There have been previous attempts to put RKBA amendements on the ballots:
This failed at the attempt to get volunteers to get signatures.
My bet is that most California gun owners are "gun shy," and I don't blame them in this cultural climate, so no one has the time or inclination to spend hours in a parking lot, risking the abuse of strangers, trying to induce strangers to sign a petition. Most firearm owners aren't known to their neighbors, friends and co-workers as "gun nuts" and they have no desire to "out" themselves.
I'm betting the success of this approach on such "fellow travellers" being willing to simply print out and post and email friends who they know to be sympathetic.