Lets agree...
You should not be allowed to buy gun until you show membership to a 'well regulated militia'....
I presume this Militia will be registered with the Government be it Local, State or Fed... Constitution doesn't say that.
Now if it is well regulated it would have a registry of all the guns in their militia.
So it is up to the Government to up hold the law and make sure the US Citizen is part of Well Regulated Militia and a Well Regulated Militia would register guns... And that is the constitution...
You are making the same mistake that the Trumpians make with regards to a "high crime" and using today's verbiage to determine what was meant in the Constitution.
There was no registry of all the guns held by citizens in the 1700s, there were no serial numbers on Muskets to register them by.
That is not what they are talking about at all.