I also live in OC and there is no place close by that I know of. I know some folks go out to Chino to the dairy's to help the farmers out, I haven't done that yet but hope to someday. If you go this route, get a written permission to do so signed and carried on your body with you. There could be places in Riverside but don't know for sure. Like Todd said there are Fed lands out toward Palmdale Lancaster areas. A close friend lives in Bear Valley that has a private range, who is also an airgunner but because of the pandemic it's a no go. If anyone finds a place post it here and maybe we can all meet, greet, eat and shoot together! Stay safe and healthy!
How about the Cleveland National Forest latimes.com/archives/la-xpm-1986-08-27-me-14282-story.html#:~:text=Concern for public safety and the preservation of,U.S. Rules Limit Target Shooting to 8