Jim Kennan,
Maybe I explained too much. Here's the short version:
A 7.62x51 chamber headspace is longer than a .308 Winchester chamber by enough to make a difference.
7.62x51 cartridge headspace is just a little bit longer than .308 Winchester cartridge headspace on average, though it's in the upper end of the >308 Winchester cartridge headspace specs.
So a .308 Winchester cartridge will probably be too short to function safely in a 7.62x51 chamber, especially considering that .308 Winchester brass isn't as sturdy as mil-surp 7.62x51 brass.
A 7.62x51 cartridge will be too long to function safely in a minimum headspace .308 Winchester chamber, but it should be fine
as long as the headspace is 1.632" or longer.
Now a lot of people exchange the two different types of ammo freely in both kinds of chambers & you do not hear about blow ups everyday. In fact they're pretty rare. That doesn't mean it's safe to switch the two cartridges, it means a lot of people have better luck than they think.
But like I said, a lot of people redline loads (i.e. load above maximum powder charge, seat bullet too deep for a given powder charge, etc...) & you don't hear about them blowing up rifles & pistols everyday. That doesn't mean it's safe to redline a load, it just means a lot of people get away with it.
But what I do bet is common is very short brass life for the reloader who uses .308 Winchester ammo in a 7.62x51 chamber. & that's something that you wouldn't hear about as most people would blame it on that particular rifle or lot of brass rather than the headspace difference between chamber & cartridge.
& when it comes to headspace, 0.01 is a big deal. There's only a 0.01 difference between minimum & maximum headspace in the .308 Winchester. But don't take my word for it; ask any gunsmith you know if a .308 Winchester chamber .01 longer minimum headspace is safe to continue using.
& for the record I don't get on anyone who polishes their chamber until it gets to be .00000003" too large.
Dave R,
Reloading manuals deal with cartridge dimensions, not chamber dimensions. Any 7.62x51 brass you resize will be resized to .308 Winchester specs. The only difference in cartridge headspace between the two is that 7.62x51 brass is within the upper end of the .308 Winchester cartridge length range.
Reloading manuals do not get into detailed discussions about chamber headspace. It's not their purpose; they just deal with cartridges. & since the reloading process will flatten out any differences in cartridge headspace it's a moot point for them to get into. The main difference is the chamber dimensions, not so much the cartridge dimensions.
& from a certain point of view they are right that the two cartridges are basically the same, since the 7.62x51 is within the cartridge headspace range of the .308 Winchester; it's just held to the upper end of that range. It's kinda like saying match ammo is the same as regular ammo in that cartridge; it's within specs but held more towards one end than the other.
But it's important to realize that 7.62x51 cartridges are held on the upper end of the acceptable range.
If you've fired 7.62x51 ammo in your bolt gun, I'd say chances are your chamber is at 1.632 or longer, though it could be right at minimum. Bolt guns are much more forgiving of cartridge differences than semi's are. & when you resize those 7.62x51 cases, they go through the same die (adjusted the same way) as your .308 Winchester cases right? That means they're resized within the .308 Winchester tolerances (which they were within to begin with). Then you trim the brass right? & you trim it to the recommended length for .308 Winchester. So for all the reloads the only difference between commercial .308 Winchester cases & your resized 7.62x51 cases is that the 7.62x51 cases are a little thicker.
& most gunsmiths & gun makers that I know don't make .308 Winchester chambers right at minimum unless it's a special order. Usually it's 1.632 or so instead of 1.630. This would eliminate any problems with firing mil-surp 7.62x51 in a .308 chamber.
The most important thing is to know the headspace of your chamber. That'll let you know if you should be concerned about it or not. & it's important to know the differences between the cartridges so you can decide if your chamber is safe to interchange ammo.
I'm not saying you can't fire 7.62x51 in a .308 chamber; I'm simply saying you should know exactly what you're working with before you do. Make sure your chamber headspace is 1.632 or longer & you should be fine.
I am saying you shouldn't fire .308 Winchester in a 7.62x51 chamber. It's not worth the risk.