Just got in on that 4-for-3 ammo deal?
The only one I've found is the Mossberg MVP series. 'Someone' said that the Savage use the '.223 Wylde' chamber, but under the FAQ on the Savage web site, they say that you should use ammo as per the data stamp, so no go on the 5.56.
Of course, people say they've done it, but I'm a little leery too. It'd be nice to hear from other CGNers who have tried this.