This is one thkng that never made sense to me and seems (at least to me) counterintuitive even though the historical precedents speak for themselves (case in point, the long-barrelled Sherman Firefly variant which became a very effective weapon against WWII German armor that the ordinary Sherman wouldn’t normally be able to penetrate)
The way I see it, the projectile is in the barrel (and therefore engaging the rifling) for that much longer – in that time, more of the shell’s kinetic energy is lost as friction in the barrel than it would be normally, wouldn’t it?
Is the effect of friction negligible and outweighed by the increased rotation imparted by the long barrel? I just don’t understand how it makes as significant a difference as it does.
In: Engineering
It’s a complex interaction between a bunch of things. The fundamental fuel is the combustion of the gunpowder, which burns and produces gas at a particular rate. Exactly how much there is and how fast it burns varies. Meanwhile, there’s a certain base pressure needed to get the bullet moving. After that pressure is reached, any additional pressure means extra force to accelerate the bullet’s velocity faster. But as the bullet moves down the barrel, it increases the volume for the combustion gas, which tends to decrease the pressure. This needs to balance out with the combustion process increasing the gas volume, which tends to increase the pressure.
Ideally, your gunpowder would build up to the maximum pressure the gun can physically handle safely and burn at just the right rate to maintain that full pressure until the bullet leaves the barrel. It’s tough to do that exactly, but you can arrange things to get as close as you can. Burn too fast, and you build up too much pressure and break things and/or run out of powder before the bullet really gets going, so you don’t get much velocity. Too slow, and most of your burning may take place after the bullet already left the barrel and be wasted.
Gunpowder burn rates and cartridge volumes are designed around the expected peak pressure, bullet weight, and barrel length. Usually there’s some margin of error such that a longer barrel would still increase the velocity. If you stretch enough things, it is possible to get in situation where lengthening the barrel further would lead to negligible velocity increase or even small decrease, like putting a way excessive long barrel with a pistol round designed for short barrels, but that’s gonna be pretty rare in practice. Presumably anybody actually designing a modification to a firearm knows perfectly well what velocity change to expect from it, or may even specify additional gunpowder be used to take better advantage of a longer barrel.
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