Suppose the Second Amendment were repealed tomorrow. The stars align and Congress, the SCOTUS, and the White House all agree that gun ownership shall no longer be a constitutional right.
What would then happen to the millions of guns currently in possession of American civilians? Would they be retroactively confiscated (which would probably be impossible to enforce reliably), or would they be grandfathered in?
Making our own weapons might be outlawed next and then the ability to make them. For example it might be required that in order to operate lathes a machine should might have to prove it was not using that lathe to produce weapons.
In addition other technologies which could be considered weapons could also be banned, theoretically. For example the government could ban technological knowledge and books about it. We'd need a new amendment to cover things like that. Probably we do, anyway; however at this time they can be considered protected for two reasons. Firstly technology is a weapon. Secondly anything not expressly excluded in the USA constitution is automatically a right of the public. The second reason is a little thin, as it has proven easier for the government to ban rights not mentioned in the constitution. I predict bans of knowledge (about weapons or anything dangerous) to follow though not immediately.
I imagine there would also be a lot of patting on the back "We did it" kind of stuff by the anti gun groups. Perhaps there would be some tears of joy, but across the aisle there would be grimaces and worries.
I do not think it would change the murder rate which in the USA is pretty low, however it would probably decrease the suicide rate. The murder rate tends to be related to other factors as can be seen in comparisons between murder rates in USA and murders in English speaking countries that do not have gun rights, adjusted for population differences. The suicide rate, however, is a different story. More people are willing to commit suicide with a gun than with a kitchen knife.
It would change the number of people shooting themselves by mistake.
Overall not much would change at first. I think that the repeal of such an amendment would not last, because the desire and the demand for it would keep it in public discussion. It would be another endless conversation like the one we are continuing to have about abortion.
Should the US government ever be taken over by some crazy person, the US population would have to rely upon the good will of the police and military to affect a coup. We citizens would not have much ability to be any threat ourselves.
Protests would not have the same leverage. Protestors needs some kind of leverage such as money or unity or connections or fame or something like that. If they haven't got anything then they will likely be ignored. One strong lever for peaceful protests is the threat of unpeaceful protests. It is a very strong lever, too. Its what makes peaceful protests impressive. There is a difference between peaceful protests by an armed populace that chooses peace and peaceful protests by an unarmed one which simply has no ability to do anything.
Gun companies would lose a lot of business. A lot of shops would close.
Gun collecting would come under threat. The government might try to confiscate older guns. Most likely it would lay down arbitrary rules about which guns were protected antiques, and it might require that their firing pins be removed or that they were in some other way disabled.
Potential invaders of the USA would be encouraged. This is an issue, because invaders often wish to subdue a population. They reconsider when facing an armed populace which can perform guerilla warfare.