I'm pretty sure it has. Certainly civilians own anti tank cannon, .50 BMG M2 machine guns, and .30 caliber machine guns as well. Don't know for certain about an all up tank, but it would shock me to find that no one owned one.
All of the above can be seen, owned by civilians, which can be seen on a episode of "Mail Call" which I happened upon on "youtube" last night. (Can't find the link as it's blocked from here, but just search on "guns" on youtube).
Oh, there are privately owned tanks, and at least one I've heard of that is licensed for on-road. But I think the weapons systems have been de-milled.