The BATFE decides what constitutes a silencer or a machinegun. They have demonstrated their arbitrary and capricious definitions in the past.

Clinton told them a Garand was an assault weapon, whatever the hell that is, but also said that Norinco AKs were sporting rifiles, whatever the hell that is.

What can’t Trump tell them that a silencer is defined as an item that completely silences the report of a rifle?

Because checks and balances exist. The Congress makes the law and the judges choose who goes to jail, regardless of what the ATF decided is and isnt legal this week.

I see the picture your painting, but I think it would backfire. If Trump used an executive order or other such executive power to tell a bureau within the executive branch to do their job in a way we like, that would be cool...... until the other shoe drops. Might not be until the White House changes hands, might be as soon as anti Trump swamp dwellers see an opportunity, but one way or another the rule would change back plus interest. Guys who acted in good faith under the new, friendly rule would get pineapples shoved up their kiesters in no time.

Nope. It would be covered under no ex post facto.