I bought a T/C Hawken through midwayusa 3 years ago and it was mailed to Upper Michigan, perfectly legal.
2 years ago, I bought a Ruger Old Army and it needed a 10 day buyers permit in order to purchase it and mail it to Michigan.
Muzzleloading long guns (rifles or shotguns) are can be bought and sold in Michigan without a FFL requirement, but even muzzleloading handguns or blackpowder revolvers are treated as modern handguns/pistol or revolvers.
Please check with the laws yourself through the proper authorities.
yooper77