They may be legal already, if one applies the logic that machine guns are legal if licensed by the Feds:... or am I reading this incorrectly:
MCL 750.224 Weapons; manufacture, sale, or possession as felony; violation as felony; penalty; exceptions; "muffler" or "silencer" defined.
Sec. 224.
(1) A person shall not manufacture, sell, offer for sale, or possess any of the following:
(a) A machine gun or firearm that shoots or is designed to shoot automatically more than 1 shot without manual reloading, by a single function of the trigger.
(b)
A muffler or silencer.
(c) A bomb or bombshell.
(d) A blackjack, slungshot, billy, metallic knuckles, sand club, sand bag, or bludgeon.
(e) A device, weapon, cartridge, container, or contrivance designed to render a person temporarily or permanently disabled by the ejection, release, or emission of a gas or other substance.
(2) A person who violates subsection (1) is guilty of a felony, punishable by imprisonment for not more than 5 years, or a fine of not more than $2,500.00, or both.
(3) Subsection (1) does not apply to any of the following:
(a) A self-defense spray or foam device as defined in section 224d.
(b) A person manufacturing firearms, explosives, or munitions of war by virtue of a contract with a department of the government of the United States.
(c) A person licensed by the secretary of the treasury of the United States or the secretary's delegate to manufacture, sell, or possess a machine gun, or a device, weapon, cartridge, container, or contrivance described in subsection (1).
(4) As used in this chapter, "muffler" or "silencer" means 1 or more of the following:
(a) A device for muffling, silencing, or deadening the report of a firearm.
(b) A combination of parts, designed or redesigned, and intended for use in assembling or fabricating a muffler or silencer.
(c) A part, designed or redesigned, and intended only for use in assembling or fabricating a muffler or silencer.