All Federal Agencies are under the Executive Branch of the Govt. which sets policies for Federal agencies which also implement Federal law.
That being said, those who are appointed to run the agencies do so within the authority of the Executive Branch and the policies of the POTUS.
So an EO can go into effect immediately, via the federal agencies, unless the Legislative Branch and/or Courts over-turn an EO based on the EO not being consistent with the law it's tied to.
Also, the policies of the Executive Branch runs down hill to the agencies so Agency policies can change immediately, unless the above-actions are taken if policies are found to be arbitrary and capricious.
So, immediate effects can be felt, unless someone, and/or the legislature, goes into action and a court ruling is made. Plus, legislation can be vetoed without an overide vote by Congress.
Then there's the gray-zone when it comes to the NFA. Case-in-point the bump stock crossed the line into the NFA world by altering the firing rate of a firearm beyond it's original design intent. Plus, a bump stock is an accessory and not a firearm. The Obama Admin didn't do anything about bump stocks since they likely flew under the radar of the admin. Once Las Vegas happened that changed. No court has enjoined a bump-stock ban since no firearm is being banned.