A stolen weapon used to commit a crime does not constitute a crime on the part of the original owner. In my state there is no legally required way to store a firearm. Shooting someone accidentally certainly leaves the person operating the firearm open to civil litigation, and possibly criminal prosecution. I don't think anyone has every implied otherwise. All the regulations that have been mentioned refer to purchasing a firearm. If you want to require rifles be locked up at night, make the argument that it will help prevent theft. Don't make the argument that someone can't purchase it in the first place.

I'm not a "2nd Amendment absolutist" as you put it, no more than I am a free speech or free press absolutist. I have no problem with licensure or registration so long as it's not prohibitive, and you will be hard pressed to find gun owners who say there are no responsibilities associated with gun ownership. To get a license to carry a concealed firearm in my state requires $20 and a few weeks while they do a cursory background check, depending on the county. To get a handgun or rifle takes half an hour for a phone call background check.

If politicians like Sen. Feinstein would stop talking about banning weapons wholesale that had nothing to do with any recent tragedy, and instead talk about a national level of firearms reciprocity (while requiring locations like NYC, Chicago and DC to come up to par), you'd see folks on the Right move quickly and decisively to supporting more common sense gun regulations.