This article is specific for NICS(Leep) and does not cover any state systems.
The excerpt below was copied and pasted from the fbi.gov website on 7/1/21. Please visit their website directly for the most up to date information.
I Was Denied a Firearm—What Happens Now?
If you believe you have been wrongfully denied a firearm, you can request a firearm-related challenge (appeal). You may only challenge a denied firearm transaction, not a delayed one.
This process will provide you:
the reason your background check was denied, and
the opportunity to challenge your denied background check online – including a Nuclear Regulatory Commission background check.
The FBI does not accept challenges or provide reasons for delays or denials on state-issued firearms permits. Those questions should be directed to the state agency. The NICS Participation Map explains whether your state, or the FBI, conducts firearms buyers’ background checks.
How to File a Firearm-Related Challenge
To challenge your firearm denial, visit https://www.edo.cjis.gov.
To submit a challenge through the mail, send a request to:
FBI CJIS Division
Attn: Criminal History Analysis Team 1
1000 Custer Hollow Road
Clarksburg, WV 26306
When submitting a challenge via mail, include your NICS Transaction Number (NTN) or State Transaction Number (STN). Failure to include these numbers will delay processing.
Help with Your Firearm-Related Challenge
If you need help or have questions on firearm-related challenges, call (304) 625-5590 or email email@example.com. (This contact information is for help with firearm-related challenges only, not the Voluntary Appeal File.)