Which brings me to my main issue with these kinds of laws: they always feel like stopgap measures

The reason for that is laws themselves. You and I may have different definitions of what an “assault rifle” is but could probably come to some agreement. Perhaps some combination of the caliber, rate of fire, magazine capacity, etc. Finding that form of consensus across the electorate would be impossible. The people who write the laws have to write specific wording which will be immediately and voraciously attached by the well-financed pro-gun lobby. It will also be attacked by ingenious (and I mean that sincerely) hobbyists who will do their best to circumvent the laws. I’m sure there are a few hardcore 2A enthusiasts who hate Trump for banning bump stocks, which serve no purpose other than to make a slow-firing weapon into a fast one… and just happened to be used in a mass killing.