Fine I will do your research for you. Here you go:
There is also a restriction to only .177 caliber in Michigan. Anything larger is consider a firearm.
Here is the relevant wording in law:
28.421 definitions.
“Firearm” means a weapon from which a dangerous projectile may be propelled by an explosive, or by
gas or
air. Firearm
does not include a smooth bore rifle or handgun designed and manufactured exclusively for propelling by a spring, or by gas or
air, BB’s not exceeding .177 caliber.
“Pistol” means a loaded or unloaded firearm that is 30 inches or less in length, or a loaded or unloaded firearm that by its
construction and appearance conceals it as a firearm.
Here is a link for you, page 14 gives the definition.
http://www.legislature.mi.gov/documents/Publications/Firearms.pdf
So additional unfound laws notwithstanding, it appears air rifles exceeding 30 inches which are in .177 caliber (or less) and fire
BBs are legal without restriction. Air rifles exceeding .177 caliber or firing anything other than BBs are firearms, and there appears to be no exception to "firearms" shooting
pellets. That means
pellet guns are firearms and subject to the same laws as rifles and shotguns.
You are limited to .177 caliber SMOOTH BORE. The fact that some will still sell you .177 rifled pellet rifles could change in the future.
The only reason they likely still send rifled air rifles is there is no federal regulations on airguns, and your state law does not seem to require long arms to go through an FFL, and is legal if they are over 18. However they are firearms per state law, no different than a rifle or shotgun. If they will sell you .177 air rifles they should sell you any caliber air rifles, but they won't and most retailers limit you to .177 pellet rifles only likely out of misinterpretation believing they are not firearms.
3.111 Rifles and shotguns; purchases by
residents. Residents of this state may purchase
rifles and shotguns in any state contiguous
thereto if they conform to the federal gun control
act of 1968 and the regulations issued thereunder,
as administered by the secretary of the
treasury and the laws of the state where the
purchase is made.
Your handgun purchasing requirements are more restrictive, and so most won't even bother with the hassle of selling you a rifled airgun under 30 inches as that requires you to go through the pistol permit process to purchase a handgun.
The law specificly says smooth bore and .177 caliber or less and firing bbs otherwise it is considered a firearm.
A quick search online and you will find dozens of airgun retailers will only send you either .177 air rifles, and some smoothbore air pistols. They will not ship you rifled pistols or larger calibers.
Though technicaly they should probably also sell you other caliber air rifles if .177 rifled pellet rifles are already considered firearms and they are willing to sell you those
They are likely misinterpreting the law and don't realize rifled pellet rifles even .177 ones are considered firearms just like the pistols. Most web retailers appear to have just copied the interpreation of another, so a mistake by the original might have led them all to make that mistake.
That or I missed a relevant statute.
The way I see it any pellet rifle or pistol is considered a firearm with a rifled barrel (or even using pellets as the law only makes exception for BBs and smoothbores.) Your long arm purchase laws are very lax (and only even mention FFLs as having guidelines) but your handgun purchase laws are restrictive. So some are willing to sell long arms legal per state law if you are over 18,
Selling handguns to you however would be a violation of state law without the permit process.
I however would not sell Michigan residents any long arms (pellet rifles)either based on this part of the law, and I imagine many airgun retailers will wise up as well:
(2) A person who knowingly sells a firearm
more than 30 inches in length to a person under
18 years of age is guilty of a misdemeanor,
punishable by imprisonment for not more than
90 days, or a fine of not more than $500.00, or
both. A second or subsequent violation of this
subsection is a felony punishable by imprisonment
for not more than 4 years, or a fine of not
more than $2,000.00, or both. It is an affirmative
defense to a prosecution under this subsection
that the person who sold the firearm asked to
see and was shown a driver's license or
identification card issued by a state that
identified the purchaser as being 18 years of
age or older
That means a person who does so twice is guilty of a felony, and shipping airguns to people when you cannot see thier license does not entitle you to the "affirmative defense", and per your state law is no different than selling a minor a shotgun or rifle.
You are welcome
I am not a lawyer and put that together quickly so verify your own laws, most relevant are probably located at the link.