Overpressure Ammunition - +P Ammunition and Velocity

+P Ammunition and Velocity

In general, the purpose of a +P cartridge is to get a higher velocity for a given bullet weight than a standard pressure cartridge. However, the pressure rating used to determine if a round is +P is the peak pressure, which is not an accurate indication of the velocity, since it is the area under the pressure curve that determines the total energy imparted to the bullet (see internal ballistics). A large number of factors can impact the peak pressure of a load, such as:

• Bullet weight
• Bullet seating depth, which is a factor of
  • Bullet material
  • Bullet shape
• Bullet diameter
• Test barrel diameter
• Test barrel chamber shape
• Bullet hardness
• Friction in bore
• Crimp strength
• Smokeless powder burn rate
• Primer strength
• Cartridge case volume

Because of these factors, it is possible to have two loads where each is propelling the same bullet weight at the same velocity, but one is a standard pressure load and one is a +P load. Even in the same firearm, with the same components, cartridges with low powder capacity and high operating pressures, such as the .40 S&W, have been shown to have a significant increase in pressure with very minor differences in bullet seating depth. One example in .40 S&W demonstrated a 20% pressure increase with a 0.05 inch (1.2 mm) change in seating depth.