Differences: .308 and 7.62x51 NATO


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MarineOne
November 14, 2009, 12:39 AM
I just picked up one of the many gun magazines and it's got a new rifle from LMT that fires the .308 Win instead of the 7.62x51 NATO. What are the differences in these two rifle rounds?


Thanks.

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TexasRifleman
November 14, 2009, 12:45 AM
There is no difference, there is a difference :)

The differences are slight and in most firearms it won't matter. In semi auto's you need to watch headspace.

I'll just repost what Walt Kulek over at Fulton Armory says about it:

They are not the same, 'cause the .308 Win was released by Winchester several years before the Army standarized the T64E3 as the 7.62MM. You'll get an endless discussion of pressure specs, endless because SAAMI and the Ordnance Dep't measured pressure in different, unrelateable ways. Howver, the chamber drawings are different.

They are the same, 'cause nobody (and Clint's been looking for many years!) makes 7.62MM ammo that isn't to the .308 "headspace" dimension spec. So 7.62MM ammo fits nicely into .308 chambers, as a rule.

But in some 7.62MM rifles the chambers are long (to the 7.62MM military spec), notably the Navy Garands with 7.62MM barrels. Thus, using commercial ammo in such a rifle is not a good idea; you need stronger brass. Use military ammo or the best commercial only, e.g., Federal Gold Medal Match.

Most of the time it's a distinction without a difference. But if you intend to shoot .308 commercial in a military arm chambered for 7.62MM, first check the headspace with .308 commercial gauges first. You may get a surprise.

Clint McKee at Fulton says this:

Whether you have a NATO chambered barrel (M14/M1 Garand G.I. ".308 Win."/7.62mm NATO barrels all have NATO chambers), or a .308 Winchester chamber, keep the headspace within SAAMI limits (1.630 GO, 1.634 NO GO, 1.638 FIELD REJECT)

You may read things about pressure differences. SAAMI uses one type of pressure measurement and the Europeans used a different method.

There is an great ongoing debate about whether the 2 pressures are 20,000 psi apart or if they are actually the same. The argument centers around what appears to be a typo in an old army field manual.

It's a very complicated and convoluted argument but I've come to the conclusion that for me personally I do not believe there is enough of a chamber pressure difference to worry about it. The whole debate and documentation is over at FAL Files if you feel the need to dig for it.

It's your gun and your life so read all the advice and choose carefully what you believe.

Mark whiz
November 14, 2009, 01:04 AM
Ditto to TexasRifleman.

Since you are chambered for .308 - you can shoot either without having to worry.

Balrog
November 14, 2009, 01:30 AM
7.62 brass tends to be a little thicker at the base, and has a little less volume capacity than commercial 308. That is important if you are a reloader.

Candiru
November 14, 2009, 03:24 AM
Regarding the pressure difference, the following is what I've come to believe. The Army manuals well back into the 50s listed the pressure for rounds as PSI, although it wasn't until the 70s that they started using the electrical pressure sensors that actually measure pounds per square inch. Up to that point, they used the copper crusher method, which actually measures CUP, or Copper Units of Pressure. CUP doesn't translate into PSI using any mathematical equation, but it's known that 50,000 CUP is roughly equal to about 60,000 PSI. These numbers match up with the apparent discrepency between 7.62 NATO and .308 very nicely.

The most convincing thing, though, is just common sense. The US Army Field Manual for the M14 lists the muzzle velocity as 2,800 FPS. (link to FM 23-8 (http://www.google.com/url?sa=t&source=web&ct=res&cd=1&ved=0CAcQFjAA&url=http%3A%2F%2Flenaweemilitia.com%2Fm-14.pdf&ei=t1n-StjTKY-isgPe25mICw&usg=AFQjCNHCmgMTNY7O66LoZ_vGHSx5F9uFoA)) and .308 ammo manufacturers publish their velocity numbers; for instance, Remington claims its .308 150-grain FMJ cartridge (link (http://www.remington.com/products/ammunition/ballistics/results/default.aspx?type=centerfire&cal=33)) gets 2,820 FPS. Would the supposed 12,000 PSI difference between 7.62 and .308, a nearly 20% increase, produce only 20 extra FPS velocity?

scythefwd
November 14, 2009, 05:21 AM
candiru, if the military is using a faster powder it very well could spike the pressure earlier and higher but have a lower velocity than if it used slower burning powders. Slower burning powders have a lower peak pressure, but a longer interval of time at that pressure (or close to it) where fast burning powders reach peak quicker and drop back off faster which results in slower bullets. For fast burning powders, it is more of a pressure spike instead of a pressure curve (to an extent.

