>>>Occam's razor would assume that the unique extent to which firearms are readily available in the US probably has a major connection to the unique extent to which mass shootings and gun violence occurs.
Well let's look at just the United States, over time, rather than the US compared to other countries. In the US, a proliferation of firearms has been a constant throughout our history. But it is only recently (really starting with Columbine) that we've had REGULAR outbursts, almost always by men under 30. I would hypothesize that the elephants in the room are a)prescription psychotropic drugs and their side-effects b)constant negative media about males/toxic masculinity c)overall ineffective child-rearing practices and extended adolescence, some of which stems from a reduction in two-parent households.
These are vague partly because so few people will take a deep dive into these subjects when so much money flows from these influences (pharmaceuticals, media, etc...).
But why aren't we looking at the variables, instead of the constant?
>>>it is absolutely progress to have mass murderers be forced to utilize something like a knife than head on down the street and pick up an automatic/semi-automatic weapon.
1. You can't "head on down the street" and pick up an automatic weapon in the US. You need a Federal Firearms License for that. And historically, FFL holders are some of the most law-abiding citizens in the country. Even if there were a ban on semi-auto weapons, the market would adapt. I've already brainstormed on how to optimize a bolt-action rifle for rapid, sustained fire and I'm not even a firearms designer.
2. Tightening the gun proliferation sounds great...in theory. How do you actually accomplish it in practice? There are 300 million+ firearms spread across the country in about 40% of households. This is a land area greater than that occupied by the Germans on the Eastern Front, with a greater number of potential "partisans", and the Germans never even came CLOSE to securing their rear areas. That anyone expects widespread gun confiscations to NOT turn into a bloodbath is naive IMO, and if the objective is saving lives than it would also be counter-productive.
3. Maybe the mass murders will switch to homemade explosives instead of knives, which would be a significantly WORSE outcome? Ever think of that? Maybe they'll get guidance from jihadis. Hell, explosives already gave us one of the worst school massacres in American history: https://en.wikipedia.org/wiki/Bath_School_disaster