What is +P and +P+ Ammunition?

In the world of firearms, some of you readers may have heard about the terms +P and +P+ (pronounced as "Plus-P" and "Plus-P-Plus"). So what is this all about anyway?

In the US, several firearms and ammunition manufacturers jointly formed a standards body called SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) at the behest of the US government in 1926. This organization was charged with setting standards for the firearms industry and contributes its specifications and documentation to the American National Standards Institute (ANSI). All major manufacturers of firearms and ammunition in America are members of the SAAMI association, as are many of the smaller manufacturers. While the SAAMI association requires its members to follow the standards and guidelines that it sets, the US government does NOT enforce the SAAMI standards by law (unlike the European CIP standards, which are a compulsory standard enforced by law in several European countries). It is also not required for a manufacturer to be a member of the SAAMI association either (and some smaller ammunition manufacturers are not and therefore not required to follow SAAMI standards). These two facts are important as we will soon see.

One of the SAAMI association's jobs is to define the maximum pressure allowed to be generated by various cartridge calibers. Some of those standards were defined a long time ago (e.g. 9 mm. Luger, .45 Colt, .38 Special etc.) and it became clear that many modern firearms could easily withstand higher pressures than what the SAAMI standards said, because of improvements in metallurgy techniques. Some law enforcement departments began to clamor for more powerful cartridges to increase the effectiveness of their handguns and some manufacturers began to manufacture +P cartridges, with the +P to indicate that the cartridge was loaded to generate higher pressures than the SAAMI standard. The +P sign would be stamped on the cartridge box and on the base of the cartridge as well.

A 9mm. Luger +P cartridge on the left and a regular 9 mm. Luger cartridge on the right. Note the letters "+P" stamped on the left.

Soon, SAAMI got into the act and established an official +P standard for a few common cartridge types. Typically +P ammunition generates about 10% more pressure than the non +P type, though this is not true for all cartridge types. For instance, plain 9 mm. Luger ammunition is supposed to generate a maximum pressure of 35000 PSI (Pounds per Square Inch), but 9 mm. +P Luger ammunition can generate a maximum pressure of 38500 PSI per the SAAMI standards. The idea behind the +P standards is that older firearms can use the regular ammunition and newer firearms which are built to higher standards may be able to use both regular and +P ammunition safely. While there are SAAMI standards in place for +P ammunition for some calibers (SAAMI only specified them for 5 cartridges types), there are manufacturers that make "+P ammunition" for other calibers as well. Since the SAAMI standards are not enforced by law, manufacturers can mark any ammunition as +P even if SAAMI doesn't have a standard for it. This is even more so for manufacturers who are not members of the SAAMI association.

Soon, some ammunition manufacturers (such as Federal, Winchester and Remington) started making even more powerful ammunition than +P and called these as "+P+". These were meant to be sold to law enforcement officials only, but this is not always enforced. Some think that this was merely a marketing tactic to make it appear as though non-law-enforcement buyers were getting something really special and forbidden.

+P+ ammunition standards are not defined by SAAMI, but general industry agreement is that it should be about 10-15% more powerful than +P ammunition. For example, 9 mm. +P+ Luger ammunition generates a maximum pressure of about 42,000 PSI.

Using overpressured ammunition increases the wear and tear of firearms. While more modern firearms can handle higher pressures, older firearms cannot handle +P and +P+ ammunition that well and may have safety issues. That is why it is advisable to check the user manual to see if +P and +P+ ammunition can be used with a particular handgun model or not.

The Effect of Wind on Ammunition Performance

In our last post, we discussed the effects of air density on ammunition performance. In this post, we will study the effects of wind on performance.

In practical outdoor shooting, especially at longer ranges, there is a good chance that wind may be present and this could affect the ballistics of the bullet. Therefore, it is a good idea to study exactly how it affects bullet trajectory.

The effect of wind on the bullet trajectory depends on the direction of the wind and the speed that it is blowing. It is easy to compensate for wind if it is blowing at a steady rate in a fixed direction. It is much harder to compensate for wind if it is sporadic and blows in gusts of differing velocities. At longer ranges (say 1000 meters), the wind may also blow in different directions along the path to the target.

There are four types of winds we need to concern ourselves with:

1. Headwind - This is wind that is blowing from the target to us.
2. Tailwind - This is wind that is blowing from behind us towards the target.
3. Crosswind - This wind blows from right to left or left to right across the line between the firearm and the target.
4. Vertical wind - This is usually encountered in mountainous areas and is caused by the wind bouncing off the side of a hill or mountain. This wind moves in a vertical direction (blowing up or down) and pushes the bullet higher or lower on the way to the target.

