What is +P or Overpressure Ammunition? – Foundry Outdoors
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What is +P or Overpressure Ammunition?

Have you ever wondered what the +P and +P+ designations mean on some ammunition and firearms? It turns out that +P, or "overpressure", is a specific designation with a precise definition that refers to the pressure inside of a round of ammunition. Read on to learn more about what +P means, and when you can use it in a given firearm.

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+P Ammunition

When buying ammo you will find a variety of manufacturers for your caliber of gun, and you will also find that some boxes are marked with a +P.

 

As mentioned, the +P represents an overpressure load, meaning more grains of powder are added to the cartridge resulting in greater velocity and internal round pressure. Intuitively, if there is more material inside a load and the same amount of air, that means the air must be under higher pressure in order to occupy the smaller available space inside the load.

The Sporting Arms and Ammunition Manufacturing Institute (SAAMI) sets the guidelines for standard pressure in pistols and rifles being produced and sold in the United States. These specifications exist for each caliber of cartridge, and they guide the manufacturing of guns as it provides the manufacturers with precise guidelines on how to build a chamber and barrel that can withstand a specific load discharge time after time.

Why Use +P Ammo?

In reality +P ammunition rarely "needs" to be used. The reasons some people prefer +P is for the additional kinetic engergy imparted into the bullet from the additional powder and internal pressure, which increases the bullet's overall knockdown power. It is a fact that a +P load will offer more velocity and a more energetic impact, which can be appealing to law enforcement agencies and home defenders. Generally speaking, a +P load possesses significantly (10-100%) more pressure than a standard cartridge’s pressure. For example a standard 9mm round will have a rated pressure of around 35,000 psi, while +P 9mm round will have 38,500 psi. 

 

Safety Precautions & When to Use +P

Adding more pressure to an already powerful cartridge can present safety concerns. Perhaps the biggest concern involves how well your gun is made, and whether it can withstand +P ammo. Some guns are manufactured well enough to withstand +P loads while others are not. With that said, always check your firearm's documentation or contact the manufacturer to see if your weapon is rated for +P ammujnition.

Furthermore, +P ammunition will wear down your gun faster then a standard load. Using +P ammo is not recommended for target practicing, and really should only be used when it is absolutely necessary. Over-use of +P ammo or usage of +P ammo on a firearm not rated for it can result in damage to the weapon, or worse yet an explosion in your hand and personal injury (or worse)! Always use the utmost caution with overpressure ammo and be knowledgeable of your gun’s +P rating or lack thereof.

Lastly, there is also a +P+ load that is under even greater pressure then +P. This load can be very dangerous as the pressure increases drastically. Going back to a 9mm +P round at 38,500 psi, a +P+ might have a rated pressure of around 46,000 psi.

 

The jump in pressure to +P+ is dramatic and there are no well-defined guidelines, meaning you are playing with fire and have to place your trust in your gun maker and ammunition manufacturer - something I don’t personally think I’m willing to bet my hand on!

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