There’s no such thing as a free lunch — or free velocity. It has to come from somewhere.
“Velocity is a function of pressure,” said Jayden Quinlan, Hornady® senior ballistician. “The reason you get the velocity that you get is because you generated enough pressure.”
Pressure and volume are inexorably linked, so if you understand the relationship of volume, pressure and velocity, you’re a long way toward understanding internal ballistics.
Seth Swerczek, marketing communications manager, defined internal ballistics as, “What happens from the moment you pull the trigger until that bullet uncorks from the barrel.”
It starts with the firing pin impacting the primer itself. The firing pin pushes itself into the cup, which crushes the anvil and ignites the priming compound, causing it to ignite.
Of course, not all primers are alike. Obtaining the desired velocity depends on using the right primer with the right powder for the cartridge you’re shooting.
Small pistol, small rifle, large rifle, large pistol and magnum primers are designed with a purpose to reliably ignite the powder in the cartridge. As the spark travels from the primer pocket through the flash hole to the body of the case, it begins generating pressure.
“In some circumstances, the bullet will start to move in the cartridge case before the powder is burning,” Quinlan said. “That movement increases the volume of space behind the bullet.”
Think about it like this… you have an air compressor, and as you let the air out, you’re taking it from that fixed volume and letting the volume out into the atmosphere in a much bigger way. What happens to the pressure? Pressure drops. Pressure and volume are tied together.
Different propellants have different shapes and chemical makeups that affect how they burn.
Various Powder Types
“Stick, ball and flake are what we’re probably most familiar with,” Quinlan said. “The different shapes or geometries of propellant influence how it will burn.”
Single-base powders are made of nitrocellulose. Double-base powders would be when nitrocellulose and nitroglycerin is added to up the energy content of the powder. Essentially, it can give more velocity for the same level of pressure.
As the propellant burns, it changes from a solid state into a gas, building pressure in the system.
Once enough pressure is generated to overcome the forces of the case holding the bullet, the bullet starts to move. With the increased volume, pressure should drop, but powders are manufactured to remedy that.
As the pressure goes up, the burn rate goes up. As the pressure increases, the case seals against the chamber wall. That keeps all of the gas going down the bore and none of it escapes.
The pressure created by the burning powder propels the bullet into the freebore – the unrifled portion of the barrel. If it’s held straight through the freebore, it will enter the rifling straight. A tight freebore (not a ton of room to wiggle), definitely helps in terms of sensitivity for accuracy.
Next, the bullet enters the lead, an angled step into the rifling.
As the bullet is entering, if it’s tipped or crooked when it tries to get engraved into the rifling through the lead, it’s going to stay crooked. As the bullet is engraving into the rifling, a large amount of resistance is present. You will reach maximum pressure within those first few inches of the rifling.
As the bullet travels down the bore, volume increases. The more the volume increases, the faster the pressure generation needs to be in order to continue to accelerate the bullet. Eventually, the powder cannot generate enough pressure to match the increasing volume and pressure will begin to drop.
The amount of pressure remaining in the bore when the bullet exits has a large effect on accuracy and dispersion. Shortening the barrel can make that even worse. As you cut the barrel back, the bullet is going to uncork from the muzzle with more and more pressure available behind it.
The pressure that’s present at the moment the bullet exits the muzzle can push the bullet and negatively influence accuracy if it is not uniform. Shorter barrels and larger charge weights of powder typically produce larger group sizes.
While there is no velocity without pressure, too much pressure can wear out components, damage your firearm and even lead to injury.
Looking at the primer or checking for ejector swipes can offer some visual clues. But the main thing is the velocity that it produced, because that will tell you if you’re in trouble or not. If you have a velocity measurement tool, use that.
Go to SAAMI.org and look at the bullet weight and velocity recommendations. If you’re getting velocity results that are well outside SAAMI numbers, you’re likely experiencing high pressure.
To learn more about velocity, pressure and powders, tune into the Hornady Podcast, episodes 28, 57 and 76.
EP. 28 Internal Ballistics
Ep. 57 One Hole Groups? | Dispersion
EP 76 Let’s Talk Powder