One question that seems to pop up with every pistol video is, "Is it +P rated?". I'm surprised to see this so often, especially with mid-to-long barreled guns. It's occurred to me that there might be some misinformation out there about "+P" ammo, so let's try to set the record straight.
What Does "+P" Mean? The short answer is "+P" means over pressure. "+P+" is even worse. For simplicity's sake we'll use 9mm as our example. Ammunition can be dangerous, especially if there aren't standards for dimensions and pressures for the firearm or the ammunition. For this reason in 1926 SAAMI was established. The Small Arms and Ammunition Manufacturers' Institute was founded in the US to set standards for important things like chamber designs and ammunition specifications. A similar organization was founded in Europe in 1914 and is known as "CIP", and English-language summary of the name, "Commission internationale permanente pour l'épreuve des armes à feu portatives" or, "Permanent International Commission for the Proof of Small Arms". If you have a European-made firearm (14 countries currently comply) or European ammunition chances are you've seen a small "CIP" stamp somewhere on it.
To keep guns from blowing up standards were set. For 9mm ammunition the images below show what SAAMI has agreed 9mm should be. These images were taken directly from SAAMI Z299.3 - 2015 "Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Pistol and Revolver Ammunition for the Use of Commercial Manufacturers".
As you can see, the nominal velocity from a test barrel remains about the same. A SAAMI 9mm test barrel is 4". There is little added benefit to +P ammunition in a barrel of 4" in length.
Specifications have also been set for the case and overall loaded round specifications. This is how you can be sure that ammunition labeled "9mm" with a SAAMI specification will fit in your 9mm firearm.
For 9mm SAAMI dictates a maximum pressure of 35,000psi and so that's what most barrels are made for. Interestingly enough this is slightly higher than the European CIP standard of 34,084psi. When 9mm is loaded to +P it has a maximum pressure of 38,500psi.
What Is It Good For? The idea behind +P is to gain extra velocity. Most 9mm defensive rounds were developed with a 4" barrel in mind. When the barrel gets shorter the round leaves the barrel at a velocity below what the hollow point was developed for. As a result it doesn't hit the target hard enough to begin proper expansion and can fail to operate as a hollow point at all. We've experienced this even with some 10mm loads that failed to open in gel because they weren't moving fast enough. When concealed carry started taking off in popularity smaller and smaller handguns began hitting the market. Hollow point ammunition intended for a 4"+ barrel was failing to perform as intended. The quickest solution was to make up some of that lost speed by loading the bullet hotter with more pressure. Think of it like a squirt of nitrous in a small-engine car to momentarily gain bigger-engine performance (rough analogy).
Does It Work? Yes and no. Unfortunately ammunition manufacturers have no way of knowing what barrel length the ammo they make is going to end up in. The only thing they do know is that it will be in a SAMMI-spec chamber and barrel. While they can select the powder and burn rate, they don't know what you're going to do with the ammunition. As a result some barrel lengths see vast increases in velocities while others don't. What every barrel does see however is pressure that is potentially stressing the limits of what the gun can hold. The increased pressure also results in harder and faster cycling, potentially causing premature wear on the firearm. It can also cause failures to feed if the magazine can't keep up. Most shooters will also notice more recoil as the slide slams harder against the frame.
Can It Be Bad? Most certainly yes. Increased wear and tear on the firearm and increased recoil aren't the only issues. If the chamber and barrel aren't rated for it you run the risk of the chamber cracking, or erupting. If the gun is rated for it, but the projectile not designed to function at higher speeds (for example a defensive +P round fired through a normal-to-long barrel) the round can hit the target too fast to perform properly. Bullet jackets shed, over-expansion leads to disintegration of the projectile, and reduced penetration. There is also a chance of the round traveling too quickly to properly stabilize and hitting the target sideways or at an angle.
The Modern Answer Fortunately the ammunition industry has created solutions to the short-barrel problem. Modern ammunition makers have begun designing projectiles specifically designed to perform well at lower speeds. These loads are often labeled "compact", "carry gun", or "micro" such as this load from Remington, or this specialty version of the Speer Gold Dot. When such a round is not available "+P" can be the answer for your sub 4" gun, but there's no real guarantee. Unfortunately the only way to really know is to test the ammunition of your choice in the gun of your choice for the circumstances that you may need.