I'm not familiar with California law on this matter.
"Target practice" with a loaded weapon is not illegal, obviously.
But, is driving with a loaded weapon in a vehicle, even if the weapon is legally owned, not legal in California?
And, he had a silencer, also?
Are silencers illegal for all private citizens to own in California, even if they own guns legally?
(In Texas, would he even be guilty of a weapon-related crime related to the above circumstances?)
What, do you think, brought him to the attention of LE (who discovered these weapons/silencers) in the first place? Does LE look at all subjects in possession of loaded weapons and a silencer as potential criminals, and cross reference everything they can about them and their weapons into a database in the hope that other crimes can be solved?
Maybe he was speeding, and they fortuitously searched his car?
Why would LE have probable cause to search his car?