38 spl pressure v 9mm pressure


July 31, 2010, 09:04 AM
Why does it take 24,400 psi to push a 9mm, 125 gr bullet to 972 fps while it only takes 13,300 psi to push a 38 spl, 125 gr bullet to 974? This difference in pressure required for a given velocity is apparent regardless of bullet type/weight and powder used.

Question 2: What could I have searched for to find the answer posted in an existing thread?

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July 31, 2010, 09:11 AM
I don't know for sure, but my guess is that the .38 has a larger case and can make use of slower powders. Using the same powder, isn't the 9mm faster than the .38?

July 31, 2010, 09:20 AM
Revolver and automatic barrels are measured differently. A 4" barrel on an auto is 4" from the muzzle to the breech face. A 4" revolver barel is 4" from the muzzle to the front of the cylinder. The cylinder adds about 1.6" of length, so a revolver will have a longer "effective" barrel length.

There's also the fact that I'm betting the loadings for the .38 use much larger charges of much slower powders. That's to take advantage of both the longer "effective" test barrel, and to make use of the larger case. A bunch of a slow powder, even in the exact same cartridge and gun, can match the velocity of a smaller charge of faster powder.

Think throwing a baseball at 70 mph vs. putting a baseball on a T-ball tee and whacking it with a bat, making it also go 70 mph.

July 31, 2010, 09:40 AM

I'm not sure I understand your question. But if you're saying that it takes less powder in the 9 to do the same thing in the 3, then, yes, that appears to be correct. In the example I used above I arbitrarily chose IMR SR4756 as the powder. The 9 requires 4.2 gr to go 972 fps (24,400 psi) while the 38 requires 5.4 gr to go 974 fps (13,300 psi). I understand why the same amount of powder in a 9 would create more pressure than it would in a 38 due to the difference in volume. But my question, I think, ... ... oh ... maybe that IS the answer... i.e., the 38 has almost 3(?) times the volume of the 9. So that's a lot more gas expanding out for a longer period. So it takes less pressure? Does that make sense? Can someone verify my theory rand perhaps restate it in clearer terms?

And again... what would I have searched for to find this in an existing thread?

July 31, 2010, 12:34 PM
I have no idea where you are getting your load data, but.

Hodgdon shows a .38 Spl 125 grain JHP using SR-4756.
5.3 grains = 939 FPS at 14,700 PSI.
6.0 grains = 1,046 FPS at 16,400 PSI.
This out of a 7.7" barrel. (6" revolver barrel + cylinder)

A 9mm using a 125 grain JHP:
4.5 grains = 973 FPS at 25,700 PSI.
4.9 grains = 1,037 FPS at 28,700 PSI.
This out of a 4" barrel. (auto pistol barrel & chamber)

It seems you might be leaving test barrel length out of the equation.


July 31, 2010, 12:49 PM
You are looking at pressure the wrong way. Pressure is a byproduct of combustion. The burning propellant exerts energy on the bullet pusing it forward. It is not pressure that drives a bullet to a specified velocity, but energy from the combusting powder. How the powder combusts and how that energy is applied depends on chamber volume.

In a 9mm, chamber volume is very small, pressure goes-up quickly, and powder burns more efficiently. In a 38 Special, the volume is twice as big, pressure builds-up slower, and the powder combusts less efficiently, so you do not get the same velocity per grain of powder in the same barrel length.

To compound the issue, 38 test barrels are vented which simulates velocity loss due to a barrel/cylinder gap. Then you have the difference between barrels, bore condition, twist-rate, etc. which further add to the difference.

4756 is basically too inefficient in the 38 at 13000 PSI.

August 2, 2010, 10:42 AM
I put a 9mm powder charge in a 38 sp case and seated at 9mm bullet to a 9mm type depth.

It should have been like shooting a 9mm cartridge in a 38 special.

But the cylinder split, and so I lost another really nice 38 special.

August 3, 2010, 12:16 AM
Which 38 did you explode?

August 3, 2010, 12:18 AM
One of the half dozen Colt Police positive surplus revolvers I got from AIM surplus for destructive tests, but they are too nice to blow up.

August 3, 2010, 12:41 AM
Ah, the Police Positive. Was there a difference in steel alloy used in the pre-war guns vs. those made in the 50's? You know what I'm getting at.

August 3, 2010, 02:16 AM
These were surplussed ~ 2002.
They had the name of the police department welded over.
I heard they were 1950's vintage.
They lots of holster wear, and no mechanical wear.
I cut an inch off the frame in one of them, so it would talk Agent grips.
I blew one up with AA#5 and one with 20 gr Longshot, but they push 158 gr at 1173 fps with LIL'GUN just fine.

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