There is no transfer tax from a private individual to a dealer.
The dealer then will transfer the MG and the suppressor to the new buyer with two tax stamps. The suppressor as one; the gun as the other. if the suppressor was welded to the barrel and the barrel not removable, as in my Savage with an Outback on it, there would be only one tax stamp needed.
BTW; If you sell a Class III weapon, the tax stamp has a collector's value of $50. I've sold a number of old ones to people.