Originally Posted by etisll40
I have an FNar that reads 7.62x51
I have another Ar-15 that reads 5.56x45
Question; assuming the answer is the same for both these examples.
I can shoot 7.62x51 or 308 in my gun because it can handle the higher pressure of the 7.62x51 and the same for my 5.56x45 use 223 too?
It's a popular myth that the military 7.62x51mm cartridge has unsafe higher pressure than the commercial .308 Winchester. The reason is that some specs are listed in the old copper units of pressure (CUP) and other specs in modern piezoelectric strain gages' pounds per square inch (PSI) . 52,000 CUP has virtually the exact same pressure as 61,000 PSI. The conversion's not exact but this is within 1%.
Worse yet, is the fact that some places (web pages, manuals, word of mouth, etc) state pressure in PSI when the value is really CUP. The reverse happens too. Best examples are in older books published before piezoelectric strain gages were invented. Peak pressure valuse are often listed in PSI when the only methos used to measure peak pressure was copper disks crushed in a special pressure test gun.
Both the commercial and military versions of each cartridge have virtually the same peak pressure specs. The difference between them is less than the difference between maximum average and maximum peak pressures. Check out SAAMI's web site and look at your favorite cartridge maximum average pressure in one of the documents listed under "SAAMI/ANSI Standards:"
Google for "MIL-C- 7.62" then pick one of the military specs for the 7.62 NATO round for its peak pressure. Virtually all are listed in PSI but were actually measured with CUP numbers.