It's a slippery slope fallacy to say that reloading attacks will be launched as follow-ons to this law.
Reloading actually *broadens* the investigative net instead of narrowing it, and so helps with the stated purpose of this law.
Let's say you, Sid Citizen, have a microstamping gun. Joe Crazy doesn't. You go to a range to fire off a few rounds. The range polices your brass, reloads them, and sells them to Joe Crazy who leaves them scattered around a crime scene. Your stamps, his crime scene. Bad for you, right? But it's good for the cops.
You become a suspect. They take your gun, run ballistic tests which (hopefully for you) prove that your gun didn't fire the terminal bullets... you are less of a suspect now. So they ask, "How did your cartridges get into his gun?" (they are cops, don't expect perfect shooting jargon from them) You say, "I don't know... I just go to Ed's Shooting Sports once a month to punch holes in paper." "Ed's, huh?" The cops go to Eds, finds out that yes in fact they do reload range brass (or sell it), and there are sales records. If Ed's is in Los Angeles County those records include driver's license numbers even. They have a trail, they can narrow the list of suspects down to people who bought ammo from Ed's. They can cross-reference that with the other suspects they've been looking at. Has the boyfriend ever bought anything from Ed's? You know how it goes.
That's why I don't think you'll see this used as an attack on reloading.
Revolvers, though.... It would be quite Californian to say that revolvers no longer pass the DOJ safety test because they don't leave evidence at crime scenes. That's a more realistic outcome of this law because it doesn't necessarily require additional legislation, just a regulatory redefinition of "safe".
Reloading has already been attacked by other bills such as the ammunition component tax bill that would charge a punitive tax for each component (bullet, primer, shell, etc) used in reloading. Expect new attacks of that sort. They won't be tied to the microstamping measure though because reloading actually helps microstamping somewhat... if you do reload you'll end up making many imprints and if you use some of your reloaded ammo there is more likely to be a recognizable imprint.
Anyway.. don't be too hard on Arnold. These bills have been coming up every year. Arnold vetoed some last year. The problem isn't that he didn't keep vetoing them... it's that the voters didn't do their part. If the legislators had been replaced Arnold wouldn't have been faced with the choice of vetoing the same bill over and over again or going along with the expressed will of the majority. The fault lies not only with California gun owners but with ex-Californians who chose to leave the state instead of fighting and becoming the positive face of gun ownership. When all the good gun owners left, those that remained were either bad gun owners (criminals and nuts) or hiding gun owners. I'm condemning myself with that by the way -- I left California a year ago and part of my motivation for leaving was the promise of fewer anti-gun laws. The first thing I did was to start erecting barriers to ever going back as well... buying products (guns included) that aren't legal in CA, so that even though I still own a house in CA it's unlikely I'll move back there. That's a loss to California's gun owners and in the long run it will be a loss to everyone. Each time we lose a part of the country it is much harder to ever get it back.
Arnold would be FAR better for California and the country than Boxer of Feinstein. At least Arnold vetoed most of the anti-gun bills last time around (perhaps before it was obvious to him that if he kept vetoing they'd just keep coming to his desk). He's not rabidly anti, he's just not as rabidly pro as you'd like. For California that's a major step in the right direction.