7,62x25 мах fps

[jeff-portnoy]

Hi.
I read about the PPS submachine , with its barrel 11" - standard bullet velocity 1600 fps!
Please explain why?
After all shorty TT-33 with a barrel 4"... - standard bullet velocity 1450 fps!

 

[Gryffydd]

150fps gain in 7 inches sounds about right for a round with a relatively small amount of relatively fast burning powder.

Is there a reason this is not what you expected? Did you expect more or less gain?

 

[56hawk]

Yeah, not sure what you are surprised about. Non magnum pistol cartridges generally gain velocity with longer barrels up to 10-12 inches. They will level out after that and even start to loose velocity with barrels over 16-18 inches due to the pressure dropping below the frictional force exerted by the barrel.

 

[jeff-portnoy]

150fps gain in 7 inches sounds about right for a round with a relatively small amount of relatively fast burning powder.

Is there a reason this is not what you expected? Did you expect more or less gain?

of course I expect more !
same cartriges for tt33 and pps , _ velocity same too

 

[jeff-portnoy]

Yeah, not sure what you are surprised about. Non magnum pistol cartridges generally gain velocity with longer barrels up to 10-12 inches. They will level out after that and even start to loose velocity with barrels over 16-18 inches due to the pressure dropping below the frictional force exerted by the barrel.

I don't see the logic!
Anybody who pushed bullet across full length barrel with a ramrod, he felt - 99.99% efforts in initial bullet motion during entrance to the barrel grooves!

 

[56hawk]

Hmmm. I've never tried hammering a bullet through a barrel from the breach end. Have hammered a few squib rounds out, and if that is only .01% of the effort I would hate to think what it would take to get the bullet started.

Anyway, as to why more velocity isn't gained I think you are confusing pistol rounds with rifle rounds. With a high capacity rifle round the pressure in the barrel tapers off much slower than a low capacity pistol round. Check out the difference in the Quickload graphs below. You can see how quick the pressure drops in the Tokarev and how much faster the Nagant gains velocity.

 

[Gryffydd]

of course I expect more !

OK, but why exactly?

same cartriges for tt33 and pps , _ velocity same too

I'm not really sure what this says, but according to your own post, it gained 150fps going from the tt33 to the ppsh.

56Hawk's graphs are an excellent illustration of the principles which govern velocity gain relative to barrel length.

I would also suggest taking a look at Ballistics By The Inch:
http://www.ballisticsbytheinch.com/index.html

While they don't feature the 7.62x25, you can get a feel for how barrel length affects different types of cartridges.
Just for one example, the .357 Sig using a 125gr bullet gains 239fps going from 4" to 11". The 357 Mag on the other hand, gains 462fps using the same bullet weight between 4" and 11". This is consistent with the differences in case capacities and suitable powders between these two rounds. With the same parameters the 9mm +P gains only 146fps.
The .223 using a 55gr bullet gains over 1100fps over the same range of barrel lengths, again matching what 56Hawk illustrated with his graphs.