The first three types of wind are considered to be horizontal winds and the last one is a vertical wind. The headwind and tailwind are easy to account for. Recall that in our previous article, we discussed a term called "drag force", which serves to slow down a bullet horizontally, as it travels through the air. As was discussed in the previous article, drag force is proportional on the velocity of a bullet through the air. Therefore, if a bullet is fired through a headwind, it is moving faster relative to the air than if it were to be fired with no wind. Therefore, this causes an increase in drag forces and the bullet travels slower. Similarly, if a bullet is fired through a tailwind, it is moving slower relative to the air, than if it were to be fired with no wind. Therefore, this causes a decrease in the drag forces and the bullet travels faster. This means that when firing into a headwind, it is necessary to slightly raise the angle of the barrel and when firing into a tailwind, it is necessary to slightly lower the angle of the barrel to compensate.

The famous English author W.W. Greener cites the following formula to calculate how much to compensate for headwinds or tailwinds:

R = V * D/4

where

R = range distance to compensate by (in yards)

V = velocity of the wind (in miles per hour)

D = distance to the target (in hundreds of yards)

Say we are shooting at a target 300 yards away and there is a tailwind of 10 miles/hour blowing behind us. Then we calculate:

R = 10 * 3/4 = 7.5 yards

This means that if the target is 300 yards away, we should really treat it as though it is 300 - 7.5 = 292.5 yards away and adjust our sights accordingly. If this was a headwind instead of a tailwind, we would treat the sights as though the target was 300 + 7.5 = 307.5 yards away. If you remember our discussion on Minutes of Arc (MOA) from a few months ago, the range adjustment translates to a 1/4 MOA adjustment for this distance (usually one or two clicks on the scope, depending on model). While this seems like a small adjustment amount, the compensation amount increases with distance to target. If we were shooting at 900 yards instead of 300 yards, then the same 10 miles/hour wind will require 1.75 MOAs correction.

A crosswind causes a bullet to move in a large deflection horizontally and also causes a small vertical deflection. A vertical wind causes a large deflection in the vertical direction and a small horizontal deflection. We can easily understand the large deflections, but why are the small deflections caused? The reason for these small deflections is because a spinning bullet tends to turn to face the wind, due to spin stabilization and the resulting torque causes a small deflection.

In reality, we often do not get a true headwind or tailwind, but rather a wind that blows at an angle to the shooter. Therefore, we must break this wind up into (1) headwind or tailwind component (2) crosswind component and (3) vertical wind component. In this case, we must compute the effects of each of these components separately and then add the results in horizontal and vertical directions to figure out how much to compensate for.

The Effect of Air Density on Ammunition Performance

A couple of months ago, we'd discussed how temperature affects ammunition performance. In this post, we will study the effect of air density on ammunition.

When a cartridge is fired, the bullet travels through the air and is slowed down by air resistance. This force acting on the bullet is called aerodynamic drag. The equation to compute drag force is:

where:
F_D is the drag force
ρ is the density of the air
v is the velocity of the bullet relative to the air
C_d is the the drag coefficient (a dimensionless constant for an object of a given shape)
A is the cross sectional area of the bullet.

As we can see from the above equation, the drag force depends on cross-sectional area of the bullet, the velocity of the bullet and the air density. Of these, we are mainly concerned with the air density in this article. If the air density decreases, the drag forces decrease and therefore the bullet can move faster through the air. If the air density increases, the drag forces on the bullet increase and the bullet slows down quicker. This makes sense as there is less air to slow down the bullet. Air density decreases as we go higher in altitude, this means that bullets travel further when we are at higher altitudes.

At this point, it may be a good idea to introduce a term: ballistic coefficient. Briefly, the ballistic coefficient of a bullet is its ability to resist the aerodynamic drag. The ballistic coefficient of bullets are measured under standard conditions and available from most manufacturers. If the ballistic coefficient at standard conditions is known and the velocity of the bullet and current air density are known, it is possible to predict bullet performance under any conditions.

There are two different standards that ammunition manufacturers use to measure ballistic coefficient. Some use the Standard Metro conditions where the temperature is 59 degrees Fahrenheit, 78% humidity, 29.5275" (750 mm.) of mercury pressure and altitude of sea level. This standard is used by some manufacturers such as Sierra and Hornady. Some other manufacturers, such as Speer and Nosler, use the International Civil Aviation Organization standard (ICAO), which defines the standard conditions to be 59 degrees Fahrenheit, 0% humidity, 29.921" (760 mm.) of mercury pressure and altitude of sea level. The difference between the values of ballistic coefficients obtained by these two standards is less than 2% though.

