7.62 vs 308


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ABroken11b
May 25, 2011, 04:38 PM
Ok, forgive me if this is in the wrong area, new to the forum but not to guns or shooting. A buddy and I heard this the other day and we need some clarification cause it doesn't make any sense to us. We were told that you can fire 7.62 x51 in a 308 without problems, but firing a 308 in a 762 is dangerous. We know not to shoot 5.56 in a 223 cause of the pressures. Wouldn't it be the same for 762 and 308 as well?

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Bill2e
May 25, 2011, 04:44 PM
Do a search, there is plenty of info.

My understanding is the .308 has higher pressure than the 7.62.

also the case thickness and shoulder angles are slightly different.

ABroken11b
May 25, 2011, 04:52 PM
I had done one earlier and alot the posts I had found were contradictory. Out of all the firearms forums that I have read, this and one other seem to have the most intelligent responses, that's why I asked.

ABroken11b
May 25, 2011, 04:55 PM
Sorry guys, I just noticed the similar threads listed below. I also meant I searched other forums and found contradictions.

mgmorden
May 25, 2011, 05:02 PM
7.62 NATO is chambered a bit lower than .308 Winchester, despite the two rounds being geometrically identical.

As such, a gun chambered for .308 can handle either. A gun chambered for 7.62 may experience "issues" (explosion seemed to strong a word :)). Case in point - a lot of small ring Spanish mausers were rebarreled for 7.62 NATO which a small-ring Mauser action can take, but .308 Winchester is too much for it (and needs a large-ring mauser action or stronger).

This is particularly confusing as when comparing military for commercial for .223 Rem vs 5.56 NATO, the situation is reversed and 5.56 NATO - the military round - is the hotter of the two.

MEHavey
May 25, 2011, 05:04 PM
See:
http://www.smellysmleshooters.net/ammopressure.htm
Military 7.62 slightly higher pressure than commercial

See:
http://www.thefirearmblog.com/blog/2007/09/03/308-winchester-vs-762x51-nato/
Military 7.62 is lower pressure than commercial

ABroken11b
May 25, 2011, 05:05 PM
Mgmordan, thanks that is almost exactly what I needed to know.

CutMan
May 25, 2011, 05:21 PM
i was always told that unlike .223 and 5.56, that .308 was interchangeable with 7.62 x 51. I've never had a problem.

http://en.wikipedia.org/wiki/.308_Winchester

Shadow 7D
May 25, 2011, 05:40 PM
^^^^^^^^
thats what
I understand too

06
May 25, 2011, 05:46 PM
Have fired them both ways in both kinds of rifles-no problems experienced. Have never heard of any personally. All kinds of rumors but has anyone actually experienced a "blow"?

EC47
May 25, 2011, 06:00 PM
I have heard that the some .308 ammo has softer primers than
the 7.62x51 thus could cause an AD in an auto or semi-auto
rifle like the M1A.

jeepguy
May 25, 2011, 06:55 PM
i have shot some 150gr. 308's in my m1a's but i normally shot 7.62 nato surplus. mostly becuse it is cheaper, comes in bulk, & is what the rifle was made for.

ABroken11b
May 25, 2011, 08:33 PM
As far as I can tell, after reading the links given by others what I've determined is 7.62 or 308 is usable in a modern well made rifle. Where the problem lies is with older rifles. I originally posted the question because I have fire both from my Remington 700, and my friends do the same from their m1a. When we heard the statement we were concerned because we didn't want to be unsafe and injure ourselves or anyone else, nor fifer want to damage some of our favorite rifles.

RugerMcMarlin
May 25, 2011, 08:33 PM
2 weeks ago we shot 2 boxes marked Federal .308 in brand new Spingfield M1A, being newgun we checked the brass over pretty good. seemed just hunky, no evidence of soft primers. I have heard something about harder primers but it applied to some belted up M60 ammo. In general I have experienced military to always be hotter than factory commercial.

Wait a minute whats the Spanish round/ G3 clone ammo same as 7.62dimensions but not same CUP?

Robert
May 25, 2011, 08:49 PM
They are pretty much one in the same. There are some minor differences. 7.62x51 bass is thicker but that only changes the interior dimensions. Were most folks get in a bind is the pressure issue. SAMMI uses PSI to measure pressure while NATO uses CUP. The two methods are not interchangeable. There is really no way to convert PSI to CUP or CUP to PSI with any reliability. As long as your firearm is in good shape you are free to use either one.

jonnyc
May 25, 2011, 09:47 PM
Bingo, Gus is right. The two are the same and perfectly interchangeable EXCEPT for reloaders. You can't load a hot .308 load into a 7.62x51 case, it has slightly less internal capacity.