MarineOne
November 14, 2009, 07:24 AM
I was thinking about picking up this new rifle from LMT, but I'll stick with an M1A/M14 from Springfield. My wife said she'd get me something for Christmas, and since I won't be home until late summer I'd rather her get a deal now than for me to hunt for it while I'm home and limited on time.

I appreciate the input.



Kris

TexasRifleman
November 14, 2009, 09:10 AM
andiru, if the military is using a faster powder it very well could spike the pressure earlier and higher but have a lower velocity than if it used slower burning powders. Slower burning powders have a lower peak pressure, but a longer interval of time at that pressure (or close to it) where fast burning powders reach peak quicker and drop back off faster which results in slower bullets. For fast burning powders, it is more of a pressure spike instead of a pressure curve (to an extent.


Well this is why it's a can of worms debate, there are lots of arguments on each side.

So the argument the other way goes that since a very common load for 7.62 uses BL(C) powder then we should be able to duplicate pretty close the load of the military rounds.

BL(C) in Hodgdon's loading data shows 50,000 psi with a 150gr FMJ, at roughly the same velocity as the NATO load.

So, if BLC2 was developed for NATO loads, and NATO loads use the same bullet as a .308 load, and the velocities are the same, how can there be a 10-20,000 psi chamber difference?

That's why I personally have decided to ignore that whole pressure thing, but it's certainly a personal choice.

scythefwd
November 14, 2009, 09:32 AM
Well Tex, thats only if the military round uses BLC2. I can't say either way. In general, the 7.62x51 is considered a higher pressure round, but negligibly so. I'd say it's your gun but I'd use either if I were you.

TexasRifleman
November 14, 2009, 09:45 AM
Well Tex, thats only if the military round uses BLC2. I can't say either way

Well, TM 43 says that 7.62 M80 Ball ammo uses WC846. (Page 11-17) 46 grains of it.
http://www.ar15.com/content/manuals/TM43-0001-27.pdf

Pretty much everything I can find says that commercially that powder is sold as BLC-2 or H335. H335 is salvaged/surplus WC846 and BLC2 is newly manufactured at least according to what I can find.

So who knows. It's close enough that for me personally I follow the advice of the guys at Fulton and as long as the headspace is OK I don't worry about using either one. But if someone wasn't absolutely certain themselves I wouldn't fault them at all for not interchanging the things.

jonnyc
November 14, 2009, 10:12 AM
They are the same round and factory ammo of either flavor is perfectly interchangeable. A "match grade" commercial .308 chamber might not like the looser manufacturing tolerances of military 7.62 ammo, but that's a rifle issue...not an ammo issue. The only difference between .308 and military 7.62 is the internal construction of the case. Military brass has a thicker web to withstand MG extraction, thus there is a slightly reduced internal volume. One might have an issue if they tried to put a hot .308 load into a military case.

Candiru
November 14, 2009, 11:45 AM
candiru, if the military is using a faster powder it very well could spike the pressure earlier and higher but have a lower velocity than if it used slower burning powders. Slower burning powders have a lower peak pressure, but a longer interval of time at that pressure (or close to it) where fast burning powders reach peak quicker and drop back off faster which results in slower bullets. For fast burning powders, it is more of a pressure spike instead of a pressure curve (to an extent.


Yeah, that follows. If the military loads produced slower muzzle velocities, that would likely be the cause. However, the military loads have everything working against them in terms of velocity (reduced case size and older, faster powder), so you'd expect them to be clearly slower if commercial loads had an extra 12,000 PSI to play with.

It's possible to argue that commercial loads in the 150-grain FMJ range are underloaded to make them safe in military arms, but if that were the case you'd expect to see hunting loads with heavier bullet weights at the same speed as the 150-grainers.



BL(C) in Hodgdon's loading data shows 50,000 psi with a 150gr FMJ, at roughly the same velocity as the NATO load.

So, if BLC2 was developed for NATO loads, and NATO loads use the same bullet as a .308 load, and the velocities are the same, how can there be a 10-20,000 psi chamber difference?


That's CUP they're using, and 50,000 CUP is about 60,000 PSI.

TexasRifleman
November 14, 2009, 12:05 PM
That's CUP they're using, and 50,000 CUP is about 60,000 PSI.

Yeah, keeping them straight is always the problem lol.

