Gus Fisher did a whole series on headspace and you can find it at
http://m14tfl.com/upload/showthread.php?t=66601&highlight=headspace
Also, if you can get a copy of Jerry Kuhnhausen's book
The U.S. .30 Caliber Gas Operated Service Rifles
A Shop Manual
Volumes I & II
pages 43 & 44 have specification drawings for the 7.62 and .308 cartridges and chambers
A larger picture of the 7.62 specs
That simplified drawing in a previous post of the chamber differences between a 7.62 and .308 chamber can cause a little confusion without some explanation. The headspace values are measured at the same point on the shoulder of the cartridge and the chamber on both the 7.62 and the .308 but the specs are slightly different. The cartridges are basically the same (with a few minor differences here and there) but the chambers are different enough to cause problems with the 7.62 military chamber having longer dimensions.
Both chambers have a range of headspace dimensions that they can fall within;
- 7.62
min - 1.6355"
max - 1.6405"
- .308
min - 1.630"
max - 1.640"
So to ensure you know exactly what kind of chamber you have you should measure the headspace dimension on your chamber rather than take somebody's word for what it is.
There are two worst case scenarios to worry about.
The first possible case is if you have a tight .308 chamber (1.630") and you want to shoot foreign made rejected military ammo that measures the NATO recommended maximum headspace dimension of 1.634" at the datum line on the shoulder. Your cartridge will be 0.004" too long for your chamber. This could result in the neck of the case being compressed around the seated portion of the bullet (due to the neck of the case being pushed in to the free bore area of the barrel which is a slightly smaller diameter than the neck). In turn, this would increase the pressures experienced by the cartridge and chamber and you might blow out the primer which would then allow hot gases to reach the bolt face which might blow up your rifle or force those hot gases in to your face.
The other case is isn't much better. This would be where you have a 7.62 maximum chamber dimension (1.6405") and you want to shoot .308 ammo that is manufactured with the minimum headspace dimension of 1.627". In this case you would have at least 0.0135" of clearance between the case and either the chamber's shoulder or the bolt face or split between the two. In this case you would get a lot of expansion and, depending a lot of variables, you would either get very short life spans for your cases due to working the brass a lot or the case head will separate from the case and gases might blow back in to the receiver and blow up your rifle.
If you want your rifle to reliably fire both 7.62 and .308 then you want a chamber headspace dimension of 1.6355" but even with a chamber dimension of 1.631" (which is pretty common with the SAI NM M1A) you would be able to shoot standard .308 ammo and most military surplus, with U.S. surplus being preferred. Even though most 7.62 and .308 cartridges use the same specs (with a headspace dimension of 1.630" being common) there will be some foreign made ammo that will exceed that dimension and if you have a tight .308 chamber (1.630" to say 1.632") then some of that foreign military surplus will not work.
One final note, don't use any of the new high velocity ammo in your M14 style rifles. The M14 style rifle was designed for chamber pressures that are not as high as what the SAAMI standard is for commercial .308 ammo. Standard .308 ammo is safe but if you use a "magnum" style cartridge in the M1A you might just blow it apart.