Your standard AR shoots .223/5.56 which is a 22 caliber bullet. Same size in diameter as a 22lr bullet that's typically used for small varmint like squirrels, raccoons,etc... It has more powder in it's cartridge allowing to travel faster and farther hence the kinetic energy and impact. However for larger animals like deer, the bullet is too small to do more than wound unless you have precise shot placement. That's why it's not allowed in most states for deer hunting. The round isn't strong enough to penetrate and take a deer down without multiple shots and ends up making the animal suffer as a result. Most hunters will use calibers like .308 or 30_06. Both are larger 30 caliber sized bullets that also travel much faster and further than a standard AR round.
Most people associate ARs with the term "assault weapon" based off aesthetics which have no bearing whatsoever on the ability of the firearm to fire. Good example is the type of gun that was just used in Tennessee which also happened to also have been used in the Chicago parade shooting. The Keltec Sub 2000 is a 9mm pistol caliber carbine. The only thing it has in common with an AR is that it has an M-Lok rail on the handguard (the area surrounding the barrel that has slots in it) which is used for mounting accessories like lights or grips for comfort. There are other rifles that use similar types of fittings that are not AR platform rifles but upon initial looks would immediately get called an AR style weapon.
Unfortunately a lot of information that is provided during the initial time frame around a shooting, is inaccurate and only piggybacks off the high emotions ppl have in reaction to a shooting. By the time, more information is received about the shooting and/or weapon type, ppl are already in their feelings and in some cases they don't even hear the updated information. And unfortunately these shootings also get politicized by both sides of the political line.