OK, I'll start. I'm a moderate with experience with firearms that is probably middle of the road. I am somewhat familiar with them, have been to shooting ranges now and then but I don't carry and I don't routinely practice. I have lived in TX, AZ and CO (where I live now) and I guess I would say I think CO's laws are reasonable in that there are some checks against folks buying them but if you really have a lot of fear for your life you can go through reasonable steps to get a concealed carry permit. I know a few people with conceal carry permits, and I support their right to have those because basically it makes them feel better. In at least one case, it probably vastly improves the quality of life for this person, because she'd be pretty fearful without it. OTOH, I really do not think there are a lot of mass shooting cases in which arming the populace will prevent gun violence death. I haven't seen all the stuff on the news about whether people in the theater were armed (likely some were) but it's pretty certain people were armed during the Tucson shooting a couple of years ago. AZ has extremely lax laws about guns, not even bothering to require any sort of permit for conceal-carry. I believe at least one of the people who tackled the shooter in that case to the ground was armed, but tackled rather than shot. Very few other mass shooting incidents have been stopped because of an armed bystander, most notably the Fort Hood shooting which was *on a military base* meaning nearly everyone around had had firearm training. At one time I was in a situation when I walked out of the house at night from a recessed door, to hear and see close-range gunfire (this was in AZ). I did have a loaded gun in the house, but my instinct was not to run out of the house with a gun but rather get inside quickly and call the police. I'm certain some people are different because there are trained soldiers and police officers, but after that whole thing I really, viscerally realized how unlikely it would be that I would be prepared to engage in a fire fight. Now, if someone were in my house and I had the advantage of knowing the layout and possibly the element of surprise I might use a gun. More likely I'd prefer some other tool such as a large knife, baseball bat, etc.
As far as gun laws, I don't particularly see how changing them would have changed the outcome in many of these cases (other than the loophole by which the Columbine shooters were able to obtain guns) unless we made it extremely onerous to obtain guns which will never happen. Even if the US did make very strict gun laws, aren't the laws of Norway much more strict about guns? What about England? Yet they have had mass shootings too. These types of incidents are carried out by people with particular thought processes and possible mental illnesses. There are things the press could do to help, like not sensationalizing the whole thing esp the shooter(s) names and faces. Having a stronger mental health system and more/better intervention would likely help as well. Probably this would not completely end incidents like this, because it's sometimes difficult to predict who will carry an act out. But that's a whole other debate that isn't even brought up most of the time.
Having some checks and balances in gun laws will probably have the greatest effect on gun accidents, which claim quite a few lives each year. I don't think it's onerous to expect some level of gun familiarity with both laws and operation of a firearm, prior to obtaining one. After all, everyone is required to have a driver's license prior to driving and much of that is because cars are deadly weapons too. And yes, some people will have guns without a license just like they drive without one, but saying "well some people won't bother/criminals don't care" is a BS argument for the same reason in either case.
There's my opinion, have at it!