Tmorone everything you said is correct, there is no dealer to dealer tax, this is not the situation of the original poster, he is not a dealer, he bought the silencer and machinegun on a tax paid form 4, he is not a dealer, he can sell to a dealer in his state or in any state, or an individual in his own state on tax paid form 4. then it can go through as many dealers as needed on tax free form 3 but if his final customer is not a dealer it goes from the final dealer to the individual on tax paid form 4 again.
Your silencers are no different that machineguns except machineguns due to the ban have a value in the used market because they appreciate fast enough to cover the transfer tax times 1 in state or times 2 out of state. Where since your silencers can be bought new with one tax, there is no used market on silencers. Sometimes in state they will sell with one tax, but unless some special antique historical silencer no one is going to pay double NFA tax on a used silencer when they can order brand new from a dealer or manuf. on one tax.