I understand the tactical considerations of getting misidentified as the bad guy, and believe me, unless me or mine is in danger, I'm probably not going to do the hero thing. Also, I'd do my best to make sure that the responders knew who I was (call 911, tape a "good guy" sign on my back or something). A tactical discussion isn't what I was planning on.
Coronach, I did mention in the post that I was going to use a high-quality cable lock to secure the rifle in the trunk, and that is precisely for the reason that you stated.
Thanks for all the suggestions, but the wife has just nixed the idea of a bolt action rifle. Not fast enough for her. She is worried about a riot situation and doesn't think she could do the bolt fast enough to dissuade a crowd closing in. And since she's the more restrictive of the two of us, I have to abide by her choices.
I wouldn't feel comfortable making a 100 yard shot with a 12 gauge slug, and I know she won't shoot it anyway. An AR is just plain too expensive to leave in the car. I've personally never seen an M1 carbine for under $500, and for two cars, even that is too expensive, same for the Kel-Tec SU-16. A pistol-caliber carbine wouldn't get through soft body armor - I feel that if Mark Wilson had grabbed a rifle instead of his pistol, a good man would probably still be alive today (not judging his choices or his actions - but trying to learn from them).
An SKS is what I thought of, but I haven't seen one for under $200 in years. And most I see are around $250. For two rifles, 500 rounds of ammo plus enough ammo to "break in" both rifles and ensure proper function, I could more than save that $50 difference and just get a WASR-2 in the currently much cheaper 5.45x39 caliber. But then there's mags... I'll probably just get the SKSs, but was hoping for more suggestions, and then she got all down on the bolt actions. Oh well. Thanks for all the help!