So, thanks to Mr. Smith, I looked for awhile on the ATF website itself and found the 5320.20 form and the following:
"A person, other than an FFL/SOT, may not lawfully transport in interstate or foreign commerce any destructive device, machinegun, short-barreled shotgun, or short-barreled rifle, without prior written approval of ATF, specifically the NFA Branch.214 For definitions of these firearms and
devices, refer to 27 CFR 478.11."
Referring to 27 CFR 478.11, the definitions of these devices appear to leave silencers out...so I think Zak is right, but I'm a little nervous relying on my probably poor interpretation of BATFE regulations.
It'd be so much nice if they put in bold red print "SUPPRESSORS/SILENCERS DO NOT REQUIRE PRIOR WRITTEN APPROVAL FOR TRANSPORT" !!!
Maybe in the next re-write to try to make things more understandable for dummies.