[SauronOfMordor]

5. The choice of a .223 round is unusual. A person with a high knowledge of firearms would realize that they are seriously undergunned with a .223. Most people will survive hits from this type of weapon. A .30 caliber cartridge would be a surer bet for killing people.

A .30 cal cartridge would generate much more muzzle blast, which would increase the difficulty of evading detection. If the objective is to create terror, he doesn't need a 100% guaranteed kill. And he's killing the majority of who he hits

[relee]

The choice of a .223 round is unusual

It may just be that you go with what you know. If he is used to shooting a .223, that's what he will use.