This is a discussion on Polytech ammo choice within the Ammunition forums, part of the M14 M1A Forum category; I have had read mixed things on weather or not you can shoot 308 through her. I have checked head space and she was good.
The way that I understand it is that the Polys have a longer headspace than the M1a. And 7.62 Nato is .005 longer than .308. To be on the safe side I bought a .308 and 7.62 Nato gage. My bolt will close on the .308 gage but will not close on th 7.62 nato. That means my headspace is to long for .308 but is fine on 7.62. I also read that the reason for this it the spec on a Poly is the same as M14 which is chambered in 7.62.
I will add this. My Poly is the sweetest shooting rifle I ever laid down on. Keep check on the head space, keep it lubed and have fun.
1. Chamber pressure differences between 308 and 7.62 is nonexistent in terms of safety for the design of this weapon; bullets heavier than 175 grain can have to much port pressure for the M1A/M14 platform; NM shooters solved this by putting a groove in the gas piston to relieve it. Chamber pressure is different than port pressure, high port pressure leads to battered receivers, cracks in the receiver heel, bent op rod, and this is the big one for this discussion, case head separation. The extraction process becomes more violent because of higher port pressure.
2. Headspace is what rules here. Brass in 308 is thinner resulting in about a 10% increase in volume, this thinner case wall is easier to pull the head off. Case separation is due to chamber size compared to case size, how far the case stretches, the violence of the action during cycling, surface condition of the chamber, pressure curve in the combustion process, op rod timing, and gas cylinder port pressure.
3. The M14 has a violent action during extraction; its design included a chrome chamber to assist in extraction, ease of chambering a round, dirt resistance, easier cleaning and errorsion suppression.
4. The headspace for the 308= 1.630-1.634
The headspace for the 7.62= 1.635-1.640
If you shoot a 308 round in a 7.62 chamber, if the headspace is long in that 7.62 chamber, say 1.339 and the brass used in the 308 is thin and hard it can lead to case head separation during action extraction cycle, reloaded 308 brass resized can make this problem express itself more often.
You can shoot 308 in a 7.62 chamber, you can shoot 7.62 in a 308 chamber, I would not shoot 308 ammo using heavy bullets, thin hard brass, in a non chrome chamber because the risk level has been raised for a case head separation.
I would not shoot a surplus 7.62 in a tight chambered SAI action, read 1.362 or less, non chrome lined chamber, the possibility increases that an out of battery firing can occur. Chrome not only aids in extraction it also aids in chambering a round.
I have said and will say, 1.634-1.635 is the ideal chamber size to fire both, it does not prevent case head separation but it lowers the risk, it does not prevent out of battery firing but it lowers the risk.
The safety factor designed into the M14/M1A is remarkable.
For all of the failures, I have never heard nor seen a serious injury. I consider serious as loss of life, loss of eye sight, missing fingers, hands, or other body parts.
This debate revolves around part fact and part myth. There is a difference between 308 and 7.62x51, it lies in the case wall thickness, case head design, case hardness, and chamber dimension. The safety features inherent in the receiver design protects us fools who push the limits of this incredible design.
You should realize that a measurement of chamber dimensions requires 2 settings. The best way I know to set your chamber, use a 308 nogo, the bolt should close and a 7.62 go, the bolt should not close. Knowing one dimension is only half the question.
Originally Posted by mb3
I had the USGI bolt conversion done to mine, and I shoot what ever I want.
A USGI bolt conversion is a nice improvement but it doesn't answer the question, what is your headspace? An improper headspace set on a USGI conversion is no more safe than a nonconverted weapon.
The headspace is what counts. That is why there are standards, gauges, and proceedures.
The simple question, can the pressure be contained inside the action safely?
If the case lets go for any reason it can be a problem. The design of the 7.62x51 by the military was to provide the safety for a machine gun in combat.
The design of the 308 was for sporting purposes, it doesn't mean the 308 is bad or unsafe, it means the 2 were design for different purposes.
Understand how they relate to each other is what this and other discussion are about.
Know you headspace, understand how that relates to the different cases and you can make your choice as to what is safe or unsafe.
Like I said above, there is alot of myth that surrounds this discussion.
Here's a quote from one of my old posts about this issue.
There are a lot of opinions about this issue but the bottom line is this; commercial .308 ammo can be used safely in an American made M1A style rifle that is in a safe operating condition. Since the American made rifles meet SAAMI safety standards, the .308 ammo, which also meets SAAMI safety standards, will work. That's why SAAMI exists, to set safety standards that allow weapons and ammunition to function safely together.
I had made the blanket statement that all M14 clones can shoot .308 Win ammo without issue, I think I generalized too much.
After reviewing some of the very conversations that I refer other to, I realize now that there is one exception, and you Canadians are the guys that caused all this trouble.
I didn't realize that the polys have a military 7.62x51mm chamber. That means that they have long headspace and throats compared to most American made rifles (those that meet .308 Win SAAMI specs). That can create a situation where the thinner case of the commercial .308 Win cartridge will stretch too much and have head separations.
So the problem isn't one of the commercial ammo being high pressured, it's the fact that the rifle's chamber is larger than other brands of rifles and the thin walled commercial cases will expand in to that larger volume. Some cases won't be able to handle that amount of expansion and they will crack.
P.S. I guess I should have kept my nose out of this one and let the smarter guys hash it out.
Last edited by RAMMAC; June 25th, 2012 at 12:31 PM.
Reason: corrected my mistake and added comment
One of the things I really like about the M14 design is the self-regulating gas system, where the piston cuts off the gas pressure as soon as it's moved enough to give the operating rod a shove. I've tried my Chinese M14S with all different kinds of surplus 7.62mm ammo, as well as commercial Wolf, Remington, Hornady, and Winchester loads from 146gr. FMJ to 180gr. softpoints, and it works fine with all of them. I don't have a NOGO headspace gauge, but mine doesn't close on a Clymer FIELD gauge. I don't reload the brass for this, since it's mostly Berdan primed, so case stretch isn't an issue.