Walk into a well-stocked gun counter right now and you are likely to see something that felt unlikely a decade ago: gleaming, old-school wheelguns sitting front and center, with “sold” tags piling up.

For the last hour, in a backroom of a gun range in Arabi, Louisiana, I’ve been building Luigi Mangione’s gun. Well, not his, in the literal sense. The not-quite-finished firearm in my hands is very ...