Topic: 308 Win. and 7.62mm NATO in a simi auto

[J Mack]

I recently picked up a PWS MK2 Series Rifle Chambered in 7.26X51 and at first thought I had some chamber issues but after a few calls to PWS and some internets researches I found all is well with my 7.62X51 chambered rifle and should not be confused with a .308 Winchester chambered rifle as I had mistakenly done.
My first mistake was assuming that the 7.62X51 and .308 Winchester where similar enough that they were one and the same as far as rifle where concerned and that modern rifles chambered to shoot both would and could do so safely without any apprehensions as far as reloading to .308 Win specs and shooting 7.62X51 chambered rifles.
New .308 Win Winchester brass measures 1.630” from the base to the shoulder and after onetime firing in my rifle it measures 1.641” from base to shoulder or about .011” growth from factory .308 Win Min Spec and if I were to continue to reload these rounds in .308 Win Dies set to min I would start experiencing head separation sometime after the second loading, a very dangerous situation in a simiauto rifle.
My plan was to start with 150 pieces of new brass and fire 500 rounds through this new rifle this weekend as sort of a break in and had I not been reloading the .260 recently and my press was setup for one of my .308 bolt guns I would have just ran this ammo through as .308.
The brass looked a little different to me around the shoulder area and for some reason I decided to check it against my .308 go and nogo gauge (something I don’t normally do)  and It was a lucky mistake that I discovered these differences as early as I did and I’m able to safely reload this ammo now that I have reset my sizing die for the longer rounds.
       
 Note the .0155” extreme difference from .308 Min to 7.62X51 NATO Max and from what I understand PWS uses a proprietary 7.62X51 machine gun type reamer that's a bit longer than the NATO spec and  1.641"   is good to go in their rifle.     
.308 Winchester
GO: 1.630"
NOGO: 1.634"
FIELD REJECT: 1.638"

7.62 x 51mm NATO
GO: 1.635"
NOGO: 1.6385"
FIELD REJECT: 1.6455"   

Measure twice and stay safe!

Little more info:
http://www.thegunzone.com/30cal.html

[RGinIdaho]

http://www.armalite.com/images/Tech%20Notes%5CTech%20Note%2069%20Headspace%20080722.pdf

http://www.saami.org/PubResources/CC_Drawings/Rifle/308%20Winchester.pdf

An old post but, just some more info on the topic.

1.641 seems to be a bit long, even by 7.62 Nato standards. Of course that could have more to do with where your tool hits the shoulder of the fired case and how much the brass springs back after firing than the actual dimension of the chamber.

BTW, I have one 308win bbl and a 7.62 Nato bbl for one of my rifles and the measured cases using my tool are 1.628" and 1.627" respectively. Another .308win that I have measures 1.630. I've measured new brass at 1.625".

As you note, this is only an issue as far as reloading to 308 win specs and shooting in 7.62x51 chambered rifles.


I drug this up primarily because it highlights the need to measure and load separately for each particular rifle in the safe. You can load to minimum specs for a particular caliber but in some rifles you run the risk of shortened brass life and separated cases. Broken shell extractors are handy to have when you are playing with handloads and trying to size cases to run in multiple gas guns. Belted magnums can be fun as well if you don't pay attention.

[Shade OGrey]

Someone please correct me if I am mistaken, but my understanding from what I have read and heard is this:
The NATO .308 (7.62x51) differs from commercial .308 Winchester in both case capacity and in shoulder length and/or shape.
Supposedly, the commercial .308 is slightly 'hotter' than the NATO stuff (because of the thinner walls creating slightly greater case capacity?).
Also, supposedly, it is safe(r?) to fire NATO spec ammunition in a rifle chambered in .308Win. but not the reverse (i.e. .308Win in a 7.62x51 NATO chambered rifle), supposedly (I know, I seem to be using that word a lot) the opposite of the .223 and 5.56 debacle.
This question has been beaten 20 ways from Sunday on the internet since before the internet existed, but I am asking again because I have an old Savage 99E chambered in .308Win that was handed down years ago and bought used before then. I know the previous owner (my Step Father) fired a few, not more than 20 or so, 7.62x51 in this firearm (I got a lot of NATO headstamped brass both loaded and fired with the gun), and who knows what the pre-previous owner(s) may have shot in it. I have not experienced anything strange with this rifle although I have only shot it a few times using commercial .308, and I am not too concerned with reloading NATO brass (damn crimped primers), the fired brass looked OK to me at first glance but I haven't really inspected it minutely.
I guess my question boils down to, is there any danger of damage to the rifles chamber or other parts from firing NATO spec ammunition in it?
Also please correct my misunderstanding if I do indeed misunderstand the .308Win and 7.62x51 relationship.
TIA

