The .308 vs 7.62x51 Debate.
While I do not see much to debate I'll share my thoughts on the subject and toss in a few pictures too.
I think at this point that everyone believes the cartridge cases of the .308 Winchester and the 7.62 X 51 NATO cartridges are identical. Well identical in that the outside cartridge dimensions are identical and for this reason either cartridge will chamber in a .308 Winchester chamber or a 7.62 X 51 mm NATO chamber. The cartridges outside specifications are identical.
The cartridges inside dimensions are a slightly different story, I believe we know that also and we know why. The Military Brass in 7.62 X 51 is thicker and this is especially true at the base of the cartridge in the web area. Since the outside case dimensions are identical the inside case dimensions of the 7.62 X 51 NATO cartridge must be smaller. Something has to give correct? That said the NATO cartridge will have less case volume. The only time this difference comes into play is if the spent cases are later hand loaded.
Both cartridges are factory loaded to very similar chamber pressures. The only somewhat confusing part is how the numbers are derived. The NATO designated case uses CUP (Copper Units Pressure) while the .308 Winchester uses PSI (Pounds Square Inch). The difference in actual chamber pressure is pretty much insignificant. The CUP method was developed long ago before we had better and more accurate means to measure chamber pressure. There is no magic bullet or formula to accurately convert one to the other. Overall it really matters not as factory loads in either cartridge will produce the same results as to pressure or so close that any difference will not come in light of the cartridges intended use.
So let's take a look at some cartridges:
Left to Right we have a 7.62 X 51 NATO cartridge, A run of the mill .308 Winchester cartridge and a .308 Federal Gold Medal Match cartridge. All three cartridges are identical in dimensions, especially the ever so important dimensions. How do I know this? Let's take a look at a specification sheet for the .308 Winchester cartridge:
This specification sheet is kind enough to include the .308 Winchester cartridge case dimensions (top) and .308 Winchester chambering dimensions (bottom). The upper case dimensions are identical for a 7.62 X 51 NATO case or a .308 Winchester case.
The ammunition pictured above I said was identical is pretty much really identical as to key case dimensions. I know because I measured all three cases. How did I measure these cases? I used a system that is "dead on balls accurate" a very, very technical term taken from the movie My Cousin Vinny.
On the left is the RCBS .308 Precision Micrometer Set for the .308 Winchester Cartridge. On the right is a set of Forrester .308 Winchester headspace gauges. The gauges on the right were used to check the RCBS gauges on the left. I measured all three factory loaded cases and all three were 1.630 from the base of the cases to the datum point on the case shoulders. For the curious that is where the case shoulder has a .400" outside diameter. All three cases were 1.630" within .0005". Dead on balls accurate!
So much for ammunition as the dimensions are the same. The big difference lies in the rifle barrel's chamber. Again, look at the image with the dimensions. The commercial .308 Winchester cartridge shows a headspace dimension of 1.630" MIN and 1.640" MAX. The headspace dimensions for both .308 WIN and 7.62 X 51 NATO are:
7.62 X 51 NATO
1.6355" Go
1.6455" No Go
.308 Winchester
1.630" Go
1.634" No Go
1.638" Field
The above numbers for 7.62 NATO based on Forrester Gauge specifications. So in the real world it is about chamber size and not cartridge size as all cartridges be they .308 WIN or 7.62 NATO have the same case specifications.
Military rifles are dragged into every clime and place where we can take a gun. From the snow of far off Northern lands to tropic desert sun (I always liked that song). Therefore their chambers are cut to a looser tolerance for the ammunition they will fire. The guys who design these chambers want a rifle that will function reliably under a wide range of adverse conditions.
Since the initial post mentioned the M1A and a few subsequent post mention the M1A we may as well look at the M1A. My own M1A is a Springfield Armory NM rifle and the barrel is clearly stamped NM .308. My particular rifle has a headspace between 1.631" and 1.632" and I know this to be true because I checked it. So let's check the headspace of a M1A rifle chambered in .308 Winchester.
First we need to field strip the rifle down and remove the bolt:
Next we need to strip the bolt which for those who have done this we know it can be a less than rifle fun experience. I cut down a 30-06 DePrimed new brass case to an OAL of about 1/2". I use this with a 6" C clamp to compress the bolt. Then drive out the extractor using a pin punch. This will relieve the spring tension on the ejector. Gently open the C clamp and you get this:
The following images show the Stripped Bolt closing on an empty chamber, a No-Go gauge and finally a 1.632 gauge in the chamber.
Now overall, after all of this, I have no reservations whatsoever about feeding my M1A 7.62 NATO or .308 Winchester ammunition. Would I have any reservation about feeding any military surplus rifle chambered in 7.62 NATO factory commercial .308 ammunition? No I would not as the cases are identical by specification. Is the .308 Winchester a hotter round over the 7.62 X 51 NATO in factory loadings? Not in any of my observations and I have shot both, over a chronograph, in both my M1A and a Remington 700 custom bolt action I built.
Setting new factory ammunition aside, the only consideration would be for the reloader. If you are reloading .308 WIN or 7.62 X 51 NATO ammunition that has been fired in a military chamber of unknown dimensions then obviously Full Length the brass which is stupid to include since any reloader dealing with an unknown brass size will do that anyway.
Ron
