US SB3873

Requires background checks for all ammunition purchases by extending existing firearm background check requirements under Section 922 of Title 18 to cover ammunition sales

Prohibits unlicensed individuals from transferring ammunition to other unlicensed persons unless a licensed dealer first takes possession and conducts the background check

Exempts transfers between family members (spouses, domestic partners, parents/children, siblings, grandparents/grandchildren, aunts/uncles/nieces/nephews), law enforcement, estate transfers, and temporary transfers to prevent imminent harm

Allows temporary ammunition transfers without background checks at shooting ranges, for hunting/fishing, and while in the transferor's presence, provided the transferor has no reason to believe the recipient is prohibited

Takes effect 180 days after enactment and explicitly does not authorize creation of a national firearms or ammunition registry