I am just curious, when the day comes that the government wants to confiscate everyone's firearms, who do you think will be tasked with job of physically removing them from people's homes?
Well, the government banned alcohol once upon a time, which was unpopular with most in law enforcement and the US military as well as other citizens of this country. Local law enforcement was not particularly effective at controlling alcohol sales (both due to passive and sometimes overt non-compliance by local LE and/or politicians, as well as corruption once prohibition started pumping huge amounts of money into gangs and a black market). To deal with this, we saw the first rise of really intrusive federal law enforcement.
I don't think the military would be especially reliable on this issue (even assuming one can get beyond posse comitatus issues). I think that possibly outside places that are already in a state of insurrection against the US Constitution vis a vis the 2nd Amendment, local law enforcement would not be adequate or politically reliable for the mission.
Federal law enforcement assets are not adequate to round up 300+ million firearms in this country -- they might be able to make a few high profile examples, but they don't have the resources to do all the legwork necessary to hunt down hundreds of millions of firearms.
The solution? Reinvent the SS or the Iraqi Republican Guard -- an politically reliable armed force beholden directly to the ruling regime. The legality of this is dubious unless the surviving members of the Weather Underground and Symbionese Liberation Army got appointed to the SCOTUS.
Also, do you think a lot of those people will refuse the order?
Well, nobody started building IEDs the day after Prohibition became law of the land -- most just ignored it and turned to the black market as a work around for their needs. Best case scenario for the government, it would be great time to work in the shovel, PVC pipe, cosmoline or dessicant packing material industries. Worst case -- people would get shot. How many? Hard to say.