308 v 7.62x51 Pressure

[Robert]

Is 308 truly higher pressure? I've heard this repeated forever on forums but I'm never seen actual documented proof. I've always been told that the CUP measurements used for 7.62 are not easily converted to PSI measurements that 308 is measured in. Therefore the difference in pressure is not as huge as most people believe and that factory 308 and factory 7.62 are pretty much the same.

I know I'm kinda beating a dead horse on the subject but I'd love to see some actual scientific data on the subject.

 

[Pat Riot]

I have had this explained to me a couple of times. Rather than fumble through I am posting an article that gives a decent explanation of concerns folks have between the two.

https://www.ammoland.com/2017/06/7-...ster-ammo-whats-the-difference/#axzz5ySyKqJFX

 

[Robert]

I have had this explained to me a couple of times. Rather than fumble through I am posting an article that gives a decent explanation of concerns folks have between the two.

https://www.ammoland.com/2017/06/7-...ster-ammo-whats-the-difference/#axzz5ySyKqJFX

And that's the way I've always understood the issue. You won't blow up the gun, but you may trash cases.

 

[shoobe01]

I read it, I still don't get why I have this one problem, but I'd love to understand: 7.62 in the Savage 99 will lock it up. Requires a LOT of force to open the chamber up after firing.

.308s work fine. Since I mix and match in other guns (like Rem 700) what is the detail difference and why do some chambers not like the wrong version?

 

[Nature Boy]

Are we talking about a comparison of Lake City XM80 vs commercial (pick it) .308 hunting loads?

 

[jonnyc]

"I'm kinda beating a dead horse"

Yes, very much so. It is the same round with two different names.....like 7.65 ACP/.32 Auto, 9mm Luger/9mm Para, etc.
Commercial chambers tend to be tighter and military cases have a tad less capacity due to thicker webs for MG use.
That's it......completely interchangeable.

 

[1KPerDay]

That’s a significant difference! Thicker walls combined with similar exterior dimensions means less powder capacity and a lower “top end” and all else the same, lower pressure and velocity.

Interesting, because the same issue in 9mm is often cited as causing GREATER pressure due to the same powder charge being used in a case with less capacity.

 

[jmr40]

7.62 is designed to function in military semi and full auto rifles. As a result it needs to fall within a fairly narrow range of pressures to operate the actions reliably. 308 is designed for sporting rifles where a wider range of pressures can be tolerated and still function in most manually operated sporting rifles. With factory loads you MIGHT get some 308's that are loaded slightly hotter than 7.62. But just because ammo CAN be loaded to max specs doesn't mean it is. In fact most 308 factory loads will still be within 7.62 specs. You are more likely to run into problems with 308 hand loads that are on the upper end of book loads. And the most likely result will be unreliable function in semi-auto and full auto rifles. Possibly premature wear, but probably not a failure.

Thicker walls combined with similar exterior dimensions means less powder capacity and a lower “top end” and all else the same, lower pressure and velocity.

Less case capacity with equal amounts of powder results in MORE pressure and velocity, not les. You can still get the same, or greater velocity from cases with less capacity. You just need less powder to do it. It's just one of those factors that has to be considered.

 

[MistWolf]

With less case capacity, you get less velocity for the same pressure.

 

[theotherwaldo]

-And then there's the 7.62 CETME... .

 

[alsaqr]

The 7.62mm M80 Ball round generates 50,000 psi.

See Chapter 11:

http://pdf.textfiles.com/manuals/MILITARY/united_states_army_tm_43-0001-27 - 29_april_1994.pdf

 

[Jim Watson]

The US Army is not a member of SAAMI and did not then use the CUP terminology. That manual is giving readings from a crusher gauge calibrated in pounds per square inch.
If .308 were really 30% higher pressure than 7.62, where is my extra velocity? Should be about 15% or 400 fps more.

 

[Steve S.]

7.62 x 51 is a metric measure to accommodate UN membership and many different manufactures of a common chambering. By design it has very slightly different dimensions and accommodating powder charge - this round simply has to satisfy a broader population of use and application. The .308 is yet another genius invention of US birth and measure designated for a much narrower use and application.
Both the 7.62 x 51 and the .308 are essentially the same round developed for different audiences to use in different equipment. While in the military, I liked the 7.62 x 51, now a civilian I like the .308 (toe-may-toe or toe-mah-toe?)

