It is start with road blocks, car search, and false flag terror.
Now, these Ruby Ridge and Waco posts are silly examples. LEO outnumbered their victims hundreds if not thousands to one in isolated areas with local LEO enabling and aiding the Feds. This would never be the case. It would be thousands to one the other way around with little local support in flyover America and a large amount of LEO noncompliance or out an out direct resistance from local LEO. It is not ever going to happen in our lifetimes.
They could go after the manufacturers of guns and ammo but black market would take over in time if any were shut down before this played out in the courts or with a change in administration.
On New Years Eve night it is illegal to shoot in the city or surrounding county islands. In ten minutes I heard WWIII going off with every caliber known to mankind going off continuously without ceasing far and wide for at least 20 square miles. enough rounds went off in ten minutes to take out 4 divisions in one large city in the central valley of California. All those discharges were illegal. Tens of thousands breaking the law simultaneously and nobody arrested. That was for fun, what do you think happens when the SHTF and it is serious business?