Yes a 18 year old can purchase a long gun with a valid identification and pass a background check
Oregon law prohibits anyone under age 18 from possessing a gun unless possession would otherwise be lawful and (1) the gun is not a handgun and was transferred with the minor’s parent’s consent, or (2) the minor possesses the gun temporarily for a lawful purpose. ORS 166.250. Oregon law prohibits the sale or transfer of a firearm to anyone under age 18, with the same exceptions. ORS 166.470. Federal law prohibits federally licensed firearms dealers from selling a handgun to anyone under age 21. 18 USC subsection 922(b)(1).
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