This person is either misinformed or is kidding you. There is no registration of long guns in Michigan. The lack of a statute requiring such registration is the only "proof" of this you will be able to find. Of course, you can consult an attorney to research this for you if you intend to use this information as "legal advice."
On the other hand, unless you have a CPL (or its out-of-state equivalent), any person is required to obtain a license to purchase a pistol before actually purchasing, carrying, or transporting it in Michigan. Violating this statute results in a misdemeanor.