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
So the reason is that the bullet doesn’t speed up instantly. As it is pushed out the barrel it speeds up. So the longer barrel the more time for the energy of the powder going off to be transferred into the bullet in terms of speed.
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Intuitively, I’d believe it’s similar to using different lengths of rubber band. There’s more thrust time or push time on the back of a longer/more stretched band than a smaller band.
In a long barrel, the force continues to push outward excelling the speed of the bullet faster and faster. In a short barrel with the same force, that force loses its thrusting power as soon as it exits the barrel.
The gases from the propellant charge have more time to act on the projectile.
The longer the projectile stays in the barrel, the lower the pressure behind the projectile (because the gases expand) and the higher the pressure in front of the projectile (because the air that was in the barrel gets compressed). At some point, the gain isn’t worth the extra mass of the barrel.
The Bullet is pushed through the barrel by the expanding gas from the gunpowder exploding. The more time the gasses pushes on the bullet the faster it goes. As long there is a higher pressure behind the bullet than in front of it, it will accelerate.
The longer the barrel the longer the explosive gasses get to push on the projectile. The longer the shell is pushed the more energy it has, the faster it will be traveling.
As long as the barrel isn’t so long that the gasses are able to fully expend, then the projectile is going to be getting more energy.
This is why, when the projectile comes out of the end of the barrel, you see the gasses follow it out.
On a pistol, if you have a short 4 inch barrel, the gasses only get to accelerate the bullet for 4 inches. If you have an 8 inch barrel, the gasses get to accelerates the bullet for twice the distance.
Imagine blowing a spitball with a straw. Now imagine blowing a spitball with a straw half the size. Now, again imagine blowing a spitball with half the size of that.
Notice it starting to slow down yet? Why?
You will intuitively know that a stub of a straw will be difficult to hit your older brother in the head from across the room.
Your lungs want to accelerate the spitball. As long as you have forceful air in your lungs to push the spitball along its journey from start to end of the straw – the faster and bigger the splat!
Be careful though – your brother with his bigger stronger lungs can have a longer straw that shoots even faster.
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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