Owning an AR4 assault rifle

Answer 1

The AR4 was not an assault weapon, either by the true definition, or by the BATFE definition for civilian firearms. It was a survival weapon designed for use by downed airmen, and fired the .22 Long Rifle cartridge.

As for owning an assault weapon in general, it depends again by which definition you define it - the BATFE definition, or the true definition.

A true assault weapon will be a selective fire weapon, capable of firing either in bursts or full auto. Some states (California comes to mind) strictly prohibit this type of firearm. Others may not, although individual counties may refuse to grant Class III permits. Civil ownership of fully automatic weapons is covered under the National Firearms Act, and requires the purchaser to obtain the appropriate permits, as well as pay a transfer tax of $200 to the BATFE for any NFA item purchased. Also note that the Firearm Owners Protection Act, passed in 1986, prohibited the sale of any domestically manufactured automatic weapons to civilians after its passing (imported full auto weapons were banned under the Gun Control Act of 1968), so any weapon you legally buy must be manufactured prior to 1986 and legally tax stamped.

If you're referring to a military style semi-auto, most states class them the same as any other rifle, although some states do have 'assault' weapons bans in effect (California, Connecticut, New Jersey, New York..).

Depending on the specific state laws, owning a weapon of this class may be legal, but the appearance must to altered, as such laws are essentially just a ban on cosmetic features.

California rifles must be completely featureless, meaning, no pistol grip (the California Rifles U15 stock and Hammerhead stock are a couple items designed to comply with these laws), no flash suppressor (although a permanently fixed muzzle brake is allowed), no collapsing buttstock, etc. One other option would be a "bullet button", which, by legal definition, makes the magazine (with a maximum capacity of ten rounds) a fixed magazine, as it requires a tool to remove it (such as the tip of a bullet). In that instance, you're allowed all of the features covered under assault weapons ban legislation.