Elidaniel, yes the Diamondback - DB15 is rated for 5.56, so you can shoot either 5.56 or .223. Have a great day!

Does this rifle have a heavy barrel ?

Yes this barrel/ chamber ate rated for 556 pressures so can safely fire both calibers

Elidaniel, yes Sir. The Barrel is Stamped 5.56NATO so it will shoot both 5.56 and .223 no issue. If the Barrel was Stamped .223Remington then, firing 5.56 would not be advised. Cheers and stay safe!

Yes, this has a 5.56 nato Barrel so it's safe to shoot 223 threw it. You just don't want to shoot 5.56 threw a 223 Barrel. So you're good to go and hope this helps.