The ATF agent needs to chill out, because it is a bad idea to take any silencer down range. Don't forget, machineguns are registered also, are ranges going to let you take your AC556 down range so that an ATF agent won't get angry if it gets stolen while you are putting up targets? How about a Bren machinegun - are you going to lug that with you, drop it in the sand while you are taping up new targets?
What angers me more than anything is misplaced animosity toward honest people who have done nothing wrong. The guy who stole the silencer is the problem, not the guy who left his silencer where it belonged while putting up targets at a range.
Common sense says - watch your stuff while you are down range.