Case wall thickness & pressure for 9mm & 380 ACP

[C0untZer0]

Case wall thickness for 9mm varies from .027 to .037 - I think NATO M882 has a case wall thickness of .032"

Case wall thickness for 380 ACP is something like .045" right?

Why can the 9mm easily withstand pressures of 35,001 psi but the 380 ACP is only rated at 21,500 psi ?

 

[rule303]

Because the .380 was originally designed for blow pack actions. The .380 case itself would have no problem handling the higher pressure, but the guns it is chambered in- not so much.

 

[Jim Watson]

A: Wrong numbers. Case wall thickness of a couple of random (but both Speer) cases show both as around .010-012" at the mouth. The .380 was .032" thick as deep as the calipers would reach, the 9mm .040".
Or are you citing the case head web thickness?

B: As is often said in rifle threads, the case is just a gasket to seal the action, it is not doing much to contain the pressure.

 

[MutinousDoug]

I don't have the SAAMI cartridge dimension sheet in front of me but my Lyman Cast Bullet Handbook lists the 380acp case wall as .0088" (.373dia-.3555dia) and the 9mm para as .0123" (.380dia-.3555dia) at the case mouth.

I just looked at ANSI/SAMMI 299.3-1993. It doesn't seem to have any specification for internal case dimensions.

 

[C0untZer0]

For the case wall thickness I was going by my Tec-9 manual which recommends ammo with a case wall thickness of .028 or thicker, and for 380 ACP - I just pulled it from Wikipedia

 

[Robert101]

JimW, can you please explain the premise of case seal and not doing much to contain the pressure. I thought the case providing the seal in the chamber was the main vessel to contain the pressure.

 

[C0untZer0]

According to this article, Tim Sundles at Buffalo Bore says the 380 case is as strong as the 9mm case.

http://www.shootingillustrated.com/index.php/20806/new-life-for-the-380-acp/

That makes me think that the case could withstand pressures of 38,500 psi.

 

[MutinousDoug]

I'd be inclined to agree with you regarding the case psi capability. I would urge much caution regarding un-supported areas of the pistol chamber (ramp and extractor) and re-load to those levels accordingly.
Actually, having broken an AMC Backup years ago from "experimentation", I would urge a pistol in 9mm Para and start my experimentation there.
Pressures in tiny cases succumb to small variations in powder weights and bullet seating depths. It's hard to get reliable reloading pressure feedback in YOUR pistol without possibly going way overboard.

 

[JDGray]

JimW, can you please explain the premise of case seal and not doing much to contain the pressure. I thought the case providing the seal in the chamber was the main vessel to contain the pressure.

The brass case has no strength without a chamber....

 

[Steve C]

The brass case has no strength without a chamber...

JD is correct. Think of the case as acting as a gasket as it expands to seal the chamber and breach face. If the case fails then its like a failed gasket similar to what one would see when a head gasket fails on an internal combustion engine.

The brass doesn't have the strength to contain the pressure by itself. The pressure is contained in the chamber by the bolt or breach face which remains in place as its opening is delayed by spring pressure and mass of the bolt or slide in blow back designs and by the barrel bolt or slide locking mechanism in other designs until the pressure drops enough to allow ejection of the case.

The pressures of the .380 acp is limited by the targeted use in small blow back handguns. If you increase the pressure you need to increase the weight of the slide or bolt, therefore the size of the handgun and the strength of the spring of the action. The Astra 600 is an example of a blow back pistol chambered for the 9mm. Heavy pistol slide with strong spring to contain the 35,000 psi 9mm. If one would increase the pressure of the .380 you could still have PPK size handguns but they would have to be lock breach actions and since you can do that with the existing 9mm, why bother?

The really small .380's like the Keltec P3AT or the Ruger LCP are examples of going the other way and making a locked breach gun to allow an even smaller handgun to be chambered for the .380 than the typical PPK size guns.

 

[BLU]

Steve C... spot-on. Best example of a higher power blow-back for me is a Hi-Point .45, 9mm, etc. Heavy slide, heavy spring. I saw a picture of a guy who over-loaded a small blow-back pistol. The slide came back extremely hard, fractured the forward portion of the slide and continued backward.... into the shooter. Rare incident for sure. Still, I re-check the powder loads on my .380 rounds because I've never forgotten that picture.