Air density depends on three factors: air pressure, air temperature and relative humidity. Of these three, relative humidity has the least effect on ballistic coefficient -- the difference between the ballistic coefficient values of a bullet at 1% relative humidity and 100% relative humidity is about 1%. Therefore the effect of relative humidity can be ignored for most practical purposes. The other two factors (i.e.) air pressure and air temperature have much more to do with air density. A decrease in air pressure results in a decrease in air density. On the other hand, a decrease in air temperature results in an increase in air density. However, as we go up in altitude, the decrease in air density due to air pressure is far more than the increase in air density due to the temperature drop. As a result of this, the density of air decreases as we go to higher altitudes.

On the other hand, if we were to stay at the same altitude, but the air temperature decreases (say between summer and winter), then the effect of temperature decrease is an increase in air density and therefore an increase in the drag forces. This is why bullets will slow down faster in winter than in summer and the bullet trajectory changes.

Some bullet manufacturers have handloading manuals that list formulae to recalculate the ballistic coefficient for different air densities. With the aid of a barometer, the formula and a ballistic coefficient under standard conditions, it is possible to calculate the performance at current conditions.

Always Make Sure of Your Ammunition Type

In the world of firearms, ammunition comes in several calibers (e.g. .22, .357, 7.62 mm, 9 mm, .45 etc.). However, when purchasing ammunition, one must be careful to specify the exact type of ammunition. We will see the reason why in this post.

Back when we studied rimfire cartridges, we noted that there are several cartridges in .22 caliber, such as .22 Short, .22 Long and .22 Long Rifle (i.e. .22LR). Besides these three, there are other cartridges too, such as .22 Remington Jet, .22 Reed Express, .22 CB etc. Most of these, except the last one have .223 inch diameter bullets, but the length of the cartridges and the bullets differ. Therefore, if a firearm takes (say) .22 Long cartridges, the user will never be able to fit a .22 LR cartridge into the chamber.

The same thing is true with other calibers as well. For example, we have .380 vs. .38 S&W vs. .38 Special vs. .38 Short Colt vs. .38 Long Colt. In mathematics, we are taught that 0.380 = 0.38, but when it comes to cartridge sizes, they are two completely different things, as the image below shows:

.38 Special (left) vs. .380 (right)

Not only are the lengths and cartridge profiles dramatically different, the two bullets are also slightly different diameters as well: .38 special has a .357 inch diameter bullet, whereas .380 has a .355 inch diameter bullet.

Similarly, when referring to .45 caliber ammunition, it should be specified if the user wants .45 ACP, .45 GAP, .45 Webley etc. As before, all of these have dramatically different cartridge shapes and bullet weights.

.45 GAP (left) vs. .45 ACP (right)

As before, the reader may observe the difference in the sizes and shapes of the cartridges. Incidentally, ACP stands for Automatic Colt Pistol and GAP stands for Glock Automatic Pistol, the names of the manufacturers whose products these cartridges were originally designed for.

Finally, we have many cartridges in 7.62 mm: 7.62x25 mm. Tokarev, 7.62x51 mm. NATO, 7.62x39 mm. Soviet, 7.62x54 mmR etc. The two most famous ones are the 7.62 NATO (i.e. 7.62x51 mm. used by FN-FAL, M14, Heckler & Koch G3 etc.) and the 7.62 Soviet (i.e. 7.62x39 mm. used by the AK-47, AKM and Type 56 rifles).

NATO 7.62x51 mm. (top) vs. Soviet 7.62x39 mm. (bottom)

As the reader may note, it is pretty easy to tell that the two cartridges are drastically different.

The same thing applies to many other calibers as well. So, the buyer must note the exact cartridge type when purchasing new ammunition. As amazing as it may seem, quite a few people are not aware of the differences between the cartridges or that the other cartridges exist. There have been several instances where a buyer has walked into a local firearm store and asked for .38 cartridges and ended up walking out with either .38 S&W or .38 Special, when they really wanted .380 cartridges, or asked for .22 Long when they really wanted .22 Long Rifle etc. It is a source of frustration to both the buyer as well as the owner of the firearms shop.

Therefore, it is very important to note down the exact type of cartridge that a firearm accepts.

The Effect of Temperature on Ammunition Performance

Today's topic of study is going to be an interesting one: the temperature of the ammunition at can affect how it performs. We will study the correlation between these two factors in this post.

