Why Muzzle Velocity Matters

What is muzzle velocity?
The simplest answer is that muzzle velocity is the speed (velocity) that a bullet is traveling when it leaves the muzzle of a gun. The speed of a bullet travels has direct implications on its impact with a target as well as its trajectory toward the target.This might be a pretty short article. Then again, it might not, because just knowing what muzzle velocity is doesn't tell us all the details about why it's important.
It turns out that muzzle velocity, or MV, is really important when you're shooting rifles or pistols. Because bullets are essentially unpowered flying projectiles, their velocity is highest when they leave the muzzle, and drops off as they travel due to air resistance and gravity. Muzzle velocity matters. It directly affects accuracy in rifles and pistols, and also changes terminal performance.
Muzzle Velocity for Pistol Shooters
In pistols, muzzle velocity doesn't really change terminal performance, but it does in rifles. However, it can negatively change a pistol's terminal ballistics if you drive a very light projectile too fast. Lightweight pistol bullets going super fast, like north of 1500 feet per second tend to not hold together on impact, so they fragment at a shallow depth and don't penetrate deep enough to damage major structures. This is why ammo like the Liberty Civil Defense 9mm, which has a 50 grain projectile around 2000 FPS, is a joke (in our humble opinion.)Importance for Rifle Shooters

Muzzle Velocity's Impact on Accuracy

One other interesting phenomenon that affects rifles is bullets that pass from supersonic speeds into subsonic speeds while in flight. Crossing the transonic barrier into subsonic speeds happens on particularly long shots, and depending on the bullet's design can change accuracy.
While muzzle velocity is primarily a function of how much propellant is in the cartridge, it's also influenced by external factors, notably the weather. Humidity, barometric pressure, and air temperature will all change muzzle velocity. If you take a 9mm and shoot it on a cold, dry day, the rounds will be going slower than they would on a hot, humid day. This is because air temperature has an inverse effect on air density - the hotter it is, the less dense the air is. The less dense the air is, the less resistance for the bullet to encounter. Pretty simple science, but also something that seems counter-intuitive.
Muzzle velocity is important. It's an easily measured, easily understood metric of cartridge performance that helps us create accurate rounds. Knowing that muzzle velocity is so critical to terminal performance, especially in rifles, we're going to do a two part article. We've covered muzzle velocity, next installment we'll cover muzzle energy and get deep into the nerd weeds with some science!