.308 vs. 7.62 NATO cartridge dwgs

[Grump]

I've found some nice and pretty chamber dimension drawings on the Web for 7.62 NATO vs. .308 Winchester, but drawings of the CARTRIDGE dimenstions are rare! I haven't found one with min/max dimensions or tolerances from nominal, either.

Anyone have any? My ultimate goal is to calculate the maximum stretch on the case when a .308-min cartridge is fired in a 7.62mm NATO-max chamber (ignoring lug stretch, etc.).

AFAIK, most ammo/rifle warnings focus on the possibility of getting out-of-battery firing in a .308 Semiauto firing longer NATO ammo. What's the effect on case life mismatching the ammo and chambers the *other* way?

 

[Grump]

Busy Board. BTT

 

[Dave R]

This may not help, but....

Speer reloading manual No. 13 has .308 cartridge dimensions and drawing on page 278. I don't have a scanner, so can't post a copy.

Maybe someone else can?

They show max case length 2.015. Trim-to length 2.005

Max cartridge length 2.810

What semi-auto are you using that doesn't use NATO ammo? Rem 740?

I am not an expert nor a gunsmith, but AFIK, you can use NATO ammo in a .308 rifle. The only length issue are: 1) make sure the bullet is not contacting rifling, 2) Make sure case length is within spec.

 

[mechanic66]

Well I've done some searching and come up with this

http://www3.sympatico.ca/shooters/7_62vs308.htm

http://www3.sympatico.ca/shooters/7_62vs308Pt2.htm

http://www.hk94.com/hk/index.php?showtopic=978

for whatever this is worth.

 

[Grump]

Dave R:

Thanks for the dimensions from the book. I guess we can calculate the cartridge headspace from the geometry of the shoulder and the intersection of the hypotenuse and the .400 datum line. Min and max cartridge headspace are what I'm ultimately after, to calculate the maximum headspace possible for an in-spec rifle using a NATO chamber, firing a SAMMI-min cartridge.

Why? Well, my light loads (that don't set the case back against the bolt--2550 fps using M80 bullets at 78 feet and 78° F) show chamber clearance of up to .015 inch. Have chamber dimensions, including min/max and tolerances and field reject, but now need to superimpose the cartridge min/max headspace info.

Mechanic66: thanks--I'll check those in a minute and see if they are any better than what I found using Google yesterday.

 

[mechanic66]

I'll toss this in too which I found on the FAL files.

http://www.falfiles.com/forums/showthread.php?threadid=22889&perpage=30&pagenumber=1

For those that don't want to read thru the 2 pages and trust me to summarize here it is my interpretation.
Basically there isn't a significant difference in chamber pressure between .308 & 7.62 and any difference can be attributed to the measurement methodology (C.U.P vs piezo P.S.I etc). The problem/difference/danger lies in the fact that the 7.62 chambers are a little longer which coupled with thinner .308 brass can cause a "case failure because there is not enough brass to flow and seal".

 

[Grump]

Well, we've come full circle and the references now refer to other sources I read yesterday.

Lots of chamber discussions. No cartridge headspace dimensions listed. Cartridge length to the rear corner of the shoulder do appear in a few places, but never with tolerances (=/- or just x.xxx + .yyy or a.aaa - .bbb).

Thanks for the references, all! I'll keep looking. Anyone have a U.S. military ammo guide or manual of some sort?

 

[Grump]

BTW, my Wilson .308 case gage shows a total difference between min headspace/min OAL and the max values of about .015 inch. Can't tell for sure b/c it's a .01 caliper w/o a vernier...

Anyone know the headspace min/max for 7.62 NATO cartridges????? Are they even different from .308? Chambers are, but inquiring minds want to know.

Another BTW--rotgun milsurp (NOT NATO-headstamped) from ?Argentina? goes a tiny bit past the Wilson gage's max cartridge headspace indicator. There just might be a difference, but I have no factory NATO stuff around.

 

[sigman4rt]

308/762

Check the fulton armory website I think they might have what your looking for.