Shady

[J Mack]

RG
I checked the chamber with a NATO gauge set and it is in between NO-GO and field where the guys from Primary told me they chambered it I also checked the fired brass with a Hornady Lock-N-Load Cartridge Headspace Gauge zeroed on a NATO GO gauge.
In my world I agree with you R G and the chamber is out of spec but in their world of building battle riffles they say this is the chamber for reliability in dirty conditions.
I have just over 1500 rounds through this rifle now and I have not had any chamber related issues and this rifle shoots way better than it should, we easily put rounds on steel at 700 yards something I would have thought imposable from a 14” barrel and that chamber not to mention the little 1X4 scope that sits on top.
Like you noted the most important issue here is to know how measure your brass and know the spec before you before you start loading bottle neck or belted brass.

SOG
You are correct the cases have similar external dimensions but the chambers are different. It’s safe to shoot NATO 7.62X51 ammo in a .308 chamber but not always safe the other way around. The issue like noted above is the longer NATO chamber and case head separation on the thinner .308 brass.

[Shade OGrey]

Thank you J Mack for confirming that my brain thing is not entirely soggy yet.

[RGinIdaho]

Thank you J Mack for confirming that my brain thing is not entirely soggy yet.

Remember, It is a chamber dimension/headspace issue. The only way to know if you can fire one in the other is check your chamber.

Failure of the brass case is the issue.

Pressure is not the issue. 7.62 is 50,000 CUP.

308 Win is 60,000 PSI.

PSI and CUP are not interchangable and there is no consistent relationship between the two.

I have a couple of older loading manuals that show 308win listed in CUP. That is the way many labs tested ammunition years ago.

[Shade OGrey]

The difference between the older copper crusher method of pressure measurement CUPs and PSI does seem to be the source of a lot of confusion.
http://www.shootingsoftware.com/ftp/psicuparticle2.pdf outlines a method to compare the two. I do not know enough to say how meaningful this may be especially given the inconsistencies of the  older crusher method of measurement. I simply present it for perusal for those interested.
Excellent point about chamber dimensions and headspace though. Something I should look into. Thanks RG.

[RGinIdaho]

Interestingly enough through some reading I did today in Lyman's 45th, copyright 1970; PSI was used prior to CUP and LUP. PSI as it applied to chamber pressures was problematic in regards to measurement and standards within the technical realm of the firearms community. This is the reason they state for a move to CUP and LUP. Apparently to ballisticians, the CUP and LUP measurements carry a bit more surety than PSI.

If you think about it, 308win in a large Nato Chamber such as JMack has, would result in lower pressure than the same round in a 308win chamber. The combustion chamber would be bigger. "Freebore" would be greater as well.

[Idaho_Gun_Nut]

RG
I checked the chamber with a NATO gauge set and it is in between NO-GO and field where the guys from Primary told me they chambered it I also checked the fired brass with a Hornady Lock-N-Load Cartridge Headspace Gauge zeroed on a NATO GO gauge.
In my world I agree with you R G and the chamber is out of spec but in their world of building battle riffles they say this is the chamber for reliability in dirty conditions.
I have just over 1500 rounds through this rifle now and I have not had any chamber related issues and this rifle shoots way better than it should, we easily put rounds on steel at 700 yards something I would have thought imposable from a 14” barrel and that chamber not to mention the little 1X4 scope that sits on top.
Like you noted the most important issue here is to know how measure your brass and know the spec before you before you start loading bottle neck or belted brass.

SOG
You are correct the cases have similar external dimensions but the chambers are different. It’s safe to shoot NATO 7.62X51 ammo in a .308 chamber but not always safe the other way around. The issue like noted above is the longer NATO chamber and case head separation on the thinner .308 brass.

I would argue the point that if you shoot 7.62x51 in a tight headspaced .308 you could acheive higher pressures than a .308 round would produce.  If you are shooting it out of a bolt action and have to force the bolt closed you are causing the case neck to crimp down on the bullet even tighter.  If you are shooting it out of a semiauto... you run the risk of an out of battery ignition and possibly injuring yourself and damaging the firearm.  As RG stated, .308 out of a 7.62x51 chamber will produce lower pressues due to the larger chamber dimensions, but stretches the case out which is fine if you want to reload the brass for 7.62x51.

