With a floating fireing pin... yes they are.
There is a reason 5.56 and most .223 are loaded with military spec primers. To avoid a slam fire.. so in a sense it is rare with factory ammo. But it has been reported that a loose round chambered can go off from the inertia of the bcg without a magazine slowing it down.
If you used standard federal small rifle primers, you're almost guaranteed to get a slam fire at some point. Even my military primers are dimpled just from letting the gun cycle.
If they do slam fire, it should happen on a closed bolt anyway.