Yes it is, as long as you don't bother anyone with it. There are hundreds of active micronations within the United States and I have never seen one be dissolved by the US. The fact of the matter is...
"I had to guess a bit as to which M7 is being referred to in the question. If the question is referring to the WWII tank the M7, then yes, the tank would be legal here in the US, but it would not be...