FNG here to the forum, but I thought I could add a bit to the discussion.
Here in SC all FFL sales must have a completed 4473 which is sent electronically (or phoned in) to the FBI for an NCIS. However, if the buyer holds an SC CWP permit, the FFL MAY at his or her discretion, allow the buyer to take possession without waiting for the NCIS to clear. Without the CWP, the FFL has to wait for the NCIS to clear OR the mandatory 5-business day waiting period to expire before transferring custody of the firearm to the buyer. The FFL is required, of course, to retain the 4473 in his or her permanent business records to be examined by the ATF without notice at any time.
Private sales or transfers between individuals only requires that the seller check that the receiver of the firearm is an SC resident (basically check their driver's license to be sure it was issued in SC). There's no requirement to record or report the transaction to anyone or any authority.
I do work for an FFL (gun range that also has and FFL and retail firearms sales), and I can say from firsthand observation that the "instant" part of the NCIS is pretty much one of those "in name only" sorts of things. Sometimes the background check comes through in just a few minutes, but more often than not, it takes a few days. It is quite normal for the 5-business day waiting period to expire before the check comes back.
I'll avoid opining too much on the illogic and non-sensical "rationality" of extending the present waiting period or adding additional required (juvenile) criminal or mental health records checks without also appropriating more money for the FBI to increase its resources (agents and NCIS support). The NCIS system is already at the breaking point and on its best days just barely functions as is. Adding more time to do something that can't already be done by the current system or, even crazier, adding millions more checks for private sales to the system can only result in even more "instant" background checks that go uncompleted.