While I agree that a tank has limited utility for a community compared to an armoured truck or a proper APC and getting ammo for them would be pretty much impossible, there are some places in the US where heavier weapons are allowed to be owned by civilians.
For example although I don't know what state it is, you can own 40mm grenade launchers and also recoiless rifles but you cannot get explosive ammo for them (only smoke, if I remember the article correctly).
In Texas they have a collector's club for flamethrowers... should give the Mexican invasion something to think about, dozens of angry Texans wielding flamethrowers