 

[taliv]

A lot of posts here seem to imply that pressure and velocity are closely related, perhaps even proportional.

Life is much more complex. Scan through a reloading manual from a bullet mfg and you’ll see many different powders for a given bullet with different peak pressures and velocities.

 

[Slamfire]

The 7.62 Nato round must meet the requirements of Stanag 2310. See the Stanag list here: https://nso.nato.int/nso/nsdd/listpromulg.html

Nato EPVAT testing is described here: https://en.wikipedia.org/wiki/NATO_EPVAT_testing

Note, the rounds are oiled. Explain that Hatcherites.

Each weapon and component considered vulnerable to the effects of a rapid change in pressure, for example barrels, breech blocks and bolts, will be tested by firing one dry round at a corrected minimum of 25% over pressure and one oiled round at a corrected minimum of 25% over pressure. 25% over pressure means 25% in excess of the Service Pressure (Pmax). The Service Pressure is defined as the mean pressure generated by the Service Cartridge at a temperature of 21 °C (70 °F). Such a high pressure proof is conducted with both the weapon and ammunition conditioned to an ambient temperature of 21 °C (70 °F).

 

[lysanderxiii]

The 7.62 Nato round must meet the requirements of Stanag 2310. See the Stanag list here: https://nso.nato.int/nso/nsdd/listpromulg.html

Nato EPVAT testing is described here: https://en.wikipedia.org/wiki/NATO_EPVAT_testing

Note, the rounds are oiled. Explain that Hatcherites.

Each weapon and component considered vulnerable to the effects of a rapid change in pressure, for example barrels, breech blocks and bolts, will be tested by firing one dry round at a corrected minimum of 25% over pressure and one oiled round at a corrected minimum of 25% over pressure. 25% over pressure means 25% in excess of the Service Pressure (Pmax). The Service Pressure is defined as the mean pressure generated by the Service Cartridge at a temperature of 21 °C (70 °F). Such a high pressure proof is conducted with both the weapon and ammunition conditioned to an ambient temperature of 21 °C (70 °F).

1) You present that quote as if it is from some STANAG, however a perusal of all the relevant STANAGs (2310, 4090, 4172, 4173, and 4383) in my library fail to reveal that requirement anywhere. Do you have a source for this?

2) Acceptance testing requirement for all US weapons is outlined the appropriate military specifications and drawings, such as MIL-DTL-70599 and for the M4A1 Carbine. For all of these specifications, the proof test requirements are "one ... high pressure test cartridge in accordance with drawing [relevant drawing for 5.56mm, 7.62mm, 9mm, or Caliber .50 HPT round]". Further, there is no mention of a lubrication requirement. The M4 series being unique in the allowance of SAAMI-Z299.4 specification proof rounds as an acceptable alternate, but still no oil.

3) EPVAT testing of all US 7.62mm ammunition is done under the requirements of SCATP-7.62mm, Test Procedures for 7.62mm Cartridges. other that the requirements for the NATO design EPVAT Barrel and Kistler 6215 Transducer, the data listed in the Corrected Proof Pressure Requirement (Service Pressure (Pmax) + 25%) table is incorrect for all United States service ammunition. This table is actually lifted from DEF-STAN 05-101, Proof of Ordnance, Munitions, Armour and Explosives, Ministry of Defence, which is a British Military specification, and not applicable to anybody other than the British.

4) Lubricating cartridges does not increase the chamber pressure, but it does increase the bolt thrust and case stress by the loss of friction between the case and the chamber wall. In old, weak, or actions of inadequate strength this might be a problem, and especially so if the loads are at the upper end of the pressure spectrum. If you lubricate cases, you should monitor the case head and primer pocket diameters for enlargement, as this is an indicator of nearing the limits of what the case is designed to withstand.

 

[lysanderxiii]

Oh, and one last thing...

From SCATP-7.62 - Ammunition Ballistic Acceptance Test Methods - Test Procedures for 7.62mm Cartridges:

7.5 TEST PROCEDURE

7.5.I Pre-firing (Preparation for test)

7.5.1.1 The required number of test cartridges and reference cartridges shall be drilled in a press using a drill jig listed in the Inspection Equipment List, to assure that the hole is drilled in the specified position. A #47 drill (.0785 in. dia.) shall be used, exercising great care to assure that the drill does not penetrate into the case far enough to remove any propellant.

I dunno about you, but that certainly sounds like they are using mid-case direct pressure readings, not case mouth pressure readings.