Bullet size and ballistics

[Paraguy]

I am puzzled, why is it that a 38 Special +P and a 9mm +P round look identifal in all ways except that the .38 looks to be about 50% longer.  Even more puzzling, even though it is has about the same weight projectile and the room to hold about 50% more gun powder it delivers it at about 150fps slower than the 9mm.  Is there a logical explanation why one round is slower than the other (all things being equal such as brand, weight, both rated +P etc. ?)

[Fatman]

Hmmm. Great question. Is it because of the slight gap @ the cylinder releasing some pressure? Or is it the type of powder used? Or something totally different? Beuhler?

[tombogan03884]

Hmmm. Great question. Is it because of the slight gap @ the cylinder releasing some pressure? Or is it the type of powder used? Or something totally different? Beuhler?

 Just wanted to show I'm curious as well.

[Timothy]

Originally, the .38 Special replaced the .38 Long Colt and was a black powder cartridge.  Case length probably was driven by those factors.  Black powder can be loaded by volume rather than weight.

Old rounds developed for different firearms at about the same time.  We didn't adopt the 9mm in the West for some time so the .38 was the "Cartridge de jour!".

Just a wild guess...

[Big Frank]

+P for the .38 is less pressure than +P in a 9mm. 20,000 vs. 38,500 PSI.

[Sponsor]

[Fatman]

Originally, the .38 Special replaced the .38 Long Colt and was a black powder cartridge.  Case length probably was driven by those factors.

Ya, agreed on that. But given the extra case length and powder advances, one would figure the velocities would be higher for a larger volume of powder.

This is a good discussion.

[Timothy]

True but modern powders are not loaded by volume.  They're loaded by weight and and that's determined by case pressures that you'd rather not exceed for a particular firearm.  Luger, when he developed the cartridge in 1901 obviously loaded it and designed the firearm for it at exceedingly higher pressures and velocity but not at +P pressures.

In 1902, the .38 needed a better powder but it was being used in older frames.  It's size is based on that history, not the pressures of modern frames.  A 100 year old .38 special can't be loaded with +P cartridges any more than a 100 year old Luger can handle a +P cartridge.  Remember, regardless of how many grains required, you may not be coming anywhere near completely loading the case with powder.

I'm not a loader, that's the guy that's should have some knowledge.  I'm still just guessing..

[Paraguy]

In this specific case, both are Golden Sabres, rated +P and one is 124 grains the other 125, yet the velocity is 150 fps different in favor of the shorter 9mm. 

[Timothy]

In this specific case, both are Golden Sabres, rated +P and one is 124 grains the other 125, yet the velocity is 150 fps different in favor of the shorter 9mm. 

But the powder "grain" weight behind the bullet is much lower in the .38 cartridge regardless of the size of the case.  The 9mm Luger is loaded with more powder by weight.

[Pecos Bill]

+P for the .38 is less pressure than +P in a 9mm. 20,000 vs. 38,500 PSI.

Pressure is what drives the bullet. The more pressure the faster the bullet speed. The actual weight of the powder means little. It's the pressure that powder produces that causes bullet movement. If you check you'll find there is less room for powder in the 9 case than in the 38 case. Each of these cartridges are designed for a given maximum pressure. This is not exceeded by the factories.

Also understand that the closed breach of the typical 9 mm handgun allows all the propellent gases ( pressure) to propel the bullet none of this gas is lost in a barrel/cylinder gap. All of this comes into play as the limiting factures of velocity.