How many miles of US borders were closely monitored? The truth is thousands of miles of border were NOT even tightly monitored. That is how illegal guns and drugs came in US, how would background check is going to help where the border isn't 100% secured?
When Uncle Sam beefed up the regulation, it gives black market business better opportunity and these business don't give damn who the buyers are. Until the US borders are 100% secured then that is whole different story.
Don't forget include how many tunnels between US and Mexico exists? Maybe between Canada and US? I believe hundreds or maybe thousands of them are not yet found but exists, perfect for gun smuggling which will ultimate killed the point of mandatory background check.
1,081 miles of openly opportunity for illegal contraband smuggling just between US and Mexico via land alone. This includes human trafficking and other criminal activities.
U.S. Government Does Not Have
Gotta to think what criminals thinks.