Having lived through the CA magazine ban threats and then having them acquiesce to agreeing to the grandfather clause I would say fight this tooth and nail. The first magazine proposal had no grandfather clause. The grandfather clause was added later because the push back was huge. Once the grandfather clause was put it, the push back diminished greatly. I’m guessing because people felt that they have theirs and don’t care about the future. Especially with Nevada and Arizona right next door it was easy to go buy high capacity mags and bring them back. How could CA prove they were made after a certain date? Then came the legislation that all mags sold in CA have to have a date of manufacture on them. I don’t recall if this ever past or not?
My personal experience in CA with the AR bans is even worse. When the AR ban first went into effect the rules that outlawed them were so vague and and stupid, everyone just reconfigured their AR to comply with the law and AR’s continued to be bought and sold. Then, Feinstein et al started banning particular brand and model AR’s. With any of the named guns purchased before the law went into effect to be grandfathered in. I bought one of those guns that were supposed to be grandfathered in before the law went into effect. But then, they either passed another bit of legislation with this certain clause slipped in or it may have been in the original legislation and not revealed until after it went into effect, that if you wanted to keep you grandfathered in AR, you had to register it. I don’t remember exactly how it went down. Practically no one registered their guns. But what this did was to create a whole lot of AR’s that could no longer be bought or sold in CA unless it had been registered. There was also a cutoff date for registering the AR, otherwise you could be punished by law if you were caught in possession of one of these AR’s that was not registered. This, in essence, made criminals out of innocent Americans trying to own an AR. And when that law basically failed from stopping the sales of new AR’s because the manufacturers just renamed their models, even more restrictive laws were passed. All with grandfather clauses. If my memory is correct, I think CA even attempted to make registering all new sales of high capacity magazines mandatory. Meaning the mag manufacturers would once again have to stamp them with a date of manufacture and a serial number. But I don’t think that passed. Eventually, the end game is going to be this. Every AR, including those that have been grandfathered in will have to be registered if you want to keep it. If it never gets registered you cannot legally sell it and you cannot legally own it. The same will happen with magazines if you don’t push back.
CA has also passed other laws. For all manufacturers of firearms to submit (I believe 6 of) every model of their firearms to be tested for safety before they can be sold in CA. Each model consists of one caliber. For instance, a Ruger revolver that is chambered in 5 different calibers is each a model in itself. So if Ruger has to submit 6 revolvers for testing, it really needs to submit 30 revolvers for testing if it wants to sell every chambering of that revolver, ie 6 revolvers per chambering times 5 chamberings. BTW, they don’t get these guns back. But now this has created another issue. There are models of Ruger revolvers that are no longer made and Ruger would be stupid to submit them for safety testing if they don’t sell them anymore. Meaning this particular model is not on the CA safety tested list. Ruger made the SP101 revolver in 9mm many years ago and stopped making them after a while. My nephew inherited one of these guns. He went to a local gun shop and told them he wanted to sell it. And the FFL said he couldn’t buy it because it was not on the CA list of Rugers that had been safety tested. He said my nephew could legally own it. And he said my nephew couldn’t sell it legally in CA to anyone. The best the FFL could do is to put it up on the Internet for my nephew and sell it out of state for him. Now, Ruger has since started making the SP101 in 9mm again. My nephew has not explored whether Ruger has had this revolver safety tested for sale in CA or not. And if they have, does this also apply to the older models or not. I would think that if any mechanical changes have been made to this model since it was discontinued then it would not apply.
If they can force manufacture dates and serial numbers placed on mags you will see more banning with confiscation and ownership punishment being the end game.
Bottom line is NEVER ACCEPT any restrictive laws even if there is a grandfather clause. Because one day the grandfather clause will be expired and you will be turned into a criminal if you don’t register or permanently modify or dispose of that magazine. Whatever the law is going to be.