In Minnesota if you want to purchase a handgun or semi-automatic long gun you must meet the following criteria:
You will appy for a permit to purchase from the POLICE DEPT in the city where you live. 
Be at least 21 years of age to buy a handgun
Be at least 18 years of age to purchase semi-automatic long guns
Not been charged with a crime of violence
Have not been convicted of a fifth degree assault
Not convicted of domestic violence per US Code and other items
Get the application here