The issue is access to your checked luggage. Normally clearing customs means you have to claim your bag and present it to customs. Since at that point you have access to everything in your checked bag you must be re screened at any airport not just Atlanta. It is legal and permissible for your checked bag to contain weapons including firearms.
If you pre clear customs and immigration like STT or DUB you also pre clear your checked luggage and will not have access to it at the arrival US airport if connecting.
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