Hammerhead6814
November 14, 2009, 12:45 PM
No difference.

desidog
November 14, 2009, 01:34 PM
When i was researching the 7.62 CEMTE round for my FR-7 rifle, I got the impression that there IS a difference in CUP between 7.62NATO and .308Win; but that in modern rifles the difference is moot, given that guns are under-rated/over-engineered for a safety margin. Either way, it shouldn't blow up in your face!

Now, the 7.62 CEMTE is another story...it has about 20,000PSI less than the .308, so a mistake there could literally blow up in your face.

Mags
November 14, 2009, 04:45 PM
The same difference as 223/5.56 it's all in your head. If the brass was so much thicker reloaders would have to load them differently than their commercial counterpart. I don't load the military brass any differently and have never experienced any pressure signs.

mljdeckard
November 14, 2009, 06:20 PM
When I would shoot .308 in my M1A, I would get failures to extract when it started to get hot. No such problems with 7.62, which is what it was chambered for. For ARs, I only use 5.56 or Wylde chambers and I've never had a problem. They DO load them differently than their commercial counterpart.

In bolt rifles I have never had a problem. The marine scout-snipers I was working with dais that they know there is a difference, but it doesn't cause a malfunction in their M-40s. As long as they are consistent and they don't switch back and forth, they are ok for accuracy out to 1000 meters. Their ammo was marked 7.62 NATO match.

wyohome
November 14, 2009, 06:30 PM
"The same difference as 223/5.56 it's all in your head." Mags

"Reduce charges developed in commercial cases at least 3 percent when loading military brass." Speer Reloading Manual #14.
I tend to lean toward the latter..

jonnyc
November 14, 2009, 07:36 PM
"They DO load them differently than their commercial counterpart."
This is not quite accurate. There are so many different loadings of both .308 and 7.62x51 ammo that you cannot say about any given loading that one is always hotter than another. Then you can throw manufacturing differences into the mix.

Mags
November 14, 2009, 07:56 PM
"Reduce charges developed in commercial cases at least 3 percent when loading military brass." Speer Reloading Manual #14.
I tend to lean toward the latter..
I don't load the military brass any differently and have never experienced any pressure signs. It's BS, size only matters in chamber size.

Balrog
November 14, 2009, 08:01 PM
The same difference as 223/5.56 it's all in your head. If the brass was so much thicker reloaders would have to load them differently than their commercial counterpart. I don't load the military brass any differently and have never experienced any pressure signs.

You just havent had pressure signs with your load. I have loaded 223 for many many years (you may have also, so I don't mean to sound like I am talking down). If you load military brass to max loads, you will probably see pressure signs in some of your spent cases. I certainly have seen pressure signs in military loads loaded to max.

wyohome
November 14, 2009, 08:01 PM
"It's BS, size only matters in chamber size."

I am guessing that you are joking, but I am afraid you are serious.

Mags
November 14, 2009, 08:08 PM
Yes I am serious. Yes case volume does affect pressure. No the difference between 223/5.56 and 7.62/308 is not noticeable. I speak from experience, I use the start load listed in a given manual for all new brass, primers, or bullets when working up a load and have never experienced pressure signs regardless of military/commercial cases. I have even shot 5.56 in a Colt chambered in 223:eek: and did not have any negative experiences and I mean alot of 5.56 in the ol 223.

wyohome
November 14, 2009, 08:13 PM
"Yes I am serious"

Well, then I stand corrected. In will notify Speer, Hornady, Lee, Barnes, and the others about their erroneous testing and information on Monday.

Mags
November 14, 2009, 08:15 PM
Thanks, please do and I am sure their tests probably come up the same as mine, however I am sure they have encountered variables in their volume of testing and wanted to CYA so they post more conservative data.

wyohome
November 14, 2009, 08:27 PM
My point is this: I believe you can achieve an over pressure condition using commercial cases and enough powder. If that is possible and your contention that "volume does affect pressure' would the logical conclusion be that military cases could cause a more unsafe condition?

Mags
November 14, 2009, 08:36 PM
It could but it does not. At least not for me I can't speak for everyone but not for me. I gues posts #2 puts it best.

t165
November 15, 2009, 12:42 AM
I have read many threads concerning this topic over the years. I have seen many keyboard wars flame up over it as well. I have a Spanish 98 action FR-8 and have been warned not to fire commercial 308 ammo in it...only 7.62x51 Nato. And I've had several CETME's. I have had many individuals advise me not to fire commercial 308 ammunition it it also. The CETME's I have owned do have "308" stamped on the firearm though. And my Saiga 308-1's have both 7.62x51 and 308 stamped on them. I do believe both Federal High Energy and Hornady light/heavy magnum 308 commercial ammunition does come with a factory warning to not use in semi-automatic firearms. These commercial 308 cartridges are suppose to be more "energetic" and cycle the action with more velocity/force than regular commercial 308 ammunition and may damage the firearm. Except for the Federal High Energy and Hornady Light/Heavy Magnum cartridges I simply do not know if there is any difference between regular commercial 308 ammunition and 7.62x51 Nato ammunition. I have seen and read arguments both ways with experts to boot.

