Question about .40 vs 10mm?

[Bart Noir]

And yet nobody has mentioned barrel length. The velocity numbers can be rather misleading if the test barrels are not all the same length.

Bart Noir

[pinzgauer]

It's been decades since I've loaded 38 and 357 but from rusty memory the case length (and thus capacity) was not a big factor in achieving velocities.

It was more to prevent inadvertent loading of the higher pressure 357 in weaker 38.

I loaded and used very hot loads (357 level) in 38 brass with no significant difference.

It's not like rifles where case capacity is a very significant factor due to the much larger amounts of powder.

I don't recall any loadings that involved compressed loads.

Revolvers are a lot more forgiving regarding case length as they don't headspace on the case mouth. OAL is also largely a non-issue as long as they load in the cylinder will turn.

For plinking with kids I load 10 mm to sub 40 caliber velocities, just enough to cycle reliably. The difference in powder weight to do that in 10 mm versus to get the same velocity in 40 is minuscule.

So that efficiency difference (which is true), does not have near as much impact in the real world as it does in rifles

[Uni-Vibe]

It's been decades since I've loaded 38 and 357 but from rusty memory the case length (and thus capacity) was not a big factor in achieving velocities.

It was more to prevent inadvertent loading of the higher pressure 357 in weaker 38.

I loaded and used very hot loads (357 level) in 38 brass with no significant difference.

It's not like rifles where case capacity is a very significant factor due to the much larger amounts of powder.

I don't recall any loadings that involved compressed loads.

Revolvers are a lot more forgiving regarding case length as they don't headspace on the case mouth. OAL is also largely a non-issue as long as they load in the cylinder will turn.

For plinking with kids I load 10 mm to sub 40 caliber velocities, just enough to cycle reliably. The difference in powder weight to do that in 10 mm versus to get the same velocity in 40 is minuscule.

So that efficiency difference (which is true), does not have near as much impact in the real world as it does in rifles

This is what I was thinking.

You could, and some have, loaded a .38 spl case up to top .357 magnum velocities and it will do the job. In other words, the 0.13 inch case length difference is irrelevant.

So why indeed does the 10mm, at only 0.142 inch longer than the .40, have such a substantial increase in power ?

More pertinently, the original question: SAAMI pressure is only 2500 psi higher in 10mm than it is in .40. Yet the 10mm drives the same 180 grain bullet 300 fps faster, a considerable difference.

.357 magnum runs at about double the pressure of .38 spl, but it also drives that same 158 grain bullet close to double the velocity.

What's going on here?

[DG23]

There is no substitute for displacement.

My last wife was not what I would call a car / engine enthusiast but she used to say that a LOT.

Guessing she was into guns and ammo and I never knew it???

Damn. Might have kept her around if I knew that...

[Gunfixr]

Given the same firearm, with the same length barrel, adding more powder and pushing up the velocity is also going to be adding pressure. It is pressure, after all, that pushes the projectile down the barrel.
If you want more velocity, with the same weight bullet, there's really only a couple ways to get it. Use more pressure, by burning more powder, to move it faster, or, use a slower burning powder, to keep the pressure down, but you need more barrel length, to get more time to build up speed.
Depending on barrel length, the second option may not be available.
Smokeless powders are "propellants", they progressively build pressure as they burn, accelerating the bullet down the barrel.

So, pushing say, a 38spl to 357 mag velocities, particularly in a shorter barrel, is pushing, or maybe exceeding, the pressure design of the cartridge.

The 10mm was designed for specific parameters. The gun designed along with it was a fairly large gun, with full length barrel, at least for a fighting pistol.
Turned out, not many could handle either. It started getting downloaded. At that point, it was realized a smaller cartridge case could be used, and it could be chambered in smaller guns. Put a 10mm in a pocket gun, it'll likely perform about like a 40. Just with a bit more flash.

[lonestardiver]

This is what I was thinking.

You could, and some have, loaded a .38 spl case up to top .357 magnum velocities and it will do the job. In other words, the 0.13 inch case length difference is irrelevant.

So why indeed does the 10mm, at only 0.142 inch longer than the .40, have such a substantial increase in power ?

More pertinently, the original question: SAAMI pressure is only 2500 psi higher in 10mm than it is in .40. Yet the 10mm drives the same 180 grain bullet 300 fps faster, a considerable difference.

.357 magnum runs at about double the pressure of .38 spl, but it also drives that same 158 grain bullet close to double the velocity.

What's going on here?

It is not just adding more powder... In the .357 you can use a slower burning powder that gets it to the higher velocities. Same thing applies to the .44spc/.44mag. For .38spl I normally use bullseye or some other fast powder. For the magnum I switch to 2400, H110, Win296 to push the higher velocities with the heavier bullets.

What are the powder choices for .40 vs. 10mm? Are the 10mm powders slower burning?

[Uni-Vibe]

It is not just adding more powder... In the .357 you can use a slower burning powder that gets it to the higher velocities. Same thing applies to the .44spc/.44mag. For .38spl I normally use bullseye or some other fast powder. For the magnum I switch to 2400, H110, Win296 to push the higher velocities with the heavier bullets.

What are the powder choices for .40 vs. 10mm? Are the 10mm powders slower burning?

Yes. I can load 180 grain .40 up to max potential with AA#5 powder, a fairly fast burning powder that has a similar burn rate to Unique. I used this same powder to simulate some 10mm FBI lite loads.

But when I load 10mm for real, I use AA#9 which is slower burning. It's not quite as slow as 2400, but it still a magnum powder that also gives serious velocities in .357 and 44 magnum revolvers. AA#9 is widely considered THE powder for serious 10mm work.

[lonestardiver]

I have used AA#9 as well for .357mag.

The point is and your use demonstrates that while a larger powder charge is used, it is the change in powder to a slower burning powder that is the key and the answer to the OP’s question.

[1986s4]

Maybe I missed it but one of the main reasons the .357 is longer is so that it could not be mistakenly loaded into a older, weaker .38spl pistol. Lonestardriver has hit the nail on the head, slower powder in slightly larger quantity is the main reason why the 10mm and others generate more velocity than their shorter cousins. Of course a longer barrel is needed to take advantage of it.
I have loaded .38 super with a small quantity VV310, a very fast powder, to achieve mild recoil and a blown case head.....

[AndyLate]

Maybe I missed it but one of the main reasons the .357 is longer is so that it could not be mistakenly loaded into a older, weaker .38spl pistol.

That is correct and thank you for saying it. The same situation exists with the 44 Mag. They are simply not germane to the question about 10mm and 40 S&W.

Shortening a pistol cartridge and producing a new round with less velocity is just not commonly done.

The physics of how it works has been explained, larger capacity allows more (and slower) powder.