.308 in a 7.62x51 chamber = ok

7.62x51 in a .308 chamber = ... maybe maybe not depending on the headspacing

[Shade OGrey]

Would anyone like to purchase or trade NATO stamped 7.62x51? Not sure how many I have, I will count them later today after I get some sleep.
I will accept cash (make me an offer) or commercial .308 ammunition (1 for 1) or .429 and/or.355 bullets (3 for 1), or something else you care to offer.
Just want to get rid of it.

7.62x51 is gone.
(edited 11/2/11)

[J Mack]

I would argue the point that if you shoot 7.62x51 in a tight headspaced .308 you could acheive higher pressures than a .308 round would produce.  If you are shooting it out of a bolt action and have to force the bolt closed you are causing the case neck to crimp down on the bullet even tighter. 

Are you thinking that the 7.62ื51mm NATO cartridge has a thicker neck dimension? What part of the 7.62x51mm NATO cartridge dimension do you think would cause the issues you described?
I have some 7.62x51mm NATO ammo here that will chamber in a minimum spec tight .308 match chamber bolt gun and I would think this is worst case scenario and only get better for factory chambered .308's.   


From Wiki:
Although not identical, the 7.62ื51mm NATO cartridge is similar enough to the commercial .308 Winchester that the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) considers it safe to fire the NATO round in weapons chambered for the commercial round
Link: http://en.wikipedia.org/wiki/7.62%C3%9751mm_NATO

My factory loaded commercial .308 Winchester and my factory loaded 7.62x51mm NATO ammo both closely resemble this drawing.



More info; http://www.303british.com/id36.html

[J Mack]

  As RG stated, .308 out of a 7.62x51 chamber will produce lower pressues due to the larger chamber dimensions, but stretches the case out which is fine if you want to reload the brass for 7.62x51.

Sorry to break this up into two post but I wanted to address this separately.

The exact reason for my first post was to inform guys that if you shoot a 7.62x51mm NATO chambered rifle with either 7.62x51mm NATO or commercial .308 ammo and reload with .308 sizing die set to the manufactures instructions of run the die down to the shell plate and back off 1/8 to ผ turn you will have issues.

I mean no disrespect Idaho Gun Nut but you will not be fine if you don’t compensate for the longer NATO chamber when reloading with .308 dies. I have never seen 7.62x51mm specific reloading dies listed for sale and assume everyone else uses their .308 die for sizing both as every manufacture recommends. 
Remember the military rifles are chambered for reliability in harsh conditions without care to how far it stretches brass as long as it can fire it safely once hence the .0075 difference in field gauges.     

[Idaho_Gun_Nut]

7.62x51 has an oveall longer case length and therefore when you try to chamber it in a tightly head spaced .308 bolt action you have to force the bolt closed.  This will force the case neck to crimp in tighter on the bullet.  Now not all bolt actions are that tight, but in a precision rifle such as I've experienced they are.

You are correct, there are no seperate reloading dies for 7.62x51 versus .308, you just compenstate as you set them up.  But again.. it is to compensate for the longer case.  I didn't mean that .308 will shoot accurately out of a 7.62 chamber but if you do so it will fire form the case to the chambers dimensions, this has been done for decades especially with bench rest shooters.  I think you were misunderstanding my statements.

[Grumblecakes]

i will take away from this thread this: loading 308 win and 7.62 nato are like any cartridge you have to understand how the brass reacts to your chamber then take measures from there. not just assume things will be perfect forever.

[J Mack]

7.62x51 has an overall longer case length and therefore when you try to chamber it in a tightly head spaced .308 bolt action you have to force the bolt closed.  This will force the case neck to crimp in tighter on the bullet.  Now not all bolt actions are that tight, but in a precision rifle such as I've experienced they are.

Unless I’m misinformed which is entirely possible.

The 7.62x51mm NATO case length spec is 2.015” -.020”

The Winchester .308 case length spec is 2.015” -.020”

On any print I've seen the minimum .308 match type chamber spec for the neck area length is .2.025” and tapers @35.43 degrees to 2.0488” OAL. If I headspace my chamber at 0.000” I could still chamber a case that is .010” over MAX without any issues.
Because you headspace off the shoulder and you need some clearance to function reliably    headspace on most match type chambers will be +.001”to +.002” over Min meaning the case could be +.011” to +.012 over max OAL without any issues and most likely .020” over MAX before you would jam it into the 35 degree taper enough to even notice a pressure difference.
I understand that we have children beating ammo together with rocks in back alleys of some third world shit hole so I can’t speak for all NATO spec ammo so I will say anything is possible.