When people mention that nobody's coming for your guns, what they mean are the many types of guns that would remain unregulated. Your revolvers, shotguns, derringers, muskets, bolt action hunting rifles, etc would likely all remain unaffected. This type of limited weapons ban would follow the same precedent where you can't buy a nuke because it's been determined that it's too dangerous.
It may well take fifty or a hundred years until a sales ban becomes effective in minimizing the availability of particular weapons, so it's not an ideal solution. Better solutions would be to implement skill based licensing, mental health checks, background checks for every weapon sold, exclusions for people currently on terrorist watch-lists, databases of gun owners that can easily be cross-referenced by law enforcement, etc. Many of those have been partially and half-assedly implemented already . . . it would just be a matter of improving what already exists.
Skill based licensing? So that mass shooters would be even better trained? Obviously these people are pretty skilled with their weapons, judging by the death toll.
Background checks - San Bernadino, Orlando, Dylan Roof, etc. - these people passed all of the current background checks required in this country. What makes you think they wouldn't be able to pass a skill test?
Cross referencing gun owners...? none of these people committed massive crimes before their shoot-outs, which is why they were able to buy their guns legally, with a full background check. Hell, the FBI runs the NICS database at the moment; how much more could you really add?
Do you have any suggestions that are not currently being used that would prevent mass shootings? California has some of the strictest gun laws in the country; still had a mass shooting. Several, actually.
I understand that there are small windows for improvements in the current laws, but almost none of them would stop any of the latest mass shootings.
Rounding up and banning semi-automatic weapons is right up there with rounding up and banning Muslims. They would have similar reductions in public terror attacks in the United States, and be similarly encroaching upon people's rights.