In the firearms world, AR stands for ArmaLite Rifle. Unfortunately, many people think it stands for Assault Rifle. This false label has given ARs a bad reputation.
The ArmaLite Rifle was designed to be much lighter, shoot a higher velocity bullet, and hold more rounds than other guns in their class. (M1 Grand, M1/M2 Carbines, M1918 Browning Automatic Rifle, M3 “Grease Gun”, and Thompson Submachine Gun.) There are two main types of ArmaLite Rifles, the AR-15 and the AR-10.
An AR-10/M14, is designed to shoot .308 Winchester, 7.62×51 NATO, .243 Winchester, 6.5 Creedmoor, .338 Federal, 7mm-08 Remington, .300 SAUM, 6mm Creedmoor, .300 Winchester Short Mag, .416 Win Short Mag, and 260 Remington. Most AR-10 platforms work best shooting .308 Winchester with a barrel that has 1:11-inch twist. An AR-10 has more velocity that allows the bullet to travel long distances (greater than 200 yards). So, if you are looking to use your AR to hunt medium game, an AR-10 is your best option.
An AR-15/M4, is designed to shoot .223 Remington and 5.56 NATO. However, the AR-15 can be easily modified to shoot .22 Long Rifle, .204 Ruger, .224 Valkyrie, .22 Nosler, .25-45 Sharps, 6.5 Grendel, 6.8 SPC, 300 AAC Blackout, .300 Ham’r, .30 Remington, .350 Legend, .458 SOCOM, .450 Bushmaster, 9mm, 40 Smith & Wesson, 45 ACP, and 7.62×39. The most versatile AR-15 is a 5.56 NATO because they can also shoot .223 Remington ammunition. The most common barrel twist for the 5.56 NATO AR is 1:8-inch twist. So, if you are looking to shoot small game or defend your home this is the AR you want.