Sure, I remember those days. 1000 rounds was like one or two good range trips with a couple friends. Literally no different than hitting a bucket of balls or two at the golf driving range. Now, you look like a crazy man if you want to buy that amount.
Firearms are very restricted in CA... lots of new laws on the books as of 2018.
https://rocketffl.com/california-ammunition-law-2018-explained/
Google the laws surrounding 2018 AR-15 registration, featureless conversions, and Pistol Roster Microstamping if you want your mind blown.
The AR-15 as you understand it in your state, is already banned and illegal to own in CA.
Also... there is no such thing as a gun-show loophole in CA, regardless of what the media says. ALL firearms transactions are subject to background check and must go though a licensed FFL.