View Full Version : M1a
August 24, 2010, 08:42 PM
I am researching the possibility of a Springfield M1A.
I am a little confused though.
On the Springfield web site the M1a Standard is listed as 7.62x51m(.308)On the detailed specifications page of the Springfield site for the Standard the caliber is listed as .308 without the reference to 7.62x51mm
Are these two calibers interchangeable in the M1A?
Can the commercial .308 AND the so called NATO 7.62x51mm be fired in that chamber???? Are these the same ammo.
I have read (ad nauseum) about whether one can interchange the two rounds. And (ad Nauseum) there are a gazillion different answers.
August 24, 2010, 09:18 PM
I have fired surplus 7.62 X 51, my .308 reloads, and some commercial .308 through my M1a for years. Surplus has been South African, Portuguese, and Austrian. Commercial .308 was Black Hills 168 grain target rounds.
My reloads have used overrun, or pulled, 147 grain FMJ boattails and enough IMR or H4896 to move them out at 2550 (or so) fps. No issues with any of the loads and MOA accuracy with my reloads out to 200 yards.
August 24, 2010, 09:19 PM
M-14 and M-1A are designed for M80 ball (7.62x51). Thats a 147 grain FMJ bullet propelled by medium burning rate powder at 50,000 psi.
308 Winchester in heavy bullet hunting loads and slow burning powder can exceed 62,000 psi as they try to match the muzzle velocity of 30-06 180 gr hunting loads. That is well beyond what the M-14/M-1A gas system is designed to handle. With 308 Win loads using 150 gr hunting bullets, I doubt there is any issue. Just stay away from 308 Win hunting loads with heavier bullets.
August 24, 2010, 09:29 PM
I purchased a SA M1A last year. It has a warning card stating to use 7.62x51 only. Their were alot of reports online stating that you could use 7.62 nato and 308. I was a little confused so I called SA directly, I was informed that the head spacing is set up to except 7.62 nato and .308. However they did also inform to only use between 147 grain to 168 grain. I don't think this applies to older M-14's and older M1A's as far as I know. I can only speak for my 2009 model SA M!A.
If their is any doubt just call the manufacturer.
August 25, 2010, 07:34 AM
I am going to call the mfg, but knew I would get good data on this sight.
One would think that the web site would clarify.
August 25, 2010, 08:18 AM
There is a narrow headspace range were it is safe to use both .308 and 7.62 in a SA M1A. If your not sure what you have use 7.62 only. Of course when using .308 keep the bullet weight at reasonable levels.
August 25, 2010, 10:50 AM
Now to come up with several thousand bucks to get it and and several thousands rounds of ammo.
Also looking at 5.45x39 AK 74 type and several thousand rounds of ammo.
:D I gots lots to re-learn. There will be many more questions before I am done!
August 25, 2010, 11:35 AM
I just stay with military grade brass like LC and 2500 fps 150g pellets over 4895.
August 25, 2010, 11:36 AM
This topic causes endless concern and problems. The NATO chamber has headspace about 3.5 thousandths longer and, IIRC, it is about 0.001" wider. That's enough to drop pressure around 500 PSI. Not much difference out of 60,000 some psi. The freebore is 0.030" longer (0.050" instead of 0.020"), IIRC. That can lower pressure much more significantly (8,000-10,000 psi or so) if a load has its bullet is on the throat in the .308 chamber but is then fired in the NATO chamber. That's a pretty unusual situation.
I don't buy the 168 grain bullet limit. Many, many, M14's and M1A's went through matches using issued M118 Special Ball loaded with the 173 grain FMJ BT match bullet by Lake City. The last time I pulled some of those they averaged 174 grains, same as the last batch of Sierra 175 grain MatchKings I bought did. I've had no issues running the 175's in my gun.
The M14/M1A design is not as vulnerable to high port pressure as the Garand is. If Springfield is warning people to stay with 168 grain bullets and below, then it is probably because most commercial ammo above 168 grains is 180 grain ammo loaded with slower powders that raise port pressure or that have longer bullet barrel times that increase the amount of gas through the port. When you speak with Springfield ask if they really have a problem with you shooting Federal Gold Medal or HSM match loaded with the 175 grain Sierra MatchKing or shooting M118 Special Ball? I'll be very surprised if they do.
Match M1A's sometimes are given a special match chamber that was designed for M852, which was match ammo made with the 168 grain Sierra MatchKing. Those rifles may be limited to the 168 grain SMK, but it sounds like your chamber is NATO spec.
Understand that both NATO and Commercial brass are the same size when new and have the same maximum COL. The slightly bigger NATO chamber is to eliminate the slight interference fit that SAAMI specs allow in bolt guns. It is bigger for smooth feeding in full auto, and has a longer throat for specialty ammo.
