FWIW I have travelled to the US alot over the years and on most occasions have brought back to the UK new and fired brass inc 50bmg cases and miscellaneous reloading kit. (nothing with primers in,big no no)
On a couple of occasions my hold case has been opened and there has been a note from TSA to say that it was them that opened it for security reasons.
No issues at either end.
In fact last October I flew back from DC with brass,die sets and a couple of scopes and the hold case was scanned right by the check in desk in front of me with no problems.
A friend has a house in Florida and brings back lots of shooting gear and on one occasion bought,on a whim,a jar of airsoft plastic BB`s.
He had this in his carry on bag and it was confiscated as it was deemed to be live ammo!
I also use my old pistol range bag as my carry on bag and have never had a problem with it being "sniffed" and its had 20 plus years of carrying guns and ammo and fired brass.
I think its just pot luck in the end!
Tony