More than half of them are desk/bureaucracy bound. It's probably more like 3/4 of them, not to mention many of them will favor the gun owners, but let's be generous to the opposing argument and suppose all active street duty cops were in on it.
So you are saying each police officer is going to arrest and remove weapons from 10 people scattered across millions of square miles. They will also need to ostensibly put those 5.5 million 'bad Americans' somewhere. We don't even have jails for that many people. You'd need literally a few million guards just to keep them. And that's just NRA members. There's many more gun owners than there are NRA members.
Have you been in the US? Do you understand how insanely unrealistic this is?
Lists are great when the people you want to hurt are small in number and weak in force. Neither applies here.
A tyrant dictator could send the army, but again, way outnumbered. Not to mention how big the USA is, how rugged the terrain can be, and how well acquainted the locals are. Even just rounding people up is super hard when people WANT to cooperate (look at our response to natural disasters)
The tyrant dictator can take over with tanks, no doubt. But then they would face a long, protracted guerrilla war with all freedom loving Americans. If Vietnam, Afghanistan, Iraq, the founding of the USA, etc have taught us anything, is that you can conquer, but holding a territory is a whole other ball game. And holding is where it's at.