7.62 vs 308 VENT

[sqlbullet]

I get so tired of hearing how 308 winchester spec is 12,000 PSI hotter than 7.62 NATO.  And I finally found the culprit today.  It is the freakin' U.S. Military in  TM-43-0001-27.

The list M80 ball at 50,000 PSI.  And, yep, that is 12,000 PSI lower than 308 Winchester.  A whopping 25% more pressure in the commercial variant of the T-65 cartridge.

But.....

If the 308 winchester is loaded 25% hotter than the 7.62 NATO, it must be way faster too.  Let's see.

TM-43-0001-27 states that for your 50,000 PSI you get 2750 fps measured 78 ft from the muzzle.

And Federal Premium 308 Winchester 150 grain is going 2820 fps at the muzzle.  So, that is more...at the muzzle.  But at 100 yards they say 2532 fps.  And if I do some math,  -.96fps/foot * 78 feet from the 2820 at the muzzle....  2745.12 fps at 78 feet.

How can that be?

It can be as  TM-43-0001-27 is wrong. M80 ball is rated for 50,000 CUP, not PSI.  Tested ammo gets a MAP of 60,191 PSI in transducer systems.

But, sqlbullet my buddy's brother-in-laws cousins dogs sisters owners step-sons half-sister had a kaboom shooting 308 Winchester in his FAL with a 7.62 Nato chamber.  How come that happened if the pressure is the same.  And, you have to reduce powder charges in 7.62 NATO brass if using 308 Winchester data, cause pressure!

Well, here is how it happened and why the two are related.  7.62 NATO chambers are a good bit bigger than 308 Winchester chambers.  And, 7.62 NATO brass has thicker walls and case heads than 308 Winchester brass.

So, when that thin walled commercial load went off in that chamber with generous proportions, it's puny little walls just couldn't contain the 62,000 psi of pressure and something gave, the case head.  It is actually the puny 308 Win brass in a chamber that would close easily on a 308 no-go gauge that is the issue, not more pressure.  Excessive headspace = head separation even in ammo that is in spec.

And when you made the case thicker without making the outside bigger, the inside of the 7.62 NATO brass got smaller.  Same powder in smaller space = more pressure.

So, now you know.  They are really the same from a pressure perspective, US Government spec sheet not withstanding.  But, they are different in construction and in chamber dimensions which means you have to exercise care when loading ammo and feeding guns.

[The_Shadow]

Yes because the Military is using many of the ammo in fully automatic guns and not just semi auto, they use the same spec brass for all of their ammo and loaded to their NATO Spec as well so there is no confusion on the battle field...

I have read about the differences a few times and these guys have info as well that makes reference to the differences.
http://www.6mmbr.com/308Win.html

Here is another with illustrations for chambers
http://www.303british.com/id36.html

[DenStinett]

I've always argued this difference
Not so much the (Factory) Ammo but the Chamber differences
As in the difference(s) in .223 vs 5.56X45
When I load, I keeps my 7.62X51 all in NATO Cases
And my .308 in 308WIN Cases
I never mix them
Plus, I keep the .308 separated, sized and loaded as per their individual use;
Bolt Action, Lever Action and Break Action

[The_Shadow]

The stuff I load is for commercial civilian rifle, I'm using the RCBS small base resizing die.  Because my Brother in law has a Remington 742BDL semiauto, so we were looking for the most reliability with feeding.

I thing that they even recommend a small base die for loading for the military chambers as well. (correct me if I wrong) 

[EdMc]

I have no idea of the chamber size of an M-14, but many recommend the small based dies when reloading for the M1-A Springfield version. I use the same RCBS dies that you mentioned for my mine. Dillon case lube sure makes it easier when sizing surplus LC brass.

[DenStinett]

I have no idea of the chamber size of an M-14 / M1-A

The Chamber is what it says on one's M14 / M1a's Receiver

I thing that they even recommend a small base die for loading for the military chambers as well. (correct me if I wrong) 

Quite right Shadow
SB Dies are used for any Semi (Full) Auto and Lever Guns
FALs and M14 all need the NATO Cases SB resized
For best accuracy, Brass for Bolt Rifles should be segregated and reused in the Rifle it was fired in and Neck Sized only
Or Full Length if you want to, but it's not recommended (for best accuracy)
Single Shot, Break Action (i.e. T/Cs) can go either way, but may not lock closed (easily) with Neck Resizing only

[gandog56]

I have no idea of the chamber size of an M-14 / M1-A

The Chamber is what it says on one's M14 / M1a's Receiver

I thing that they even recommend a small base die for loading for the military chambers as well. (correct me if I wrong) 

Quite right Shadow
SB Dies are used for any Semi (Full) Auto and Lever Guns
FALs and M14 all need the NATO Cases SB resized
For best accuracy, Brass for Bolt Rifles should be segregated and reused in the Rifle it was fired in and Neck Sized only
Or Full Length if you want to, but it's not recommended (for best accuracy)
Single Shot, Break Action (i.e. T/Cs) can go either way, but may not lock closed (easily) with Neck Resizing only

Hard to do with my Mosin Nagants....since I have like 16 of them. And a Winchester 95 that shoots the same round.

[DenStinett]

Hard to do with my Mosin Nagants....since I have like 16 of them. And a Winchester 95 that shoots the same round.

Just takes a little added effort on your part to keep them separate   

[gandog56]

Hard to do with my Mosin Nagants....since I have like 16 of them. And a Winchester 95 that shoots the same round.

Just takes a little added effort on your part to keep them separate   

Lot of effort. I took five of them to my last range session. Too used to just throwing the empties into a bag.

[EdMc]

I have no idea of the chamber size of an M-14 / M1-A

The Chamber is what it says on one's M14 / M1a's Receiver

I was referring to a cast or mould of the individual rifles chamber, or have I missed something. It happens.....   Anyway, I never received the mount for the surplus night vision scope. Maybe 'R-Guns' isn't a reliable source of parts.