Let me educate you as to gun laws
for most of our nation's history, people buying guns were presumed to be lawful citizens-and no background check was required. In 1993, after years of Sarah Brady's yapping (about a guy who would have passed the check and still shot her husband) the Brady Bill was passed. Congress ONLY applied this law to licensed gun dealers for TWO REASONS
1) federally licensed firearms dealers hold a FEDERAL license that makes them subject to numerous record keeping requirements. Since they can receive firearms within interstate commerce, they are subject to federal regulations based on the FDR expansion of the commerce clause
2) congress specifically DECLINED to make private sellers-who by federal law-CANNOT SELL GUNS INTER-STATE exempt from this new law.
a) because private sellers do not have to keep records
b) because private sellers may well be held not to be within the purview of the commerce clause as expanded by FDR
so its NOT A LOOPHOLE. the standard model for most of our country's history is NO BACKGROUND CHECKS
so you are incorrect when you call it a loophole.
IT IS THE FFL that created federal jurisdiction to demand the background check