Sounds like they addressed a legit threat.
I have to say that 100 rounds of ammo does not constitute “a large cache”… it constitutes an hour’s practice.
This bothers me though:
[a 19-year old family member] was arrested for possession of an unregistered firearm and is being held on $35,000 bail.
Gun not listed as stolen, had a serial number that had not been altered. So he’s in jail on $35,000 bond for failing to do state registration paperwork. That infringes a constitutionally guaranteed right. 
I was at the range on Thursday for a low light handgun class. 1.5 hours on the range with 200 rounds spent. And there was very little rapid fire with the light off.
If I can fit it in my range bag, it is not a “cache of ammunition!”
And yes @Zee, that sounds unconstitutional. But, until something changes, CA will do what CA wants to do.