7.62×51 NATO Versus .308 Winchester: What’s The Difference?
BY Herschel Smith
4 years, 8 months ago
This rule is backwards from the 5.56mm versus .223 rule, in which a chambering for 5.56mm can shoot both, but not necessarily the other way around.
On April 1, 2020 at 11:59 pm, Georgiaboy61 said:
The two cartridges – 7.62×51 NATO and .308 Winchester – have a great deal in common, and usually can substitute for one another, but there are special cases when such is not recommended.
Reloading manuals stress the need to be cautious when transferring an equivalent load already used in .308 Winchester, to the 7.62 NATO case, recommending that the user back off on the powder charge by 1-2 grains (especially if near the safe pressure limit).
Some authorities claim that this is because the internal dimensions of 7.62 NATO brass are somewhat smaller, but a number of decent studies suggest that this is not always – or even typically – the case.
However, backing off a bit is still recommended, since 7.62 NATO cartridge cases are built to a uniform and somewhat more-stringent standard of case thickness, rigidity and strength than some commercially-available .308 brass. Because of this rigidity and strength, pressure spikes occur more-rapidly than in equivalent commercial .308 Winchester ammunition.
Military-spec ammunition, such as STANAG M80 Ball 147-grain FMJ 7.62×51, is required to perform reliably and well not only in rifles, but in crew-served weapons such as machine guns and mini-guns, and in infantry, armor, and aerial roles, to name a few. Such ammunition must work with cloth or linked metal belts, when boxed, and also with mechanical feeding systems such as those found in tanks, helicopters, fixed-wing aircraft, and the like.
Because feeding, extraction and ejection can be quite violent in such use, the 7.62 NATO case is designed to be tough-enough to handle such abuse and still work almost all of the time. To enhance reliability, military chamber and bore dimensions are often more-generous to enhance reliability, especially when the weapon is hot and dirty. This is another reason for the enhanced stiffness and strength of the case – it may have periods during the firing cycle when it is not supported, or inadequately supported – and must be strong-enough to handle such use.
Due to standardization agreements between the member nations, NATO-spec ammunition is produced according to strict guidelines so that – in theory, anyway – any 7.62 NATO M80 Ball – will function in any NATO member service rifles and MGs. That’s the whole point of standardization, so that a Belgian paratrooper for example can top off his M80 load at a U.S. or German supply point and be assured that the ammunition will work in his FN MAG or other .308-caliber weapon.
The commercial .308 market is another animal entirely. Some manufacturers make military-grade M80 Ball for civilian use, employing the same standards as used for STANAG production. They have discovered that some civilian shooters and sportsmen will pay well for such ammunition. There are, however, many who cut corners and make their .308 Winchester cases with only the minimum required material to meet SAAMI specs, and no more.
Contrary to popular belief, equivalent civilian versus NATO-spec loads tend to be “hotter” on the civilian side rather than the military. For example, 175-grain BTHP Match M118, 7.62 NATO rounds for military use clock in around 2580 fps muzzle velocity, whereas its civilian counterparts tend to have a 20-30 fps muzzle velocity advantage, depending on manufacturer. M80 Ball 147-grain has an MV of around 2750-2800 feet per second, but civilian makers of .308 hunting ammunition often push this considerably higher. These higher-pressure loads are safe for use in the typical bolt-action hunting rifle, which is built to handle such loads.
Often, it isn’t so much the ammunition being fired which is all that different, but the characteristics of the firearms or weapons firing them.
Military rifles and automatic weapons must be reliable and robust under field conditions, even if it costs a certain amount of accuracy to obtain. Chamber and throat dimensions tend to be more-generous to enhance reliable operation, and also to allow the weapon to function when it heats up and gets dirty/fouled. This typically costs some accuracy, the amount depending on the ammunition and firearm in question.
The typical .308 Winchester bolt-action, however, has a tighter chamber, to enhance accuracy, strength and also because it is typically not used for strings of fire longer than 2-3 shots at a time. Since long strings of fire are atypical, the dimensions can be fairly close because the rifle does not have to accommodate as much heat and dirt as the typical military weapon.
Having said that, the old salts who hunt like to work their bolt-actions over and over again, often using a bit of lapping compound, to smooth out and loosen up the action a bit. Make it easier to cycle when the piece is hot and dirty with carbon fouling.
On April 2, 2020 at 11:01 am, Gary said:
Georgiaboy61 thanks for a very informative post!
On April 2, 2020 at 12:56 pm, Georgiaboy61 said:
@ Gary:
Thanks for the kind words. Sorry for droning on so much. The nerd in me gets kind of excited by arcane technical knowledge sometimes. Get me started and you can’t shut me up!