For all assault rifles and any scary looking semi-automatic rifle you do.
In the state of mn you will needca permit to purchase an AR platform rifle.
Oddly specific asking about assault rifles (not just rifles) and oddly broad asking about semi automatics. Semi automatics are any firearm which automatically chambers the next round to be fired with the next trigger press. This includes many handguns, rifles, and shotguns. You should apply for a permit to purchase. Info here: https://dps.mn.gov/divisions/bca/bca-divisions/administrative/Pages/firearms-permit-to-purchase-transfer.aspx
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