Mags
November 15, 2009, 01:00 AM
T165 Your statements make it sound like the 308 loads are more potent than the military 7.62 loads making me think the loads can be different across the board but the brass is the same.

t165
November 15, 2009, 01:21 AM
I honestly have no idea Mags. The only reason I was warned not to fire 308 commercial ammunition in a CETME was some believe the 7.62x51 has tougher/thicker brass than what some 308 commercial cartridges are loaded with. Due to the flutes in the CETME's chamber the thinner brass of commercial loaded cartridges can expand more causing the case to either get stuck in the chamber and in some cases the case heads becoming damaged or ripped completely off resulting in a stuck case. I simply do not know. I know the CETME rifles were designed for the 7.62x51 Nato and that the ones I have owned (5) all had 308 stamped on them. Frank C. Barnes who authored "Cartridges of the World" states in part "the .308 is nothing more than the NATO 7.62x51mm military round". I'm sure there are other experts who will disagree with Mr. Barnes. The argument is above my pay grade so I'll not be entering ths fray. :)

wyohome
November 15, 2009, 01:48 AM
"some believe the 7.62x51 has tougher/thicker brass than what some 308 commercial cartridges are loaded with."

I just weighed 5 Lake City Match cases, cleaned, deprimed and trimmed. 181.1 Grs.
5 similarly prepped 'Super Speed 308 Win' cases weighed 154.2 Grs.
(Average weight)

t165
November 15, 2009, 04:48 AM
Thanks wyohome. While not an exhaustive study it does show a significant difference in overall weight between the 7.62x51 lake city match cases and commercial super speed 308 brass you have just compared. One of the threads I read actually compared different brands of commercial 308 brass and they varied. But, not to the extent exhibited in the direct comparision you made between the 7.62x51 NATO and commercial 308 brass. Perhaps there is some truth to the argument commercial 308 brass will swell more in the CETME's fluted chamber causing damage and a jammed weapon.

If the internal deminisions between the 7.62x51 NATO and commercial 308 brass are indeed different then handloaders have to be cautious when crafting reloads. I read a good article about this in a major publication within the last year or so dealing with the 257 Roberts (IIRC). Brass used for a regular 257 Roberts load had greater internal powder capacity but was loaded to a lesser pressure from the factory. Brass used from the factory for the +P 257 Roberts factory load had beefed-up brass and a smaller internal powder capacity even though it was loaded to a higher pressure from the factory. Handloaders were cautioned not to mix up the two different brass when reloading. The handload with the +P 257 Roberts brass would exhibit greater pressure than the handload with the standard 257 Roberts brass all other factors being equal. I'll see if I can find the article. I toss my subscription magazines after a month or two so I'll probably have to search for it online. It was an interesting read.

Balrog
November 15, 2009, 08:45 AM
If the internal deminisions between the 7.62x51 NATO and commercial 308 brass are indeed different then handloaders have to be cautious when crafting reloads. I

I do not understand why you start that sentence with "If". Absolutely positively without doubt the case volume of 7.62 brass is less than 308 brass. 7.62 brass is also thicker. There is not as much room for powder, period.

TexasRifleman
November 15, 2009, 10:28 AM
T165 Your statements make it sound like the 308 loads are more potent than the military 7.62 loads making me think the loads can be different across the board but the brass is the same.

Not necessarily, it's because the chamber dimensions differ slightly and if you have a rifle with excess headspace you can have a problem, even at lower pressures.

Older milsurp rifles especially tend to have more headspace simply due to use.

Rimless cartridge headspace is measured from the bolt face to a datum reference line on the shoulder of the case. This datum is set by SAAMI for commercial loads. Military headspaces tend to be more tolerant of loose fitting rounds because the brass is thicker and can handle more stretching without bursting.

If you use commercial brass in a rifle with excess headspace you are in a danger zone. If that thinner brass ruptures and the rifle has weaker metallurgy you get blown up guns or worse, blown up people.

Insufficient headspace is just as bad. It doesn't leave any room for the brass to expand any so you have an overpressure situation.

So, if you go back to the first post I put in this thread you will see the recommendations for headspacing. If you can close the bolt on a NO-GO gauge you should probably not use commercial brass at all if you even shoot the rifle. If you can close the bolt on a FIELD gauge you are holding a hand grenade.

