You're ignoring the form 4473 system which is effectively a type of registry.

Except it isn't a registry of gun owners, which is what is specifically prohibited under the law.

If they want, the feds can call up gun manufacturers and get lists of all the gun stores they sent a particular model of gun to, and call those stores for the 4473 records, and send confiscation notices to the people who own those guns. They could use the 4473 system to confiscate AR15 rifles, for example.

1) legally they can't confiscate a person's guns unless that person has the gun illegally (like is a convicted felon of a violent crime)

2) It is a bit far fetched to come up with a scenario where the government would go through the whole convoluted process you outlined in order to confiscate all gun of a particular type from every gun owner in the US. Not to mention just owning a gun wouldn't be probable cause for a search of the gun owner's home (unless the owner is already in violation of gun laws because they are barred from owning one due to past crimes). Even if in the future a law is passed banning a particular type of gun, more than likely it will be on future sales of new guns of that type. They won't confiscate guns en masse, there would be rioting in the streets.

this is no loophole

I didn't call it such. I'm a lawyer, that carve out in the Brady law is an exemption.

I grew up in Texas, I don't care if people own guns. But selling a gun to someone without bothering to find out if they are a convicted felon who can't own a gun is reckless and why I think background checks on private sales need to be made. Won't completely stop those who aren't allowed to own a gun from getting one, but it will curb a way they can get them.