I'm well aware of the fact that M33 is a ball round and that the mild steel core does not make it armor piercing.
However, the most prevalent ban on AP ammo is on projectiles with a specific percentage of their core made out of hardened material which is capable of being used in a handgun. This typically applies to handgun ammo but rifle ammo gets caught up in the AP handgun ammo bans when there are readily available handguns chambered for rifle rounds, such as 223Rem, 7.62x39mm AR15 handguns, and various hunting handguns like those in 308Winchester.
The only "AP" ammo ban I am aware of that effects Ca. residents is the Federal ban on AP handgun ammo. Any Ca. attempts to ban "AP" ammo would likely follow closely to the handgun ban where they try to ban steel core projectiles, the fact that M33 is perfectly acceptable is some indication that they haven't made any attempts to ban steel core ammo.
I own a 50BMG and have bought and loaded M33 spec ammo as well as handloading my own M2 projectiles. The bullet construction is virtually identical with the exception being overall projectile weight and the hardness of the steel core.
You can bet that if Ca. tried to ban AP ammo they'd draw zero distinction between the M33 ball round and the M2 AP round, steel core would be the qualifier in any legislation they drafted.