I can't speak to machine guns, but suppressors going from dealer to dealer DO NOT incur a $200 tax.
Dealer to dealer happens on a form 3, tax free form. Dealer to individual or trust is a form 4, which a stamp is stuck to.
You're putting info out that's flat out wrong David. I've purchased 4 suppressors this year and all from out-of-state. Dealer to dealer, then dealer to me were only $200 per can total...not $400 like you're saying it should be.