Most of America is definitively outside city limits.
Again... It depends on where you live.
And the vast majority of Americans live inside the city limits, or inside a subdivision, either way it is most often illegal to discharge a weapon. If you have close neighbors, this likely fits you.
If you live on a farm, or on a fairly large parcel of land, outside the city limits, it is probably legal for you to discharge a weapon on your property. If that fits you, you probably don't have close neighbors, either.
And most of America is not your property.
In any case, firing a warning shot is still almost certainly at least a misdemeanor, and most likely a felony.