I'll repeat it here, quoting Walt Kulek at Fulton. Notice that he recommends using the SAAMI headspace for BOTH military and commercial, and explains why.

They are the same, 'cause nobody makes 7.62MM ammo that isn't to the .308 "headspace" dimension spec. So 7.62MM ammo fits nicely into .308 chambers, as a rule.

But in some 7.62MM rifles the chambers are long (to the 7.62MM military spec), notably the Navy Garands with 7.62MM barrels. Thus, using commercial ammo in such a rifle is not a good idea; you need stronger brass. Use military ammo or the best commercial only, e.g., Federal Gold Medal Match.

Most of the time it's a distinction without a difference. But if you intend to shoot .308 commercial in a military arm chambered for 7.62MM, first check the headspace with .308 commercial gauges first. You may get a surprise.

And then Clint chimes in with the same thing:

Whether you have a NATO chambered barrel or a .308 Winchester chamber, keep the headspace within SAAMI limits (1.630 GO, 1.634 NO GO, 1.638 FIELD REJECT)

The pressure argument isn't really what matters, it's headspace that will hurt you.

As for the CETME, if you do a search for "bolt gap" you will find the problem. It's pretty common to see incorrect bolt gap on the CETME rifles. On that rifle type bolt gap can hurt you too. But that's another problem, not ammo OR headspace. Incorrect bolt gap can allow the rifle to unlock too soon, allowing the unsupported brass to explode.

If you own a G3 style rifle you should be checking the bolt gap pretty regularly. Simply trying to avoid the problem by only using thicker brass is potentially dangerous.

t165
November 16, 2009, 02:43 AM
Hello Balrog, I qualified my statement with "if" because I know of no person who has ever sectioned all 7.62x51 NATO and commercial brass 308 cases and measured/weighed them from all the different manufacturers from all the different nations. Factory cases differ from lot to lot. There are differences between the various domestic manufacturers. Many different countries also have differences in their military and commercial cases. Empirical evidence has proven this over the years. I do feel comfortable stating, at least domestically, 7.62x51 is suppose to be constructed with thicker brass. I can also say there are no ballistic scientist or firearm ammunition engineers contributing to this thread. We are all just regurgitating what we have read elsewhere from another source. Seeing as to how the professionals cannot seem to agree on many of these issues it is not surprising we amatuers on this thread cannot agree either. I freely admit I am not a scientist nor an engineer professionally employed in the firearm business. I am not qualified to offer professional advice on these matters. I have only shared what I have seen or read. The word "if" allows me to engage in the discussion with out having to stick my neck out to be chopped off. You never know, someone who actually knows what they are talking about, with the credentials to prove their expertise, may join this discussion and make a fool out of me.

MarineOne
November 17, 2009, 10:07 AM
So other than checking for headspace with a gauge, I could ideally shoot 7.62x51 NATO ammo from both a rifle chambered in that round, and a rifle chambered in .308 Win ..... however I wouldn't be able to shoot .308 Win ammo from a rifle chambered in 7.62x51 NATO due to chamber pressure.

At least it leaves a door open if I choose to go that route. I still think that I'll just have my wife find an M1A Loaded for me for Christmas.

I really appreciate the info.



Kris

USSR
November 17, 2009, 11:54 AM
So other than checking for headspace with a gauge, I could ideally shoot 7.62x51 NATO ammo from both a rifle chambered in that round, and a rifle chambered in .308 Win ..... however I wouldn't be able to shoot .308 Win ammo from a rifle chambered in 7.62x51 NATO due to chamber pressure.


No. The chamber pressure difference between .308 Win and 7.62x51 is negligible, since the published pressure spec's are the result of using two completely different methods to measure pressure. The only thing you really need to be concerned about is: some military rifles with weak action designs were converted to .308/7.62x51; and military autoloaders (such as a converted Garand) that require a certain amount of port pressure to operate the action.

Don

TexasRifleman
November 17, 2009, 12:09 PM
however I wouldn't be able to shoot .308 Win ammo from a rifle chambered in 7.62x51 NATO due to chamber pressure.

If you believe there is a significant pressure difference then sure, don't do that.

I personally don't believe there is a pressure difference. The arguments on this are endless, literally. There was at one time a thread over at FAL files that read like a PhD dissertation on the subject LOL.

MarineOne
November 18, 2009, 12:41 AM
Okay then so no worries. I hate poorly written articles.

Thanks for the help gents.




Kris

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