A 'Firearms Identification Field Guide' by the Michigan State Police
I was in a thread as relevant to carry of arms at a day care when I thought out loud that the very first thing anyone should ever do when they have question about legality is to check the state laws as applicable to the subject matter.
In doing so myself to feed my own curiosity I as a result came across this field guide.
Add this to your own reservoir of information either as studied or simply keep it as a bookmark for future reference.
Michigan State Police
Criminal Justice Information Center
Firearm Records Unit
Firearms Identification Field Guide