How well do .308 ARs tolerate surplus European Ammunition?

[rickyrick]

So with the great prices on AR10 stuff combined with the fact that I have a mountain of surplus 7.62 Nato, I started thinking about an AR10. I bought out a guys supply of gun related stuff about 5 years ago and all the ammunition was included at the same price so I took it. I do have a FAL so I know the ammunition works. I’m just not a fan of the FAL, I only keep it because it’s pretty lol.
I have no experience with AR10s but I know with other rifles some types can be picky about surplus ammunition. I could plink for months with the ammunition I have now.
Just wondering what the more experienced members know.

 

[HiBC]

Its possible the way some surplus ammo comes on the market is that its out of spec.
I once had some battle-pack Portugese 7.62 NATO that chrono'd 2900 fps.

I could use it in my FAL,but it failed to extract in an AR-10.

There is another thread where a gentleman is having issues disassembling his M1-A after shooting some hot ammo that failed to extract.OP rod damage? I do not know.
Some Garands are chambered 7.62.Might 2900 fps ammo be rough on op rods? I prefer not to find out the hard way.

I suggest caveat emptor. It may be more lucrative to sell out of spec NATO ammo than to scrap it.

I'm thinking your chronograph is your friend.

Higher than NATO spec velocity probably indicates higher than spec pressure.

As far as which gun,AR-10 vs FN-FAL can handle more overpressure abuse,I just don't know.

The FAL has the adjustable gas system.The violence can be tuned out.

Ripping AR-10 extractors over rims may not be best practice.

 

[mehavey]

Are you going for an AR (like Ruger's) that has an adjustable gas valve?

 

[rickyrick]

I may one day have more interest in my FAL in the future. I’ve shot a couple of hundred rounds of some of this ammunition. Had a doubling trigger early on but a new HTS cured that issue easily. The rifle works fine, has a pretty wood furniture set but it just never really appealed to me.
I like it for it looks and the cool factor of it so it’s staying.

I’ll look into an adjustable gas system AR.

 

[brasscollector]

My experience with AR10s has been only with the 308. 18" barrel, rifle gas functions great. Beats up the brass a bit, some loads more than others. For now it has a non adjustable gas block, but the more I play with it the more I realize it will need one. Another 18" barrel but with mid-length gas has always had an adjustable gas block. The first time I shot it the gas block being open close to 100% I'm pretty sure the brass made it downrange nearly the same time as the bullet. That particular rifle is adjusted 1-2 clicks from closed and now ejects at 3-4 o'clock. Bottom line: most every AR10 needs an adjustable gas block.

 

[davidsog]

Never had a problem with my PTR 91.

 

[MarkCO]

"Most" .308 pattern ARs are set up to run NATO spec ammo and if a run of the mill sample, will run fine with surplus ammo. But, as others mentioned, yes, adjustable gas is the key. Match barrels, not as much.

I have a few benchmark loads I use to test .308 pattern ARs, one is a 140 grain bullet that chronos about 2400 fps. If that runs 100%, which most do, you really don't want to be shooting 165s at 2700 fps in the same gun.

Realize that there are two "calibers"....308Win and 7.62Nato. The commercial .308Win is loaded to higher pressures than the NATO rounds and since they chamber the same, but the guns are typically over-gassed even for NATO rounds, therein lies the problem, and the answer for "why adjustable gas" is a good idea.

I run a Superlative Arms adjustable gas block on my DD5V1.

 

[davidsog]

The commercial .308Win is loaded to higher pressures than the NATO rounds

That is internet mythology. Aberdeen used the commercial .308 specifications and simply adopted them under the European Metric system as the 7.62mm NATO.

The confusion in the civilian sector stems from pressure measurement methodology and a lack of accurate conversion between the methods.

You can find commercial .308 that is higher pressure but only in the higher bullet grain weights not used by NATO.

Attached is pretty good article researched open source. It is spot on.

