Joe doesnt need to own any firearms to buy ammo

If Joe buys 500rnds/wk of 45acp but he doesnt own any firearms, does CA state police come asking questions?
At present, the California government probably won't have any interest in how much he buys or whether or not it's for any guns he owns or doesn't own (although he might get the attention of law enforcement if his purchases fit any profiles of selling on the black market, which is an illegal activity). It's when a gun confiscation scenario comes up that Joe's purchases, whether he owns any guns or not, could cause him some problems.
If he had purchased ANY ammo that was entered into the database, reasonable suspicion exists that he possesses at least one firearm. That's all the state needs for it to investigate. Ammo possession is generally a reliable indicator of gun possession, since one without the other usually makes no sense. So if the state is serious about confiscating all guns, why wouldn't they investigate? Joe may be able to explain why he bought 500 rnds/wk of .45 ACP but doesn't own any firearms, but he'll probably do so while state agents search his home under a search warrant.
now multiply Joe by 50,000 people who 1) dont own any firearms, 2) are not prohibited from buying ammo,
I'm not saying it's a practical thing to do. Gun confiscation in the US would not be a smart or practical thing to do.
However, it is what the anti-2A movement dreams of doing someday, and there's no shortage of smarmy politicians who keep attempting to do it in one guise or another, and will continue to try. If most of us on this forum didn't believe so, the topic wouldn't come up as often as it does, if at all. Unfortunately, California is now politically such that it may try.
3) do it because they can resell it on black market for profit
Given that such black market activity in California is illegal, I would expect law enforcement to crack down on it when it's found.
or will commiCA start to limit how much ammo one can buy?
Given the gun-purchase and mag-capacity restrictions already in place, it wouldn't surprise me if California tried it (which is a separate matter from whether SCOTUS would uphold such restrictions after the inevitable lawsuits).