What is + P Ammunition?

P Ammunition

+ P Ammunition is called “overpressure ammunition” because the “+P” stands for more pressure in the chamber.

So, a gun works by setting off an explosion behind the bullet. (The arrow.) The shell is just a little bit bigger than the barrel (technically, the bore, but whatever), so it seals off the barrel. The explosion builds up pressure behind the bullet, forcing it out of the barrel and toward the target.

Now, different cartridges and calibres are made for varying levels of pressure. The designer devised a way to send a specific projectile downrange at a certain speed, which creates a certain amount of pressure inside the chamber.

For example, it’s about 34,000 psi for a 9mm, about 21,000 psi for a.45 ACP, and about 60,000 psi for a.30-06, which is called “God’s own rifle cartridge.”

Now, the SAAMI and the CIP, which set standards for measurements and other things about ammunition, write down the pressure levels as one of the things that makes a cartridge a cartridge.

More powder adds pressure to the chamber and makes the bullet move faster.

+ P Ammunition

Proof that loads far surpass +P chamber pressure

Each gun must be tested before it leaves the factory. This means that WAY too much powder is put into the chamber to show that the barrel and chamber are good.

+ P Ammunition

Proof loads show that the barrel and chamber are more than strong enough to handle the stresses of standard pressure loadings.

Think about it this way: a car with 100 horsepower will have trouble going up hills and getting up to speed on the highway. Not in a car with 450 horsepower.

Proof testing ensures that a gun can handle more pressure in the chamber than it will usually.

Now, what are the pressures in the proof load chambers like?

For example, a 9mm proof load is usually around 48,000 psi (47,500 psi for NATO spec and around 50,000 psi for SAAMI spec), and barrels must be proof-tested before leaving the factory.

Since 9mm+P has around 38,000 psi of pressure.

We’ll talk about when and how overpressure ammunition can be dangerous or harmful for your gun and why manufacturers say you shouldn’t use it or should only use it sparingly.

But we’re also saying that +P ammo isn’t inherently dangerous as long as it’s loaded according to the rules for +P ammo and used safely. If you were packing your carry gun with +P before putting it in your concealed carry holster, that’s fine, with a few exceptions.

Where +P Ammo Can Cause Problems

So, the problem with +P ammunition isn’t that it’s inherently dangerous. It’s not true. Firearms tested at the factory can usually handle the extra chamber pressure.

There are two areas where problems could happen.

Where +P Ammo Can Cause Problems
9mm polymer tipped bullets (used for concealed carry guns) on wood background. Macro with shallow dof and copy space. Selective focus limited to first bullet.

First, some semi-automatics use +P.

So, a semi-automatic gun works: when the cartridge goes off, the slide moves back, squeezing the recoil spring. When the spring is completely compressed, the fall moves back.

For the gun to work right, the recoil spring needs to be set to a certain tension so that the slide doesn’t move too quickly or too slowly. This is the spring rate, similar to how a car’s shock absorber is set to take in shocks and vibrations from the road.

The slide moves backwards faster than the bullet leaves the barrel. The recoil must be more vital to compensate for how fast the slide moves.

Part of balancing this function is the ammunition, and most pistols are calibrated with ammo that has a standard pressure.

First, some semi-automatics use +P

Since adding more pressure speeds up the slide, this can damage the frame and fall at the end of its travel and lengthen the time when the drop is open, which could affect feeding and extraction.

Case Head Separation And The Glock Kaboom

Case head separation is the other problem that could happen. This doesn’t happen very often, but it has been known to happen.

There are several reasons for this, but most of the time, it is because of a flaw in the cartridge case and how the feed ramp and chamber are made in some guns.

So, a cartridge case is made of a material that can handle a certain amount of pressure. If the material is weak because of a flaw or because a handloader has used it before, the cartridge case may break before all of the gas made by the burning of the fuel is sent down the barrel.

Case Head Separation And The Glock Kaboom

Most of the time, the weak spot is near the case head, where the rim and primer pocket is.

Now, most pistols have something called a feed ramp. The ramp is used to move the bullet into the chamber.

Some guys don’t fully seat the cartridge in the chamber, so the case head is visible inside the room. This is important because a fully supported case has more material under the case head. This gives the director more support and keeps it from coming apart.

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What does it have to do with +P ammunition?

In a pistol that doesn’t have a fully supported chamber, cartridge cases with weak case webs can break, which could cause the case head to separate and the gun to explode.

This malfunction is called a “Glock Kaboom,” known to happen in Glock pistols.40 S&W, .45 ACP, and 10mm chambers.

Even so, how likely is it that that will happen? Not too much. Even in pistols without a fully-supported chamber, case head separation is rare. But it has been known to occur from time to time.


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