Federal law does not require a waiting period, only a background check. Last time I bought a firearm from a dealer, the background check took about 5 minutes. By law, up to 3 days are allowed for the background check, but it usually takes only minutes.

There had been a Federal waiting period, but it was ruled unconstitutional in 1997. Some states have waiting periods. See https://en.wikipedia.org/wiki/National_Instant_Criminal_Back...