Differences between 223 and 5.56 / .308 and 7.62

First off, if you are a serious gun guy, do not get offended if I do not get into this subject as deeply as Askins or some other full time – honest to goodness gun writer would – I am introducing the subject for those interested in guns, but not solely interested in guns – that being said – if you spot any factual errors or misleading statements, PLEASE comment. My intent is to discuss this issue from a basic standpoint, as one common misconception I hear VERY often is that .223 Remington is the same round as the military 5.56×45 NATO or the .308 Winchester is the same bullet as the 7.62×51 NATO

Both the .223 and the .308 are very close to their military counterparts. When reloading, a pulled bullet from a NATO round is identical in size to their caliber’s civilian counterpart.

In general, the external dimension’s between a 223 and a 5.56 or a .308 and a 7.62 are identical (more so on the 223/5.66), but internally there are differences, the NATO rounds develop much higher pressures

Additionally, the chambers in guns that are made for these two NATO cartridges have longer throats than the chambers in their commercial counterpart.

For the majority of this site’s regular readers, those that have guns as a part of their disaster preparations, but not as their ONLY disaster preparation, all you absolutely have to know about this topic is simple:

You can safely fire the .308 Win in a 7.62 NATO chamber but you cannot SAFELY fire a 7.62 NATO round in a 308 Win chamber.

It is exactly the .223 Rem. verses 5.56 NATO.

What happens is that since the throat of the civilian chamber is shorter, bullets fired in the NATO chambers have to “jump” a little farther to engage the rifling of the NATO cut chamber – which means that if you do fire a .223 in a barrel made for a 5.56 NATO round, it will have some loss of accuracy. This loss may be unnoticeable to you, especially if you shooting skills are far below your firearm’s inherent accuracy level – but rest assured it is there.

If you shoot the military round in the commercial chamber, the longer throat will cause the bullet to encounter on the rifling earlier, leading to higher chamber pressures – which can cause the receiver to fail (that should be read as explode). Once again, the word is can, like shooting wax slugs in your shotgun http://www.tngun.com/wax-slugs/, you can do it thousands of times with no incident, or once and lose a hand or an eye.

This is much more of a problem with the smaller .22 caliber, than it is with the larger .30 caliber of the 7.62, and I have never had a problem personally, nor have I met anyone that has. However, the facts are not disputed, and if you want to read more I have a chart that compares the military headspace of rifles with the SAAMI

Additionally, when reloading, know that brass will stretch more in a military chamber upon firing, thereby reducing the life of the brass and possibly promoting case head separation.
It is not all bad, with the additional length, the firearm will allow a round to chamber in an incredibly dirty weapon, which is a requirement for military applications.

To make the issue more complicated, a Go Gauge on a 7.62 is .003-inch longer than a .308 Winchester Go gauge.
The 7.62 NATO NoGo gauge is also longer, by .004-inch.

This means it is possible to chamber and fire a .308 Winchester round that would blow up 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.
If that was not bad enough, since .308 brass, is of commercial manufacture, it is thinner than the 7.62 NATO brass, it expands more, which could lead to casehead separation.

All this has been covered very well on the internet, and a simple search will take you as deep in the subject as you wish to go.
Luckily, if you are fond of poodle shooters, most all of the AR’s I find are all chambered for the 5.56 – in my experience the .223 chambers are mostly for bolt guns or single shots- so maybe this is only relevant as trivia.