This was my post from some time ago. I did a lot of research to come up with this conclusion. I have condensed and clarified my results. I have also included relevant parts of others posts on this thread and I thank you all. This is a discussion about whether 7.62x51 NATO and .308 Winchester have different pressures when fired. Some say .308 has higher pressure. Some say 7.62 NATO has higher pressure. Some say they are the same. THE ARGUMENT. The entire misconception comes from 7.62 and .308 being tested by two different methods. SAAMI testing quotes 62,000 PSI for service pressure of .308 Winchester. CUP testing quotes 50,000 CUP which is approx. 58,000 psi for service pressure of 7.62 x 51 NATO. Believe me I checked and double checked this information. .308 is at 62,000 psi max service pressure. With 83,000 to 89,000 psi for proof pressure. NATO EVPAT testing says 7.62 service pressure is at 60,190 psi. With 75,275 as the max proof pressure. This would put 7.62 at a service pressure very close to .308. Close enough not to have to worry one way or the other about higher pressure. You can see the actual NATO results here. NATO EPVAT testing - Wikipedia, the free encyclopedia THE RESULTS. So, the bottom line is .308 @ 62,000 PSI, and 7.62x51 NATO @ 60,190 PSI. This result is from tests conducted using the same testing. Technically, the NATO 7.62x 51 round is lower pressure, but for all intents and purposes, the pressures are approximately the same, and should not cause a problem with any modern firearm that is in good condition. PROOF PRESSURE. "Proof Pressure is, "The level of pressure at which a component, pipe, tube, hose or other fluid passage will not yield during application of internal pressure. ..." It is a pressure at which it has been tested to and found to be safe. All European weapons are proof tested and the test pressures stamped on the firearm along with an identifying proof house mark certifying the test. HEADSPACE. The headspace is slightly different. The .308 Win "Go Gauge" is 1.630" vs. 1.635" for the 7.62x51. The .308's "No-Go" dimension is 1.634" vs. 1.6405" for a 7.62x51 "No Go" gauge. That said, it is normally fine to shoot quality 7.62x51 NATO ammo in a gun chambered for the .308 Winchester (though not all NATO ammo is identical). However once fired in a 7.62 chamber, either case may expand large enough to not fit in a .308 Chamber again. CASE THICKNESS DIFFERENCE Also of note is the case thickness, the 7.62X51 case is much thicker than the commercial .308 brass, this can pose some problems with chambering issues and case head separation depending on the rifle used and reloading techniques. A good article on the differences between the two rds. http://www.surplusrifle.com/shooting2006/308vs762nato/index.asp As far as I am concerned they are close enough in PSI to be considered the same. Although different chambered rifles may be a problem. And I know Wikipedia is not always the best place to find intel. But this does seem to be legit to me.