Do you follow current events? Imagine small arms held by the Syrian People, against the 20th Century war planes dropping ordinance and chemical weapons. Whose winning those engagements?
Then consider the effect if citizens could own the arms capable of engaging a 21st century military force in the US? In my opinion, having served on a Destroyer in the late 60's, how that vessel would have done against the 21st Century Navy - not well and not for long.
Your reasoning is absurd. Today the cost and size of our military, and the training, cannot be defeated or even slowed by a civilian force, and even if the arms of war were legal in the US. Have you thought about how these arms might be used in small engagements against the civilian population (think criminal gangs).
The thought that a private citizen (Other than a well prepared Multi Billionaire with his own private military) can even last but a few minutes against the force of the US Military is absurd. Luckily, there is a military tradition and law written into the Military UCMJ (Uniform Code of Military Justice) that is like the Military's version of the Constitution of the US, that bars this type of military action. Not even a President can wildly expect the Military to follow that order and remain in power for very long.
The first half of the 2nd amendment has really been out of meaning since the National Guard Act of 1917. I imagine that some Multi Billionaire could buy a small country and start building a force that might last a few days, months or more but no on the continental US. Due to the various weapons laws (we can't even call them firearms anymore) they would be stopped long before they gain the weapons to have a ghost of a chance. Same goes for a state accumulating those weapons even though they can legally do so.
The Second amendment really didn't stay current much past 1850. AFter that, Firearms started accelerate to the point where only Governments could afford them. Much like the KIng and his Armory in the 12th century.
I have never demanded that we need to get rid of the 2nd amendment. What I do suggest is we need to amend it to keep it current. Make it more specific to today and less ambiguous.