The AR-15/M-16 fired the .223/5.56 mm round. In this caliber selection there exists a wide variety of bullet choices to meet tactical needs. Entire volumes have been written on bullet selection, but boiling it down to the bottom line, if only one type is to be authorized I strongly recommend a barrier penetrator type. Corbon DPX, Black Hills TSX and other brands loaded with the excellent Barnes X bullets rank at the top of the list. Federal Tactical bonded .223, Remington Bonded PSP and Hornady TAP penetrators are also available. Make sure the bullet is a "bonded" or solid projectile, as labels can be confusing. Barrier penetrator ammunition is standard street issue to agents of the FBI and DEA. Both of these respected agencies have done extensive testing to prove the need. For police patrol/SWAT functions where offenders are inside vehicles and behind cover, barrier penetrators will be most effective where fast fragmenting polymer tip, soft point and full metal jacket bullets will fail to make it through to deliver a fight stopping wound.
Barrels and bullets
Coupled with ammunition selection, a technical barrel issue must be reviewed. As stated, the AR-15/M-16 system is chambered for the most common patrol rifle caliber, the .223 Remington/5.56 x .45 mm NATO round. Rifles can be chambered for either .223 or 5.56. The commercial .223 can be fired with no issue in a military 5.56 chamber, but not the reverse. Be certain that your patrol rifle barrel has a true 5.56 mm chamber to prevent function problems that often occur when 5.56 mm ammo is fired in a .223 chamber.
In addition to projectile construction and type, bullet weight must also be matched to the barrel twist rate. Twist rate is the distance in inches the bullet must travel to complete one full turn. The rule is that heavier bullets of the same caliber must spin faster to stabilize in flight.
The early M-16 A1 has a "slower" twist rate of 12 inches for a 55 grain bullet. Therefore the bullet made a full 360-degree rotation in 12 inches of barrel to stabilize the bullet in flight. Any bullet weight in excess of 60 grains fired in a 1/12-inch twist barrel typically will not stabilize and will begin to yaw or wobble in flight. Upon hitting the target, the bullet turns sideways on impact, creating what is called a "keyhole" (as seen in Photo 5 on Page 42).
Other than the surplus M-16s, almost all AR type barrels today are manufactured with either a 1/7 or 1/9-inch twist barrel rate. While they generally will stabilize bullet weights of 50 to 77 grains, go with a 1/7 if you intend to use the heaviest bullets. Be sure when ordering your patrol rifle that you know what twist rate the manufacturer uses. Understand that the slower twist rate LESO 10-33 M-16 A1s limit bullet weight to under 60 grains, and restrict ammo choices to the lighter weight bullets.
Exotic additions go mainstream
To create the functional patrol rifle system requires a few additions to the basic rifle. What used to be exotic accessories are now mainstream items. The most basic additions include a tactical sling that allows the patrol rifle to be carried close to the body and ready for immediate use. The sling should be capable of retaining the rifle close and tight to the officer should he or she need to transition to empty hands, electric stun device or handgun. The best sling designs offer fast adjustment and allow firing from either shoulder. The Spec.-Ops Patrol and Viking VTAC tactical slings are examples I use daily.
Rifle mounted lights are another component that come in many forms and mounting solutions for the patrol rifle. Surefire and Streamlight are among the companies offering a number of possibilities. Having used these for years in all weather and conditions, I know they work on demand. The cost may be higher initially, but they hold up and are backed by solid warranties. Features to look for in any light and mount include: lightweight, powerful, long battery life, shockproof and easily accessed, positive on-off switching.