Maxwell Murder wrote:
//You know more about this than I do Eric, so why does a shorter barrel decrease muzzle velocity? I would have thought otherwise, and I'm interested to hear more about it.
//To a point you are right, there is a point where the barrel would be so long it causes unnecessary drag and slows the shot down (though gun manufacturers tend to know what's up and don't design barrels that long, you can catch of glimpse of this phenomenon in Paintball where people have 18+ in barrels and need to turn their input pressure way up to counteract the barrel drag). However, there does need to be enough length for the shot to accelerate. Remember a bullet works by the gases behind it pushing it out the barrel. Obviously to accelerate the bullet the gas needs to be behind it. As soon as the bullet leaves the barrel the gas dissipates into the air, obviously no longer accelerating the bullet. So the barrel needs to be long enough to keep the gas behind the bullet until the bullet reaches its maximum velocity but not so long that it slows the bullet down by creating drag. Sawing off the barrel would remove the length the bullet is accelerating in, thereby causing a slower muzzle velocity.
A helpful analogy might be to picture a car accelerating to its maximum speed. Let's say a certain car needs 100 feet of road to achieve its top speed (yes I am aware that is a really short distance, it's just an example). Let's say at the end of the 100 feet of road is dirt or something that no longer allows the car to accelerate. So after the car accelerates along the 100 ft of road, it reaches its maximum velocity just as the road ends, similar to how a bullet needs the barrel length to accelerate. Now cut the road to 50 ft. The car, having half as much road to accelerate on, would reach a much slower velocity before running out of surface to accelerate, just as the bullet leaves the shorter barrel slower.
I'm sure you didn't need the second explanation as everyone on this forum for the most part is quite intelligent but I felt proud of myself for thinking that up so I decided to include it
And sorry for the thread jack, if anyone has any other questions about firearms/ballistics I'd be more than happy to answer any questions through PMs.