What pressures are OK with .308 AR-type?


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Polar Express
March 9, 2011, 05:44 PM
Hi Folks,
Well, I am looking for some direction/info here, and I tried a few different searches to see if the info was already posted. While itís possible itís already here, the searches I tried didnít show me the information I am looking for.

First of all, I am under the impression that I should not use my M1a to run hunting rounds (higher performance, higher pressures) as the gun is not designed/rated for the higher pressures that can be present in higher performance loads that one might use for hunting purposes. (if this is wrong, I would like to know, just for my own knowledge) Also, the configuration of my M1a is not such that I would WANT to use it for hunting.

Now, that being said, my next plan is to build a .308 AR-type rifle. I would like to build this in a fashion that it would be able to be used for hunting, if the situation was advantageous to do so. So, will the AR-type chamber/action handle the higher pressure hunting loads? Are the pressure ratings standard, or do the pressure capabilities vary depending on material, and mfgr? How would I find out such data?

PE

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LemmyCaution
March 9, 2011, 06:20 PM
First of all, I am under the impression that I should not use my M1a to run hunting rounds (higher performance, higher pressures) as the gun is not designed/rated for the higher pressures that can be present in higher performance loads that one might use for hunting purposes.

Not wrong, but a little misguided.

The M1a has problems with bullets over 175gr, not due to chamber pressure, but due to pressure at the gas port that drives the action.

It doesn't have to do with 'hot' hunting loads. It has to do with bullet weight increasing the port pressure. Excess port pressure can bend or break your operating rod.

If you keep your chamber pressures to SAAMI specs for .308WIN and your bullet weight under 175gr, you'll be just fine with any .308WIN out there.

Polar Express
March 9, 2011, 07:28 PM
Lemmy,

Thanks for the clarification on the SAAMI specs and the M1a limits. Do you know if the "AR-Type" design has limits based on it's particular configuration?

PE

LemmyCaution
March 10, 2011, 10:27 AM
I'm not hugely knowledgable about 7.62x51 gas guns, other than the M14/M1a.

Wish I could help you, but I don't have your answer.

Z-Michigan
March 10, 2011, 10:54 AM
Most of the AR-type 308s are fine with bullets at least up to 180, possibly heavier. Excess port pressure has the potential to cause function problems but won't break anything unless it's at or past proof load pressures (far overpressure). To some extent you can play with buffer weight if you want to run loads that would otherwise cause trouble. I have a DPMS 308 and it seems to be set up for heavier loads, as with common 150gr FMJ it can marginally short-stroke if the round is weak or the carrier group is dirty.

On an AR system, operating pressure is an issue of barrel internal pressure, port location (gas system length), and port size. None of these are adjustable in operation, but the port location and size are adjustable if you're building a rifle.

Polar Express
March 11, 2011, 02:46 AM
Z,

thanks for the info. In this case, I do intend to actually build the rifle myself, can you recommend a book or guide that I can reference to determine where I want the port based on the intended purpose of that upper?

Thanks,

PE

Z-Michigan
March 11, 2011, 10:02 AM
I'm not aware of a book or guide for you, but a good barrel shop can probably advise. You would need to know what gas system length you want and give them some info about your intended load.

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