TexasRifleman
May 25, 2011, 09:52 PM
Gus is right, there is no direct conversion between CUP and PSI, however there IS good evidence to build a correlation chart. Lots of research on this has been done on this topic by shooters. The following is an interesting read on the subject, and offers some charts to help with correlation. If the math is too much skip to page 6.


http://www.shootingsoftware.com/ftp/psicuparticle2.pdf


Bottom line is there is no appreciable difference in pressure between 7.62x51 and .308. What happens is in chamber dimensions and thinner brass on commercial ammo. In a rifle with excessive headspace especially you will see case bulging or ruptures with commercial brass that LOOKS like overpressure, so that's why people will say they see higher pressures in .308, the brass simply gives out sooner without a chamber capable of supporting the brass.

Robert
May 25, 2011, 09:58 PM
I learned it all from you TR.

velocette
May 25, 2011, 09:58 PM
+1 for Gus n Jonny! They's got it right. The two cartridges are essentially the same. The only difference is the method of measuring the pressures, Copper Units of Pressure, and Pounds Per Square inch. They do NOT interchange, nor is there a formula or conversion chart.
The only real difference is that military weapons generally have much more generous chambers to permit reliable function when very dirty, sandy, and hot. Military brass generally, (but not always) has thicker walls to function well in loose chambers without problems. (read machine guns)
This is NOT the same as 5.56 military and .223 civilian, they are NOT the same.

Roger

hm
June 5, 2011, 02:13 AM
Check me on this since I want to be clear before I make an uninformed mistake. I should be able to fire Win 7.62 from my Howa 308 without worrying about firearm damage, right? If so, what kind of ballistics can I expect?

helotaxi
June 5, 2011, 08:29 AM
NATO does not and has never used CUP for pressure measurements. Being a mostly European organization, they have always used mega-Pascals (MPa) for pressure specifications. They also measure in the neck of the brass not the body. SAAMI formerly used CUP but has since switched to PSI. PSI and MPa are directly comparable; however, what is not directly comparable is the testing location. Testing at different places can result in different pressure readings so the two are not 100% interchangeable though the pressures are very close: 7.62x51 @ 415 MPa (60,200 PSI) and .308 Win @ 62,000 PSI (430 MPa).

There are three problems that one can run into:

1) The owner of a semi-automatic (or NFA fully automatic) chambered in 7.62x51 could have a weapon with very generous headspace. The NATO headspace spec has a very wide tolerance. The chamber would pass a check with a field gauge, but still be excessive per .308 Win specs. Since the .308 is made with thinner brass, firing .308 in this "loose" chamber may result in case failure. 7.62x51 has very thick brass to keep this from occuring.

2) One could find themselves with a SAAMI .308 Win chambered rifle and a box of European made NATO ammo which would be perfectly safe; however, the ammo is on the long side on the datum point and will not chamber. This is not unsafe, merely annoying.

3) A less than diligent reloader could assume that since 7.62x51 and .308 are interchangeable then they could treat the brass as 100% interchangeable (like .223 and 5.56x45) and he simply uses his normal .308 load in some 7.62x51 brass. The thicker brass will result in higher chamber pressure with the same load and he may get quite the surprise.

ETA: hm- Yes, you can safely fire 7.62x51 in your Howa and you can expect ballistics along the lines of NATO spec for the round, i.e. 2800 fps from the 150gn and 2700 fps from a 168 gn.

madcratebuilder
June 5, 2011, 08:35 AM
Chamber Size & Headspace

Headspace, there is a .013" difference in acceptability, between these two specifications. This is significant in that, for reloading purposes, brass will stretch more in a military chamber upon firing, thereby reducing the life of the brass and possibly promoting case head separation. But that additional length will allow a round to chamber in an incredibly dirty weapon, which is a requirement for military applications. This is the chamber specification and not the ammunition specification. The external dimensions of the two types of ammunition are nearly identical.

308 Winchester (SAAMI) Headspace
GO - 1.6300"
NOGO - 1.6340"
FIELD - 1.6380"

7.62 NATO (Military) Headspace
GO - 1.6350"
NOGO - 1.6405"
FIELD - 1.6455"

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.

The above is taken from a paper the FALPhil wrote a few years ago. It had been posted on-line but the site is no longer up. If anyone would like the 8 page document, PM me with your e-mail and I'll send you a copy in .pdf.

Dreamcast270mhz
June 5, 2011, 12:48 PM
This seems to be reliable, my gunsmith recommended it as a ballpark, but don't take my word for it:

psi = -17,902 + (1.51 c.u.p.)

conversely

c.u.p. = (psi + 17,902)/1.51

But overall it is best to just use care with your ammunition

helotaxi
June 5, 2011, 02:31 PM
The problem with trying to convert CUP to any actual pressure is that CUP "captures" all the area under the pressure curve. It does a very poor job of showing peak pressure. It is possible for a lower pressure load with a long duration to measure the same as a load with a higher pressure but of short duration.

If you look at the specs for a round with pressures defined in CUP, there are some interesting terms used specifically to capture the weaknesses inherent in the CUP system. Those weaknesses are also why SAAMI no longer uses that standard.

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