The majority of law enforcement precision tactical marksmen use .308 caliber rifles, but at the ranges typical of LE SWAT engagements, a .308 is almost overkill considering that the marksman is typically striving for a head shot at a distance that generally doesn’t exceed 100 yards. In fact, FBI statistics show that the average SWAT marksman engagement takes place at 51 yards. Thus, a .223 caliber rifle should be more than adequate given the average engagement ranges.
The argument can be made that the .223 isn’t as effective at longer ranges, but with the right ammunition, a rifleman with a .223 can accurately engage targets at surprisingly long distances using ammo like the military’s Mark 262. The latest version of this very accurate ammunition is the Mark 262 Mod1, the difference being that the latest version has a cannelure, while the original does not.
The Mk 262 is essentially Black Hills’ 77-grain match cartridge, so precision marksmen should have little trouble engaging targets at normal LE distances using it and the ammo has proven to be extremely effective in combat in Iraq and Afghanistan.