It was known for a while that cold ammunition does not perform as well as ammunition at a warmer temperature. The higher the temperature of the ammunition, the higher the velocity of the bullet. Of course, the increase in velocity is also accompanied by an increase in the chamber pressure. This is caused because the powder in the ammunition burns at a faster rate when it is warm and at a slower rate when it is cold.

This phenomenon affects how ammunition should be stored. For instance, in a National match in 1930, it was found that ammunition that was exposed to direct sun rays for several hours before the match, was generating excessive pressure, causing some firearms to malfunction. The problem got so bad that the organizers had to stop the match and replace the ammunition with others stored at cooler temperatures.

However, ammunition stored at colder temperatures will ignite slower and not generate as much pressure and velocity, which could cause the bullet to reduce its range. Therefore, when people publish data about velocities and pressures generated by different ammunition types, this data is always measured at a certain standard temperature (in the United States, this is generally 59 degrees fahrenheit)

Modern ammunition is not as susceptible to the effect of temperature swings, but it still happens. So what is the amount of loss or gain due to variations in temperature, the reader asks?

Several different authorities attempted to answer this question in the 1930s. According to studies conducted by the Frankford Arsenal (located in Northeast Philadelphia, Pennsylvania) and Burnside Laboratory (located in Carney's Point, New Jersey), the ammunition they tested increased in velocity on average by about 1.7 feet per second for every one degree fahrenheit rise in temperature for ammunition loaded with IMR type smokeless powders. What this meant is that ammunition heated from (say) 70 degrees fahrenheit to (say) 130 degrees fahrenheit would experience an increase in velocity of about 102 feet/sec.

Correlation between ammunition temperatures and velocities for three different types of ammunition. Click on image to enlarge

This test was conducted by the US Ordinance Department and the image is now in the public domain.

Of course, the composition of the ammunition plays a large part in this and different ammunition types react to temperature in different ways. The US Ordinance Department also conducted its own tests in the 1930s and the results are shown in the graph above. As you can see from the above graph, Dupont's 1489 powder generates about 51000 PSI (pounds per square-inch) of pressure at 70 degrees fahrenheit, but generates about 56500 PSI of pressure at 140 degrees fahrenheit. This is an increase of about 5500 PSI (or 11% increase) and some firearms may not be able to handle the excess pressure generated.

In case you're wondering why 70 degrees fahrenheit appears in the graphs and notes above, it is because that was the "standard temperature" that measurements were taken against in the 1930s. These days, the standard temperature in the US is considered to be 59 degrees fahrenheit

The following table is based on U.S. Army tests on .308 Winchester ammunition, which is the commercial variant of the 7.62x51 mm NATO standard ammunition.

Degrees Fahrenheit	Muzzle Velocity	Bullet Drop at 600 Yards (200-Yard Zero)
-10	2400 feet/sec	-109 inches
+25	2500 feet/sec	-100 inches
+59	2600 feet/sec	-91 inches
+100	2700 feet/sec	-84 inches

Bear in mind that air is a good insulator, whereas brass is a good conductor of heat. Therefore, ammunition that is exposed to direct sunlight may reach higher temperatures than the surrounding air and 130-140 degrees fahrenheit is not an unusual temperature for ammunition to reach. The excessive pressures may cause some firearms to malfunction and can also affect the accuracy of the weapon.

Consider that a hunting rifle has been zeroed in at a temperature of about 80 degrees fahrenheit (which is the average temperature on a typical day where the author lives in Southern California). Now let's say the person goes hunting with that rifle in Alaska, where the average temperature in early morning is typically about 0 degrees fahrenheit. That is a 80 degree variation in temperature, which can cause the velocity of the bullets to drop by a significant amount. This means the sights will need to be readjusted because of the drop in velocity and chamber pressures.

In the late 1800s, the British author and firearms manufacturer, W.W. Greener, came up with a pretty convenient approximate formula to account for changes in temperature. His formula is based on a standard temperature of 60 degrees fahrenheit (rather than the modern ballistic standard of 59 degrees F) and is as follows:

Range adjustment for temperature = (degrees +/- from 60 degrees fahrenheit) * (target distance in hundreds of yards) / 10

So if the temperature is above 60 degrees, we should subtract this distance from the actual range and if the temperature is below 60 degrees, we should add this distance to the actual range. For example, say the temperature is 100 degrees fahrenheit and the actual distance to the target is 800 yards. Therefore, we calculate:
Range adjustment for temperature = 40 * 8 / 10
which works out to 32 yards. What this means is that though the actual distance to the target is 800 yards, we should treat it as being at (800 - 32) = 768 yards and adjust the sights accordingly.