Twelve years ago (where the hell did the time go?) I bought a Bushmaster XM15 AR variant in Arizona. It’s a great rifle and has a fixed stock, A3-style detachable carry handle, 20″ barrel, etc.
It’s also specifically banned by make and model number by the state of California who, in their infinite wisdom, decided it was too dangerous for mere mortals to own. Possessing such an evil “assault weapon” in the state is a serious crime regardless of whether or not I install a fixed magazine.
However, if I were to strip the lower (that is, remove the fire control group, springs, pins, etc.), get a new lower such as those made by a variety of companies and which can cost as little as $50 or make my own from an 80% lower, install a fixed magazine, assemble the new lower with the parts from the stripped lower, and mount the original Bushmaster upper on that newly-assembled lower then everything would be perfectly legal so long as the lower marked “XM15” is not brought into the state.
Literally the only difference is that the lower is a chunk of aluminum — forged by the same company, no less — with different writing on the side, yet one is legal in California and one isn’t.
Hooray for stupid laws.