Issue number one is state compliance. There's quite a few states that don't actually submit their lists of prohibited persons up to the federal level. Think if you guys still allowed guns on a wider level. Someone gets a conviction bad enough to be restricted in Sydney. They move to Brisbane. Sydney didn't put their conviction into the system... now, even though THEY know they're prohibited, they go to try to buy a gun on a whim.... and get approved, because their record isn't in the system.
That happens. Federal restrictions on domestic violence can't be enforced because they don't get submitted. Oregon's legislature is chest thumping right now because they just, literally/specifically/retardedly, made a state version of the Lautenberg Amendment which makes anyone who has a domestic violence conviction OR a restraining order with DV as a reason for the order, unable to posses or purchase firearms.
The other aspect is that if you do a purchase specifically to hand the gun over to someone else out the gate, it's supposed to be illegal. Proper gifts don't apply, but if you're giving it to a restricted person that shouldn't have a gun and can't buy it themselves, then it's a straw purchase and a federal felony. Just like if you lie on the paperwork.
The feds have like 48 convictions for 48,000 current falsifications/straw purchases that were logged. How these people are still walking around is on the system, not the legal gun owner.
The system is actually rigorous and thorough in design, the problem is enforcement.