Zlatmash KSO 9 Krechet 9mm Carbine

@Alex Nicolin Not according to BBTI tests: http://www.ballisticsbythei...

@Alex Nicolin Yes, I see there are indeed increases, but very small ones. What is surprising is that the velocity seems to be going up and down slightly, as the barrel gets longer. I have no idea why.

@Alex Nicolin -The gas pressure behind the bullet being lower than the pressure in front of the bullet is what propels the bullet.
-The gas pressure behind the bullet increases as the powder burns, but there's a limited amount of powder.
-The gas pressure behind the bullet decreases as the volume inside the barrel and behind the bullet increases due to the bullet having moved forward
-Friction between the rifling and bullet decelerates the bullet.

So at a certain point, the volume inside the barrel behind the bullet has increased enough so that the pressure pushing the bullet forward is weaker than the friction slowing the bullet down.
Pistol rounds are loaded with faster burning powder than rifle rounds, since pistol rounds are designed for use in shorter barrels than rifles. So that's why this inefficiency happens when you give a pistol round a rifle length barrel. It could also happen with a rifle round, but only with an absurdly long barrel.

Going even further, the pressure behind the bullet can get even lower than the atmosphere, so pressure forces would also start to slow the bullet. If I remember correctly, this is why an M79 makes its distinctive sound. After the grenade gets out of the way, rather than a muzzle blast, the reverse happens; air gets sucked into the barrel.

A squib load takes this to the extreme; friction can completely stop a bullet, leaving it lodged in the barrel, if the round was made with extremely low propellant, such as a primer only. This can cause disaster when you fire another round. If you fire another regular round, the gun can blow up and injure the shooter. If you fire a blank, you can fire a bullet when you didn't expect to, which is the error that killed Bruce Lee's son.

@loopydupe Yes, this makes sense, and is what should be expected - the velocity increase tappers of as the gas pressure behind the bullet decreases. But that's not what is shown in the experimental data on Ballistics by the Inch. Instead, quite surprisingly, the velocity increases steadily to a point, then decreases and increases again a few times, before it stabilizes. From a point on accelerates, that decelerates, then accelerates again in a few cycles. I have a hypotheses about this: the powder does not burn steadily in the tube, but in fact a large portion of it gets compacted and stuck to the back of the bullet, and flares in fits and starts. In fact, if high-speed frames of handguns being fired show a large cloud of unburnt, and unignited powder particles being blown from the muzzle, together with the bullet. http://kuulapaa.com/home/hi...

The only way they could have failed to ignite is to have stuck together tightly so only the surface layer of this mass of powder burned. The "plug" of powder is more pronounced in pistol cartridges, because they don't have shoulders to keep the powder burning in the tube. Rifle cartridges with steeper and wider shoulders (those who are short and fat) keep a larger percentage of the powder burning in the tube, since the plug diameter is minimized. There were some guys at a site called superior ballistics that thoroughly studied the phenomenon of "plugging" powder, but unfortunately, the site is dead.