A question on pressures?

[MikeJ]

I'm trying to understand something and am at a complete loss. If the 9mm typically has a PSI pressure of 35000 and so does the 40 S&W, why is it that the 40 is considered so much more abusive on guns than the 9mm?

 

[Steve C]

If the 9mm typically has a PSI pressure of 35000 and so does the 40 S&W, why is it that the 40 is considered so much more abusive on guns than the 9mm?

The pressure of the cartridge doesn't do any damage or cause any wear to the pistol other than an extreme overpressure load that blows up the barrel. What wears on the gun is the recoil generated, the slamming, banging and hammering of the parts as the ammo is shot. The .40 cal fires significantly heavier bullets than the 9mm and this produces greater recoil energy. 40 's and 9mm's are built on a similar weight frame so the heavier recoiling round hammers the pistols frame and moving parts proportionately greater.

Think of it this way. You can swing a 6 oz tack hammer faster than you can swing a 24 oz claw hammer but which will go the most damage for each hit to what you are pounding on? ..... The heavier hammer will.

 

[45crittergitter]

Further, Force = Pressure x Area

If the 9mm has a head area of say, 1/2 square inch, and the .40 has say, 3/4 square inch, with the same pressure the .40 will produce 50% more force than the 9mm.

 

[RPCVYemen]

If the 9mm has a head area of say, 1/2 square inch, and the .40 has say, 3/4 square inch, with the same pressure the .40 will produce 50% more force than the 9mm.

Is the following correct?

If the if the .40 is approximately a 10mm, then the radius is 5mm.
The 9 will have a radius of 4.5 mm.

The difference in the area will be proportional to the squares of the radii

4.5 * 4.5 = 20.25
5 * 5 = 25.00

25.00 / 20.25 = 1.23, or about 23% more power.

Mike

 

[Ben Shepherd]

RPVCYemen:

9mm=.355. 40 is a true 10mm diameter(or .400 if you will).

Your math looks correct.

 

[mpmarty]

Also, most .40 S&W ammo is loaded to saami max of around 35,000 but nearly all domestic 9mm stuff is loaded a lot lighter for some reason. European "sub machine gun" ammo is considered too hot to shoot in some 9mms and it is, are you ready? loaded to 35,000 pressure levels.

 

[trueblue1776]

Your mathematical reasoning is flawed on a fundamental level, maybe try to keep it simple: heavier bullets on a frame that can shoot lighter bullets will cause wear faster than shooting the lighter bullets would.

But it's all just barking at the moon as far as I'm concerned, my P229 has had cases of .40, every flavor in the candy store, and shows no abnormal wear, or any wear for that matter.

 

[ugaarguy]

Your mathematical reasoning is flawed on a fundamental level, maybe try to keep it simple: heavier bullets on a frame that can shoot lighter bullets will cause wear faster than shooting the lighter bullets would.

But it's all just barking at the moon as far as I'm concerned, my P229 has had cases of .40, every flavor in the candy store, and shows no abnormal wear, or any wear for that matter.

The P229 was a ground up redesign of the P228, and it (the P229) was built for the .40 S&W and .357 SIG rounds.

 

[45crittergitter]

The math is a bit off. My original was a simplified example not using real numbers. The error is that you don't use the bore size to get the area, you use the cartridge case head size.

 

[trueblue1776]

45,
I was implying that same pressure on smaller surface is more stress not less.

 

[Candiru]

What's been said so far is pretty much accurate, although I would assign more of a role to pressure than what has been asserted so far. The Browning inertial recoil design used in most modern semi-autos uses the pressure to lock the barrel and slide together until the bullet has left the barrel, so the barrel is literally trying to tear itself out of the slide under pressure, and to that should be added the force of the bullet slamming into the rifling, which depends largely on the weight of the bullet. All Browning-pattern semi-autos have fairly tiny slivers of metal holding the barrel and slide together, and if pressures above the tensile strength of the metal are experienced or if there is excessive endplay, these tiny bits of metal will give slightly. As the metal gives way, the barrel gets more of a run-up at the locking lug(s) under pressure, increasing the amount of impact force it brings to the relationship, and so on. As headspace opens up, more of the case is left unsupported.

9mm vs. .40 S&W is an interesting comparison. Let's take some 9mm at 30K PSI versus .40 S&W at 35K PSI. 9mm case head diameter is 0.394" and .40 S&W case head diameter is 0.424". In each case, area is given by pi * (d / 2)^2, so 9mm has a case head area of 0.122 square inches and .40 S&W has 0.141 square inches. Multiplying by pressure, we get 3,660 pounds of pressure for 9mm and 4,935 pounds for .40 S&W.

The amazing thing about all this is that 9mm actually has thicker brass in its web than .40 S&W! Personally, I think .40 S&W would be an incredible cartridge if it were a lightly loaded 10mm, but don't care much for it in its shrunken and, in my opinion, ill-considered current form.

 

[45crittergitter]

Negative, trueblue. Pressure, by definition, is equal in all directions and in magnitude. 100 psi applied to 1 square inch = 100 pounds force. 100 psi applied to 2 square inches = 200 pounds force. Stress (and strain) is another matter, not computable from the given info, but it is safe to say that the same pressure applied to a smaller area (breechface, for instance) does not necessarily result in more (or less) stress.