Interesting. So that would indicate a thrill-killer too nervous to reload? That would point away from terrorists.
It may be that the weapon being used is a Thompson Contender: a large pistol with interchangable barrels. More easily concealed than a rifle, it can chamber and fire the .223 cartridge (or virtually any other). But it holds only one cartridge at a time. A 150 yard shot from a rest is within the capability of six to a dozen people at my club.
You can bet the police have looked into this possibility.
D.P.Roberts
Yes. Rifles and pistols have been made in various .22 centerfire calibers, in bolt-action, break-action, and semi-automatic configurations.
Manufacturers are too numerous to mention. The cartridge is not necessarily .223 Remington, our standard military infantry round... Bullets meeting the given description could be fired from a .22-250, .222 Remington, .222 Remington Magnum, .223 Remington, .224 Remington, .224 Weatherby, .225 Winchester, .220 Swift, even ancient cartridges like the .22 Savage High-Power... or any of a dozen different "wildcat" (custom-made) cartridges such as .22-6mm.
While the rate of rifling twist on the recovered bullets could tend to rule out some of the abovementioned calibers, it's truly anyone's guess as to what sort of firearm is involved here.