I've got a Beretta auto shotgun and I'm thinking of putting a rifled slug barrel on it. Is there any place in N. California that does that?

There is nothing to replacing the barrel, anyone can do it. The task is finding the barrel.
What model do you have?
Check with Hastings and Beretta, possibly Ithaca. They might have what you need, and it is just a quick change. If it is not a factory available barrel, and you really want one, it can get expensive. Then you might want to just pick up an 870 with a rifled barrel ($300 or less). This is what most people do because the cost of a barrel is often times more than the cost of a new gun.

I've got a Beretta auto shotgun and I'm thinking of putting a rifled slug barrel on it. Is there any place in N. California that does that?