You're operating from a position of unwarranted fear. The combined strength of the US military and every law enforcement agency, isn't enough to confiscate 400 million guns from 150 million gun owners.
Look at the numbers. It's been estimated that only 3% of the American colonists actually shouldered a weapon and fired upon the British in the Revolutionary War.
If only 3% fought back against a nationwide gun grab, that would work out to 4.5 million dedicated citizen soldiers, versus a maximum of 1.5 million federal troops. That's a rout, and honestly, I think the percentage of active resisters would be much higher.
You’re operating from the presumption that the “combined strength of the US Military and law enforcement agencies” will come for ‘everyone’s guns all at once.
I’m going with the idea that they will slow boil the frog. First they will vilify militias and patriot groups. They will reinforce that when those groups resist and events like Waco happen, and if that doesn’t happen they will probably provoke something else or if desperate enough drum up some false flag operations.
Then they will use the media to turn public opinion against all gun owners by associates. Pretty soon, everyone will be afraid to even say or post support for the second amendment.
Third, they will selectively target and implement collections slowly and methodically. There will be no need to plan and stage a massive single confiscation event.