The common miss conception with the +P designation is the bullet goes faster. This is not always true, +P only means there is an increase in pressure over the SAAMI standard.
+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. 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.
John
