Educate Me on 7.62 vs .308

[tps7742]

OK I thought I had it figured out, now I am confused.

[M16]

It all about the neck thickness of the cartridge. Military brass has a thicker neck. If you shoot a 7.62 in a tight necked .308 you could have a problem. Basically no factory rifles have a tight necked chamber. The increased pressure comes from not having enough space for the brass to expand causing a rise in chamber pressure.

[Cable]

OK I thought I had it figured out, now I am confused.

Me too. I feel like I took 2 steps forward and 3 back.

[Cable]

It all about the neck thickness of the cartridge. Military brass has a thicker neck. If you shoot a 7.62 in a tight necked .308 you could have a problem. Basically no factory rifles have a tight necked chamber. The increased pressure comes from not having enough space for the brass to expand causing a rise in chamber pressure.

Okay, So what about .308 in the 7.62 as stated in the OP LOL?

[Smell the Glove]

OK I thought I had it figured out, now I am confused.

Here's a good write up about it.

"The internet firearms and shooting culture is a relatively close knit group and very computer savvy, as hobby groups go. Many of the community are members of the several dozen discussion groups that revolve around the special interests of gun owners.

Because of the nature of the internet and the inherent tendency of human beings towards believing anything that sounds reasonable, without applying critical thinking skills (probably a result of trends in government school systems – but that is another treatise), there is much misinformation available to the casual gun enthusiast about a variety of subjects concerning firearms.

One of the most pernicious of these “urban legends” is that there is a significant difference in the pressures between the 7.62x51mm NATO cartridge and the 308 Winchester cartridge. The misinformation indicates that using the commercial offering in a military weapon will visit death and
destruction of biblical proportions upon the miscreant who would attempt such a thing.

The real problem is the confusion between the old and the new methods of pressure testing. The old pressure testing method used for the 7.62 NATO cartridge started out life in the 1950s and is still published today in the US Army Technical Manuals. The figures are based on the copper crusher
method in CUP, but are published as PSI. The new method is the piezoelectric strain gauge transducer method; it is the same technology used today to show an automobile’s oil pressure. The piezoelectric strain gauge transducer pressure method is a direct pressure reading based on an absolute standard, where the older copper crusher method a conversion based on a relative measure and a conversion chart. And this is why you see the difference in the pressure readings, but the older 52,000 CUP is equal to 62,000 PSI (piezoelectric transducer method).

Today, these two methods are called CUP and PSI and the readings are different, but 52,000 CUP equals 62,000 PSI and both are the same pressure, similar to the way 60 MPH equals 100 KPH.

Conclusions

The pressure difference between the two rounds is insignificant, the real problem is commercial ammunition has thinner cases that were not designed to shoot in military chambers BUT we do it all the time anyway and this why you see more case head separations on commercial cases fired in military chambers.

The M118 special long range round is loaded to 52,000 CUP (all other U.S. 7.62mm are 50,000 CUP) which would be equal to the pressure levels of commercial ammunition, this means actually there is no pressure difference between the .308 and 7.62 NATO for the M118 cartridge.

No accurate conversion between copper crusher and true pressure exists, but approximations can be made. In all the conversions outlined above, pressures are in thousands of PSI (KPSI). Expect errors of several KPSI, or about 15%, with such formulas. Many factors determine how much the indicated pressure reading from a crusher misses the true pressure, and the error varies among cartridges and even among different loads for one cartridge. The conversions might be accurate enough for many practical purposes.

So, to sum everything up, the pressure difference between the 308 Winchester and the 7.62x51mm NATO is less than 2,000 PSI which is statistically insignificant. The same pressure variation may be achieved by firing any rifle on a hot day and on a cold day or by changing brands of primers. It is safe to shoot 308 Winchester in your 7.62x51 rifles (even the Ishapores) and vice versa. Handloaders should be aware that they should reduce the amount of powder when using military 7.62 NATO cases by about 10-12% and work up to safe pressures with corresponding velocities."

[Matteo]

Stamped 5.56 can shoot .223 & 5.56 not vice versa

Stamped .308 can shoot 7.62 & .308 not vice versa

X2

[Tommyh]

Okay, So what about .308 in the 7.62 as stated in the OP LOL?

if it is truly a 7.62x51 chamber, 308 rounds will load but the casing is thinner. what will happen, is upon powder ignition, the brass will heat up, thin out more, and flow towards the rifling because of chamber dimensions being a little longer in 7.62x51. this can, and has, cause case rupturing and head separation on top of body parts damage. the brass on 7.62x51 NATO brass is thicker to fit in looser toleranced MG chambers and not flow forward.

[Cable]

Alright. So I won't shoot .308 rounds out of it. Glad I asked since the gun dealer said any ol .308 round would be fine.

[M16]

Okay, So what about .308 in the 7.62 as stated in the OP LOL?

Won't be a problem. Go for it.

[Buckhunter75]

Guess I had it wrong. The higher pressures explanation makes a lot of sense.

[grtwht]

Cable you can do it. PM sent