Some background. If you engage in the buying and selling of firearms for profit, you're a dealer. If you're a dealer, you must be federally licensed (FFL) and conduct background checks.
If you only sell guns occassionally, you're probably not a dealer. But per the ATF, dumb fucks have been convicted after selling as few as 2 firearms because they present themselves as dealers. Anyway, the legitimate, occasional, dude to dude sales are the only sales today that don't require background checks. As just dudes, they don't have access to the database to run a check. Per politifact, 13% of firearms are sold this way.
So there are 2 ways to get expanded background checks:
1 - give everyone access to NICS? Maybe every SSN is allowed to run a free criminal background check once a year? NICS was established in '98 when this shit was nearly all done over the phone so only giving FFLs access to NICS made sense. Given the internet, it seems like it could be expanded without requiring more man power.
2 - (this is what's commonly proposed) require every sale to go through a licensed dealer. This will cost $25-$35 and be added to the purchase price paid by the buyer. Seems reasonable.
Some will argue it's unenforceable without a registry. They're currently largely correct:
Today if an investigator finds a firearm used in a crime and they want to trace it, they contact the manufacturer who tells them which wholesaler or dealer (FFL) it went to. Then the investigator contacts the dealer to see who bought it. He finds the relevant 4473 form. Then they contact that dude to see what he knows... Texas does not require a person to report lost or stolen guns. Only ~11 states did as of 2017... So unless that changes, the dude could simply lie. That's a lot of effort for what may be a dead end.
But of course there's more!
When an FFL dies or goes out of business their pile of 4473s go to the ATF. The ATF has allegedly been digitizing these. Annually, FFLs are subject to inspection. The right wing claims the ATF is taking pictures of 4473s during these inspections to digitize recent purchases. That may all be total bullshit but the ATF is providing FFLs with an eForm 4473 so if they're not already digitized, they will be one day.
It's not exactly a huge leap to see how the ATF could end up with a database that's maybe 1 year behind new purchases.
And that's exactly why gun owners want to retain the private sale. It makes it more difficult to create an accurate database.
So let's say universal background checks pass along with a new AWB. It grandfathers in existing firearms, and the actual impact of these new laws is small... So then there's another terrible school shooting that requires more laws. The ATF has a pretty accurate database now. The AWB is amended or the ATF simply changes one of their definitions so a certain firearm or fire control group or whatever is now considered a machine gun and must be turned in... It seems crazy but this is where many gun owners are when it comes to trusting the ATF. So thats why there will be big push back against something seemingly very minor like universal background checks.
If that sounds fucking dumb to you, Google AutoKeyCard to see a recent example of what can be considered a machine gun.
ALTERNATIVELY, If you simply require lost or stolen firearms to be reported AND give the public limited access to NICS you can require universal background checks without any additional concerns (warranted or not) of an ATF registry.