Read FALPhil's post, here (http://thefiringline.com/forums/showthread.php?t=326877). It covers how the pressure specs got historically confused between the two, but are actually the same for all practical purposes.
August 26, 2010, 07:37 AM
Great post UncleNick. Sadly there is a lot of misinformation on the net, it's nice to read the facts!
August 30, 2010, 05:47 PM
I just stay with military grade brass like LC and 2500 fps 150g pellets over 4895.
Why this velocity? M80 travels around 2700fps, or so my chronometer says.
I just picked up an M1a and will be loading 150gr. Hornady FMJ in it.
August 30, 2010, 06:07 PM
.308/7.62x51 is the same as .223/5.56....except in reverse.
for a gun marked .223 its dangerous to fire 5.56 as 5.56 nato has higher preasure and could have catostophic failure in a .223 gun. but a 5.56 gun is safe for both.
.308 is actually higher (can be) than 7.62nato so its safe to use either in a .308 gun, but MAY not be safe to use .308 in a 7.62 gun. the SPringer M1As are all .308 and can use either type of ammo safely
August 30, 2010, 07:23 PM
Not really. The .308 and .223 were developed for the military and introduced as civilian rounds. Nothing was changed. Not the case and not the pressure. The differences people believe exist are the result of a comedy of measuring unit discrepancies, with the exception of one 7.62 NATO and two 5.56 NATO loads that run about 5.5% over truly comparable SAAMI pressure.
Go back and read read FALPhil's paper I linked to in my earlier post. The problem was that the military continued to report the results of copper crusher tests as "psi" until the mid 90's sometime, decades after the commercial firearms industry had started to distinguish copper crusher and non-conformal piezo transducer readings as cup and psi, respectively. Thus, the army's psi were actually cup and comparing them to the commercial 62,000 psi spec was not valid. They would properly be compared to the SAAMI copper crusher .308 spec to get an apples to apples comparison.
The SAAMI copper crusher pressure limit for .308 was 52,000 cup. 52,000 SAAMI cup = 52,000 army psi. The army specified most of its 7.62×51 ammo as 50,000 psi (actually cup), making it quite safe in commercial 52,000 cup/62,000 psi chambers. The one exception is M993 AP, which is loaded to 55,000 army psi which would equal 55,000 SAAMI cup, and that one is likely to exceed 62,000 SAAMI psi by 5.5% if someone measures it. The fact military guns can shoot it means they will all be fine with SAAMI spec ammo.
Specialty ammo is always something to be cautious of. But most of it is within the same ballpark. Keep in mind that SAAMI specs are for an average peak pressure and that maximum peak pressure within a sample of 10 can be 4% higher. Given the confidence limits of a sample of ten, no doubt if the same loads were fired for hundreds of samples, a number of them would exceed even 5.5% above average. And all this is still well below proof pressures.
The same measuring system mix-up happens with .223 Remington chambers and 5.56×45 NATO rounds except there is a twist to it. The SAAMI cup limit is 52,000 cup for the .223 cartridge as well. M193 Ball and M196 tracer are both rated in original Army specs as 52,000 psi (again, actually cup) and are therefore a perfect match. M995 AP is rated at 50,250 original Army psi, so it is even lower pressure. M855 ball and M856 tracer are rated at 55,000 original Army psi, and so, again, exceed the commercial spec by 5.5%.
The twist is that where all the other rounds SAAMI originally rated at 52,000 cup were given new Piezo pressure ratings of 60,000-63,000 psi, the .223 was given only 55,000 psi. This is likely a lower real pressure than the original rating. We know this because the European CIP, which got its .223 Remington pressure standard loads from us originally and tested them and published ratings of 54,000 cup and later 62,000 psi in their copper crusher and Piezo transducer equipment, respectively.
So, something caused SAAMI to lower the spec when they switched to psi. An instrumentation anomaly is perfectly possible, but nobody will ever reveal what actually happened because of liability concerns. In the meantime, lots of FC M193 gets sold by Gander Mountain and other commercial dealers without pressure issues being reported in civilian guns, and you know perfectly well Joe Blow fires them in anything they'll fit into.
Also consider the same rifle models that are chambered in the 62,000 psi .308 often get chambered in .223, where the bolt face is smaller and the chamber metal is thicker, and now, suddenly, they're going to tolerate less pressure than in the .308 configuration? If .223 brass had weaker or thinner heads, that might be arguable, but it doesn't. Unlike .308, It's civilian brand brass weights are the same as military.
So, as I said at the start, this all seems to be a comedy of unit errors. Unless you get a specialty round that jams into the throat at the end of the shorter SAAMI chamber freebore, you should be good to go.
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