Firearms transactions between residents of different states are governed by Federal Law. Title 18 Chapter 44 Section 922 also known as 18 USC 922 governs interstate firearms transactions, even between private individuals. First step is to download your own copy from here so that you can follow along:
http://www4.law.cornell.edu/uscode/1...2----000-.html
Here we go regarding private sales:
922(a)(3) makes it illegal for a private party to obtain any firearm from any out of state source with a couple of exceptions: firearms acquired by inheritance and long guns obtained from only FFLs and only under certain conditions.
922(a)(5) makes it illegal for a private party to transfer, sell or give away any firearm to another private party who does not reside in the same state, with two exceptions: again, firearms transferred by inheritance and the loan or rental of a firearm to a private party for temporary use for hunting or sporting.
922(b)(3) makes it illegal for a FFL to sell or deliver any firearm to any person outside of the state their business is located in except: they may sell or deliver long guns to out of state residents provided that the transfer occurs at the FFL's place of business, in person, and that the sale or transfer meets the requirements of both states' (the FFL and the recipient's) laws. Also excepted is the loan or rental of a firearm for temporary use for hunting or other sporting.
Now, state laws will define the scope of the exception to 922(b)(3) above. For instance, Texas state law says that a Texas resident may purchase long guns in contiguous states (as does Florida law as well). That does NOT mean a Texas resident may do a private transaction with an Oklahoma resident. What it does is define the limitation to the 922(b)(3) exception: an Oklahoma FFL can sell a long gun to a Texas resident, but a Montana FFL cannot, because of Texas law.
I do not know what restrictions are in place because of Michigan/Indiana laws. The easiest thing to do would be to call an FFL in the other state and ask them if they can sell long guns to residents of your state.
To sum it up:
A handgun obtained from an out of state source must be transferred to the recipient by an FFL in the recipient's state of residence.
A long gun obtained from an out of state source may always be transferred to the recipient by an FFL in the recipient's state of residence. In some cases, the long gun may be transferred to the recipient by an out of state FFL, at the location of the FFL's business, in person, if both state laws allow it.
Hanguns nor longguns may never legally transfer between two private parties who are residents of different states. The only exception is by inheritance when someone dies or temporary loans or rentals for SPORTING purposes. Not even gifts from parents to children are excepted.
Out of state handgun sales do NOT have to go FFL to FFL. The private party seller can ship the handgun to the recipient's FFL for transfer, if the recipient's FFL will receive the handgun from a private party. Most won't, but there is no law against it.
note what is in the red, this is federal law