Hello everyone from South of the border. My first post. Being that Canada allows importation of Chinese rifles there must be a ot of expertise with M14 clones on the forum. I just aquired a Polytech M14-S and have a couple of questions about the chamber and use of .308 and Nato rounds. The receiver is marked .308 but it's my understanding that the receivers were not marked correctly as it should have been 7.62x51 Nato. I have used a Nato Max guage of 1.6455" and it does not close all the way. How can I check to see it the chamber will also handle the 308 Win or should I not use 308 Win in this rifle? Thanks for any help given.
M14-S Polytech Chamber ID
- Thread starter LX Kid
- Start date
Blastattack
CGN Ultra frequent flyer
- Location
- Winnipeg MB
Seeing as .308 and 7.62x51 are practically identical and fully interchangeable, I wouldn't worry and would just shoot the thing. Stick to bullets weighing approximately 150 grains and you'll be fine.
Potato boy
Regular
- Location
- North of the mountain
The chamber pressure of the 7.62 is over 60000psi. It is SLIGHTLY lower than the 308. The difference is so slight in fact that SAMMI considers both rounds interchangeable. Let’s put this high pressure myth to bed. Springfield also list them as interchangeable and clearly stated that the M1A is good to go with rounds up to and including 180 grains. The US military uses 174’s on a regular basis. 6 grains won’t make a difference. You do not need to stick to 150’s. I’m pretty certain the US military, Sringfield armoury and SAMMI know what they are doing. People confuse the Garand with the M14 design.
Potato boy
Regular
- Location
- North of the mountain
Yes
Blastattack
CGN Ultra frequent flyer
- Location
- Winnipeg MB
No, it isn't. Pressures are nearly identical when you actually do some reading. Copper Crusher pressure and PSI pressure are not the same things.
correct
to the OP , welcome
we have a Main Battle Rifle forum here where all facets of the m14 family of rifles has been regulary discussed for over a decade. Lot's of good information on the chinese rifles here.
as far as chambers go and pressures, there is a lot of misinformation out there.
Generally the .308 and 7.62x51 is interchangeable in this rifles as long as headspace is within reason. By that I mean , as long as the chamber headspace is not excessive, both ammunition can be used.
So in your case, the bolt isn't closing on the 7.62 field gauge but this is not telling you much. It would be assumed that 7.62 x 51 ammunition will be safe though.
.308 = 1.630" to 1.635" chamber and 7.62 x 51 = 1.6345 to 1.6455
It is my strong opinion based on my time working with these firearms that ammunition using .308 commercial brass, be they factory or reloads, should ONLY be used in M14 type rifle chambers that fall between 1.630" and 1.638" ..... regardless of manufacturer of the m14 type rifle.
walleyed99
CGN frequent flyer
- Location
- I'm not telling, Sask
Chamber pressure is similar, it's just one is measured in PSI and the other in CUP and the numbers are different because of the different units of measure. However, the actual pressure is very similar
The pressure difference is not the part that make it dangerous its the headspace and powder loading. 7.62x51 loaded with less powder and slightly longer by about 0.012 thou with thicker case design. The .308 is shorter with thinner case walls and loaded hotter potentially using a .308 in a 7.62 chamber you could have a hotter load with thinner case in a oversized chamber a very bad combination.
jmichelin84
CGN Ultra frequent flyer
- Location
- Big Land, nL
Someone had to argue pressures again, in my Norinco I fire both, hunt with Winchester, plink with Winchester and surplus, have at her
I have the identical rifle albeit with GI furniture and I have never hesitated to fire 308 in it. Any commercial 308 will be fine. Heavy for caliber bullets and heavy powder charges might give you some issues.
My experiences with .308 chambers and 7.62 ammo is that a relatively 'tight' .308 chamber - closing on a go gauge, not closing on a no go gauge and a normal throat often will not handle 7.62 ammo. The ammo is so bent it doesn't fit in a nice chamber.
Haven't come across a 7.62 chamber that wouldn't handle .308.
Haven't come across a 7.62 chamber that wouldn't handle .308.
Potato boy
Regular
- Location
- North of the mountain
I guess SAMMI has no clue what they’re doing because they say they’re interchangeable,you have any other pearls of wisdom that you care to correct SAMMI on. I wish this nonsense between those two chamberings would just end,it is not dangerous
walleyed99
CGN frequent flyer
- Location
- I'm not telling, Sask
If you follow 45acpking's advice of not using 308 ammo in rifles with larger headspace than, 1.638 then you should be golden.. I have shot 308 in rifles with headspace of upto 1.640 in the past with no issue, but wouldn't necessarily recommend it. With larger headspace I'd look more to handloading ammo using Lake City Brass and you should have no issues
south of the border..... don't think the batf would allow his polytech across the border to attend a clinic but good advice nonetheless
hmmm, I'm not as sharp as I think I used to be.
when I cut chambers in m14/m1a barrels I use the Clymer M118 , .308 match reamer. After my own experimentations with these rifles and aftermarket barrels I stopped cutting those chambers to 1.630" and instead I cut them to 1.632".
I have yet to find an example of military surplus 7.62 x 51 that is damaged or bent in any way by being fed, fired or extracted from that chamber dimension. None of my paying customers reported any issue either and I do get correspondence from many folks i built these rifles for. They do need to be cleaned more often and this is signalled by poor extraction.
I realize your experience in the gun smithing and rifle building is vast compared to my own , but when it comes to this platform specifically, I can't agree with your first comment. I'm not trying to debate you either
Your second comment I mostly agree with but recommend to shooters of the M14/M1A to watch for any negative signs if they are shooting .308 commercial in an M14/M1A chamber measuring over 1.638".I'm speaking from personal experience, to this platform only.
I did not mention ammo damaged by being fed or fired... The ammunition I am referring to was surplus and it would not chamber... and it was not a headspace problem... if you rolled a round on a smooth plate of a piece of glass it wobbled very badly... the run out was huge... so much so you couldn't close a bolt on it in a .308 chamber. This was on a rifle I just built for best accuracy with .308 ammo... he thought he would blast off some surplus ammo. I told him I could make the chamber larger and increase the headspace slightly but he would be wasting the accuracy potential with good ammo...
