Selecting the appropriate ammunition for the police-patrol carbine is key to obtaining best accuracy and terminal performance. When choosing the bullet weight, much depends on matching it to the barrel twist rate (i.e., the rifling) of your weapon. As a general rule, faster twist rifling rates can stabilize heavy bullets, while slower twist rates cannot. If you go with a heavy bullet the barrel twist rate can t stabilize, you will encounter accuracy and performance problems.
The .223-caliber (or 5.56mm NATO designation) round is the predominant caliber in use today in law enforcement, used mainly in the AR-15/M-16 type weapon system. Barrel twist rates are noted as a number, such as 1/7-inch, 1/9-inch or 1/12-inch. These numbers represent one full rotation of the bullet in a barrel s rifling for every 7, 9 or 12 inches of barrel length. The 1/7-inch and 1/9-inch twist rates will stabilize heavier bullets used in .223-caliber or 5.56mm barrels, such as the 62 77 grain bullets currently in use. (This faster barrel twist rate also allows the use of the original M-193 55-grain full-metal jacket (FMJ) bullet.) So, if you use a recent-production .223 or 5.56mm rifle with a twist rate from 1/7 1/9, you can use a wide range of bullet weights. Some will prove more accurate than others, requiring testing of the individual weapon to determine the best load.
Some barrels are visibly stamped with the twist rate, but with others you must contact the manufacturer. Most current commercial barrels are manufactured with a 1/9-inch twist. Current military-spec barrels feature a 1/7-inch twist.
If you have a slower-twist barrel, such as the 1/12-inch twist found in the original AR-15 SP1/M-16 A1 barrel, I recommend using bullets weighing 55 grains or less. Bullets 60-grain and heavier often do not stabilize in flight when fired in a 1/12-inch twist barrel. This lack of stability produces a yawing effect. Such a bullet does not spin like a drill bit but wobbles or turns on its base in flight. The bullet may turn sideways, impacting the target in what is known as a keyhole (see photo). Lack of bullet stability greatly reduces accuracy and limits the bullet s ability to penetrate objects, particularly if the bullet impacts the target sideways.
What you choose for practice ammo must not only remain reasonably accurate, it should provide the same point of aim/point of impact as your duty ammo. If, for instance, you practice with 55-grain FMJ and deploy 69-grain boattail hollow-point on the street, you may find a significant difference in the point of aim and point of impact between the two.
In a series of tests, I used a very accurate, scoped, custom, match AR-15 to compare various bullet weights and types. The target results varied from a centered small ragged hole to a group measuring more than 4 inches at 80 yards. A variance of 4 inches at a possible street confrontation distance is not acceptable. It shows you must test your ammunition choices in the rifle you will use in the field.
Finally, you must know what you load in your magazines. It s easy to mix ammunition type, and physical appearance is not always an accurate indicator. In the photo, the round to the left of the 62-grain green tip is a 55-grain M-193 FMJ. It could easily be a 62-grain FMJ, however, and if you unknowingly fired it in a 1/12-inch twist barrel, you would face the described accuracy and penetration issues.
Know your ammo, and store it in properly marked containers or magazines. And, understand the reason for choosing bullet weight and bullet type your safety depends on it.