Since domestic experiences are being shared, I figured I'd relate my recent one. I took three separate flights:
BWI > DEN
DEN > AUS
AUS > BWI
I checked the SIRT without declaring it and thankfully had no problems. I placed the SIRT in its factory plastic case with two dummy mags. Just to play it safe, I also included the factory manual in the case, and a hand-written note:
"This is NOT a firearm. If there are any issues, please contact me at XXX-XXX-XXXX"
On the DEN > AUS leg of the trip, I did find a TSA notice of baggage inspection upon arrival, but I was never contacted or delayed in any way.
For whatever it's worth, I was not traveling with any live firearms; just the SIRT.
Following up to this discussion, a related conversation with some folks relayed that one particular individual apparently forgot to fully clean out his training gear from his checked baggage when flying. Apparently, this person missed a SIRT training mag. Flew through two major airports without it ever being noticed.
Not as awkward as finding an MP5 mag forgotten in checked baggage whilst in transit through a Middle Eastern country, but not terribly reassuring either. This individual was likely just lucky not to have had worse challenges... certainly not something he would ever repeat deliberately.
My only guess is that the training weights inside the mag body are so different from the size / shape of cartridges that they don't look anything alike on the screener's terminal. Again, not something one would want to test, however.
In 2003 I forgot a loaded .40 cal mag from my P7M10 in my backpack that I carried on. I went through security in AUS, ORD, and LGA - and then home again. Only then did I notice it.
Makes you feel safe, eh?
I flew from Dulles International to Manchester NH with a complete SIG slide in my backpack carry-on.
But the security guy in Manchester sure noticed it, and that made for an interesting afternoon...