Reader Question: Why do longer barrels make the bullets go faster?
A reader, who asked not to be named, asked the question “Why do longer barrels make the bullets go faster?”
That’s actually a bit of a loaded question (get it? Loaded? like a rifle? oh nevermind). Maybe it’s not so much loaded but it is more complicated than you would think. I’ll give you the short answer real quick then get on to the sciency stuff. The short answer won’t be the whole answer though.
The TL;DR* answer is because generally the bullet has more time to accelerate, but that’s not the whole story.
From this point on, we’re going to be talking science. There will be discussions on physics. I must warn you, there might even be an equation or two. Oh, how I love equations!
As we’ve discussed in other articles and guides, a bullet doesn’t do anything until there is an explosion behind it. Being that the bullet is in a confined area, there is only one way for it to go: out the barrel (and hopefully into the bullseye at the target). Now, if you take the force of that explosion and divide it by the weight of the bullet, you get the acceleration of it (a = F/m).
So what does that mean? As long as force is being applied to that bullet, it’s going to accelerate. That is, what we call in the science world, a “no duh”. So with a short barrel, there is a very small amount of time that the expanding gasses have to push that bullet out. So, on my Glock 19, the gasses only have about 4 inches worth of barrel to push that bullet. Put that same 9mm round in a Kel-Tec Sub-2000 with a 16 inch barrel, that 9mm is going to be going a lot faster…up to a point.
You see, barrel length is kind of a “diminishing returns” type of thing. In a perfect world where equations are simple and all encompassing, you could put a 300 foot barrel on to a .338 Lapua Magnum rifle and use the moon to sight your new optics. After all, the moon deserves it. Look at it up there…all smug. “Look at me! I’m the moon! I change your tides and stare at you all night.” Someone needs to teach that rock a lesson.
Sorry, got away from myself for a moment. Where was I? Oh yeah, barrels and such. There are quite a few factors that prevent you from being able to put any length barrel on to a gun. One of these is the butterfly effect. This is an element of Chaos Theory (remember Jurassic Park?) that basically states “often times small factors can create huge changes down the line”. In this case, the rifling, minute imperfections on the inner surface of the gun, friction, bullet composition and more affect how each and every bullet travels inside that barrel. The longer it spends in that barrel, the more time those factors have to work.
The other main factor, and the biggest, is the gunpowder. There is a finite amount of powder in the cartridge. That powder can only create so much gas. That gas can only expand so much. After a certain length, that bullet is going to start decelerating in the barrel. Make the barrel long enough, and the bullet will never be able to exit.
So with every rifle manufacturer, when a new bullet is created or a new rifle is designed, there are groups of people with impressive sounding initials after their name wearing lab coats and holding clipboards that spend hours calculating the ideal barrel length so that the bullet will come out at the highest velocity and keep its accuracy.
So there’s the full answer: A bullet has more time to get up to speed in a longer barrel but after a certain point, which depends on the bullet size and cartridge load, the barrel needs to end lest ye slow down the bullet.
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*TL;DR = “Too long; Didn’t read.” which is the current internet vernacular for “sum it up for me”.
Images used under creative commons license from Wikipedia and Idovermani.
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