grayfox,
There are several ways to get what you're looking for, and a heavy-barreled .223 with lighter bullets is one, though in my experience not as effective as dropping down in cartridge, or using a muzzle brake or suppressor.
I don't even the .223 much on PD's anymore, mostly shooting the .17 Hornet and .204 Ruger, because I can see the bullet hit through the scope. There is some controversy over whether the .204 makes much difference, and there have been long threads on the Campfire on the subject, but many people find it helps. Another good round is the .17 Fireball, which shoots much flatter than the .221 Fireball.