 

[T. O'Heir]

"...thinking about an AR10..." Depends mostly on who made it. A real AR-10 was intended to use standard 7.62NATO ammo. Current commercial AR-10's usually are too.
"...a lack of accurate conversion between the methods..." There is no mathematical way of converting PSI to CUP or vice versa. Accurate or otherwise. Those are 2 different physical methods of measuring pressure.
"...have a mountain of surplus 7.62NATO..." Whose? Not all milsurp 7.62mm Ball actually is NATO. No wee circled cross on the head stamp means it's not NATO.
"...a 140 grain bullet..." Isn't NATO. Neither is 2400 FPS. 7.62NATO Ball is a 147 grain bullet at ~ 2700 FPS. Max pressure is 60,200 PSI.(that's CIP Max. CIP is the European equivalent to SAAMI.). SAAMI Max for .308 Win is 60,000 PSI.

 

[davidsog]

There is no mathematical way of converting PSI to CUP or vice versa. Accurate or otherwise. Those are 2 different physical methods of measuring pressure.

Exactly. There is only approximations none of which are accurate.

NATO uses a completely different methodology which more comprehensive and accurate than the civilian methods in use.

What is a fact is:

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.

The pressure measurements for .308 vs 7.62mm NATO are exactly the same when placed on the same scale no matter which measurement you use.

 

[MarkCO]

Reading comprehension strikes again.

The actual pressure difference between factory loaded 7.62 Nato and .308 Win is in the range of 6 to 7Kpsi. That is enough of a difference to affect operational performance of a DI large pattern AR. Yes, there are misconceptions on the actual numbers, but there are differences in the actual pressures in actual ammo on the actual shelves. Some surplus ammo is even lower pressure. Manufacturers set up their ARs to run cheap steel cased, and lower pressure surplus ammo to keep complaints to a manageable level. Might not be best for the crowd with the higher end pressures, but it is what it is.

T. O'Heir, you said

"...a 140 grain bullet..." Isn't NATO. Neither is 2400 FPS. 7.62NATO Ball is a 147 grain bullet at ~ 2700 FPS. Max pressure is 60,200 PSI.(that's CIP Max. CIP is the European equivalent to SAAMI.). SAAMI Max for .308 Win is 60,000 PSI.

Never implied that a 140 at 2400 fps is anything other than a load I use "to test .308 pattern ARs".

SAAMI max is 62 Kpsi, lets at least get that right.

 

[davidsog]

Reading comprehension strikes again.

No problem man, take your time.

I understand your reply was reference a gas operated AR-10 as that is the topic of thread.

Your statement that .308 and 7.62mm NATO exhibit a pressure difference is fundamentally not true and a common internet myth. Simply google FR-8 and see all the misinformation out there about this topic.

.308 and 7.62mm NATO are the same for all intensive purposes. Yes, you are correct in that 110 grain weight bullet will have a different pressure from a 175 grain weight bullet all things being equal.

However you meant your post, I think the average reader would infer that you meant you should not shoot 146 grain .308 ammunition out of a rifle designed for 146 grain 7.62mm because the pressure is higher in the .308.

 

[MarkCO]

Davidsog, while I agree with you for the most part, realize that the OP is asking about a large frame AR, in which 55-62Kpsi loads and anything that is a good bit less than that, will not have the best function, either over or under gassed for one or the other. In bolt guns, not a worry at all.

I pressure tested several of the Nato surplus ammo, a few samples had pressures just below 40 Kpsi, and actually ran most ARs. Add 20Kpsi to that, in the same gun, it is extremely over-gassed. Not an "unsafe" pressure issue at all, just an operational issue that I have personally seen drive several Large Pattern AR owners to exasperation until they realized what was going on.

 

[bfoosh006]

The difference between 7.62x51 v. .308 isn't just about the ammo... it is more about how the chambers are cut.

Adding a longer throat / freebore and a less abrupt leade, ( a different angle to the rifling ), makes a world of difference pressure wise.

The same holds true for 5.56 chambers.

That is also how Weatherby achieves such high velocities.

