In nearly all states, the form 4473 that you fill out for the background check which has the serial number of the gun on it is kept at the dealer and not sent to the government. In order to access the information on that form, a law enforcement agency must be looking for a particular gun for a specific reason - such as it was used to commit a crime with.
Gun registration is providing the government with the serial numbers and current owners of all firearms which the government can access anytime they want, for any reason they want.
The purpose of the 2nd Amendment is not about self defense. It is about the right of the people to have the means available to them to protect themselves from the government should it become necessary. The 2nd Amendment must be read in light of the Declaration of Independence.
Declaration of Independence - Text Transcript
The 2nd Amendment:
Second Amendment | Wex Legal Dictionary / Encyclopedia | LII / Legal Information Institute
The "well regulated Militia" is nothing more than a group of citizens banding together for a common purpose. According to the Declaration of Independence, it is the duty of the citizen to band together to form a "well regulated militia" in the event that their government becomes tyrannical for the purpose of abolishing that government and establishing a new government of their own making, representative of the citizens, just like the founding fathers did. "Well regulated" does not mean government regulations concerning who can have what guns and where they can carry them and permit requirements. What "well regulated" means in the 2nd Amendment is regulated towards a common purpose - just like the high pressure air in a scuba diver's tank is "well regulated" so that the diver can use it to breathe. What better definition of "necessary to the security of a free State" could there be than that provided by our founding fathers in the Declaration of Independence? In order to be able to achieve that goal it is absolutely necessary that the right to bear arms of the citizens not be infringed upon by the government.
Given the actual reason for the Second Amendment, in light of the Declaration of Independence, it is clear to see that firearms registration - providing the government with a list of exactly who owns what guns - is completely, 100% contrary to it.