAMI/ANSI maximum pressure of 62,000PSI. While not every manufacturer may load it to this level, this is the industry established maximum. This is also measured using the piezo transducer method. The proof cartridge pressure is 83,000 to 89,000 PSI. Note the differences between the military test and operational standard vs the commercial one.

The worst case scenario for shooters of 7.62 NATO rifles is as follows: Using a commercially made, maximum pressure cartridge with long headspace, fired from a weak action (ex. converted 93/95 Mausers).

The weaker action is a possible final piece of the 308 vs 7.62 NATO puzzle.

Perhaps no single element will cause your rifle to fail. Maybe you’ve used your rifle for a while with no ill effects. Usually a rifle will take some abuse before failing. Like a lot of things in life, Murphy’s Law – If something can go wrong, it will – could very well catch up with you.

Personally, I don’t like the odds.