Look at some of their throat lengths / freebore...

Cartridge......................... Throat length (free bore)
224 Weatherby Magnum .162
240 Weatherby Magnum .169
257 Weatherby Magnum .378
6.5-300 Weatherby Magnum .2037
270 Weatherby Magnum .378
7MM Weatherby Magnum .378
300 Weatherby Magnum .361
340 Weatherby Magnum .373
375 Weatherby Magnum .373
378 Weatherby Magnum .756
416 Weatherby Magnum .239
460 Weatherby Magnum .756
30-378 Weatherby Magnum .361
338-378 Weatherby Magnum .361

With out the added throat length... ( essentially allowing for a larger chamber ) ... most higher pressure rounds would show excess pressure signs. )

So.... any time someone starts talking about ammo differences... remember to take into account the place it is being fired from... IE the barrel and chamber is really what makes a difference.

The US military and NATO have thoroughly documented the differences... and take measurements at different places on a chamber then SAMMI.

 

[stagpanther]

NATO uses a completely different methodology which more comprehensive and accurate than the civilian methods in use.

How so? (just curious)

 

[davidsog]

EPVAT is an abbreviation for "Electronic Pressure Velocity and Action Time". Action Time here means the (short amount of) time required between the ignition of the primer and the projectile leaving the barrel. This is a comprehensive procedure for testing ammunition using state-of-the-art instruments and computers. The procedure itself is described in NATO document AC/225 (Com. III/SC.1)D/200.

Unlike the C.I.P. procedures aiming only at the user's safety, the NATO procedures for ammunition testing also includes comprehensive functional quality testing in relation with the intended use. That is, not only the soldier's safety is looked at, but also his capacity to incapacitate the enemy. As a result, for every ammunition order by NATO, a complete acceptance approval on both safety and functionality is performed by both NATO and the relevant ammunition manufacturers in a contradictory fashion.

https://en.wikipedia.org/wiki/NATO_EPVAT_testing


https://web.archive.org/web/2013071...11smallarms/WednesdayInter12315Pellegrino.pdf

 

[davidsog]

it is more about how the chambers are cut.

Yes. If the headspace passes a .308 field gauge, you can shoot .308 all day long in your 7.62mm NATO rifle.

There are some differences in the cases as the civilian ammo is not subject to as rigorous testing. As such, cases can be thinner than the NATO standards.

Again, passing a field gauge is all that is required.

In terms of chamber pressure......

146 grain .308 = 7.62mm NATO M80 ball.

It is pretty cut and dry. Aberdeen Proving Ground used civilian .308 as the specification for NATO 7.62mm.

 

[davidsog]

The US military and NATO have thoroughly documented the differences... and take measurements at different places on a chamber then SAMMI.

Exactly.....

That is the root of all the internet hooey that .308 and 7.62mm NATO are not interchangeable.

 

[stagpanther]

I found the biggest challenge is getting compatible parts if your mix/matching components. The next biggest task is carefully balancing the gas system--more of a factor than in most AR15 builds IMO because the bigger cartridges tend to vary more in bullet and charge weights. The recent trend in 22 +/- inch barrels with xl+2" gas systems is no coincidence IMO--it tends to deliver well managed and timed gas impulses which don't bang the system as hard and are easily tuned up and down with a decent adjustable block. Shorter barrels and gas systems are going to be a bit harder to manage and time the gas impulse--they tend to hit harder (aka over-gassed) and rely on what I call "component musical chairs" of switching out heavier buffers, springs etc to dampen the gas hit. Related to the careful matching of components--a high-quality BCG in an AR10 build I've found is of more consequence than it might be in an AR15 build. All that said--a lot of fun, I think (and if you're a compulsive AR15 builder, that's just a ladder drug to building AR10's).

 

[stagpanther]

That is the root of all the internet hooey that .308 and 7.62mm NATO are not interchangeable.

Have you done much reloading using both 308 and Nato 7.62 x 51 cases interchangeably for the same cartridge configuration?