Because some people (Camnator) believe that the government will just use that to round up guns so that nobody has any kind of firearm. Of course, no sane minded person has even suggested such a thing.

Like the man below me said "Its more about potential for abuse." People have to register guns now depending on the state you live in. But with a national registry one of the things it would do is go into higher detail. Example, I live in California and own an AR15. When I bought my AR I had to register it. How ever it is registered as a "long rifle." It doesn't not tell the government I own an AR15. With a national registry they would include deeper detail. So if a tyrannical government did ever happen here in the US the government would be able to target people based on what weapon they own. I don't mind having to register my weapon. I just don't want the government to know what I have so they can target me because some nutcase shoot's up a school.

Its more about he potential for abuse than what it is.

It is incredibly costly and simply would not work. There are about 88 guns per every 100 people currently in the US. Okay, this is assuming that every one who ones one will register it, so an unlikely scenario. The ATF, or what ever bureaucratic body that would be in charge of this, would have to file a form for every one of those guns. Now you have to hold that data somewhere, either physically or digitally. Or the smart way, both. You now have filing systems of millions and millions of guns that need to be stored in a cool, dry environment. Then you have to have a data bank that would also need constant maintenance. You would have to hire staffers just to keep the server up and make sure all the files are not being destroyed by time or animal. Then you would have to set up a network that would allow FFLs to input the data to the main server of what type of gun it is, who bought it, serial number, etc... This connection would need to be secured. The server would need to be secured. Say the server goes down and you need to pull records. Well you have to go through the physical copies to find that record. Then of course if someone changes address, or sells the gun, or gives it as a gift the records would have to be amended both Physically and digitally. It is just cost after cost after cost after cost. We really have better things we could spend our money on. Canada tried this with long guns and it failed miserably. The program was shut down due to massive costs, major inefficiencies, and the fact that people were simply not registering their guns. It's really just a waste of time.

By Gun Registry, do you mean something like keep check of which gun owner has possession of which guns? Like do a sweep every year to make sure they know who owns what firearms and knowing who the firearm is with at any given time?

I could see it create more headaches for people who legally own guns that end up in criminal hands without their knowledge/intent. Also, privacy is a huge concern.