Completely disarming a place the size of the USA?? Not even remotely viable. They took hand guns off licensed gun owners here in the UK, much to their chagrin. There was a dramatic change in gun crime using hand guns afterwards. It went up!
Just dont be stupid to do it all at once.
Say first, repeal the grandfathering clauses for automatic wepaons so they have to be turned in. Most of the public would support that ine.
Then over the next few decades slowly increase barrel/total length requirements, to start to remive short shotguns and carbines.
Continute to do that while starting to reduce the maximum alowed magazine sizes making it a criminal offence to posses one over the limit.
Again easy to sell to the public "why do they need short arms and big magazines for target shooting/hunting??"
This also means magazine posession which many would keep would get them a conviction and remove thier right tonown fire arms further limiting the number especialy amongst die hards (also lets you go "most opposition to new gun laws comming from criminals)
Then you can eother start going after mechanism, ie must be bolt or straight lull for rifles not semi auto.
Or you can start rasing the cost of fire arm ownership.
On registration forearm must be metalurigcally tested for later isentification at owners cost (and again at any new barrel purchase)
Forearms license aplication fres go up along with increasing the frequency of renewals.
And then after a few decades. You basicaly only have rich/upper middle class types owning long shotguns and hunting rifles.
Every step is reasonable sonits hsrd to gain momentum against it without looking bad
