This bill, titled the "ATF Transparency Act," aims to enhance the fairness and efficiency of firearm transfer and registration processes by amending the Internal Revenue Code of 1986. It introduces a new administrative relief process for individuals whose applications for firearm transfer and registration are denied by the Secretary, typically due to prohibited person status under federal or state law. This process requires the Secretary to provide the relevant NICS transaction number, allow for appeals similar to existing regulations, and permit transferees to submit information to prevent future erroneous denials or delays, akin to the Voluntary Appeal File program. Furthermore, the bill stipulates that if an appeal is successful, the Secretary must reimburse the transferee for any reasonable and necessary attorney fees incurred. A significant provision of the bill establishes a 3-business-day deemed approval for firearm transfer and making applications. If the Secretary fails to approve or deny an application within this timeframe, it is automatically considered approved, allowing the transfer or making of the firearm to proceed, and denials cannot be based solely on processing delays. Furthermore, a safe harbor provision protects individuals: if a deemed-approved transfer is later found to be for a prohibited person, the transferor is not liable for certain violations, and the transferee is exempt from liability if the firearm is returned within 14 days of notice. Finally, the legislation mandates several reports and agreements to improve the National Instant Criminal Background Check System (NICS). The Comptroller General, in conjunction with the Inspector General of the Department of Justice, must report on unresolved NICS inquiries and recommend actions to minimize them. Additionally, the Directors of the ATF and FBI are required to enter into a memorandum of understanding regarding NICS inquiry administration and processing.