Real MP5 mags may be prohibited. IRC mags for pistols that exceeding 10rnd capacity are prohibited so it'd stand to reason that 30rnd mags for prohib machine guns would also be prohibited.
I'm guessing that the disassembly would be to determine if your mags had an actual floor plate under the plastic top and a spring to drive it.
There are a bunch of messy things that happens at borders that are somehow ok. I hear that if your laptop fails to power up when airport security asks to inspect it, they brutally take it apart to see if it's concealing a bomb.
Want nearly free GBB gas?