I this the stats suggested in the Dungeon Master's Guide (p.268) is pretty fine, though.
Also, a rifle definitely would not do radiant damage.
Why not? Its magical crystal beam shooter, not a blackpowder firearm.
That's not quite true.Well, as I understand it, radiant damage refers to divine magic...
That's not quite true.
Radiant damage, as defined in the game "sears the flesh like fire and overloads the spirit with power." but is not inherently tied to only happening because of divine magic.
In fact, as far as I can tell "divine magic" as 5th edition uses it only means "the spells of clerics, druids, paladins, and rangers," which even things like angels attacks don't fall under the category of.
Also, there is at least one spell that is only available to sorcerers and wizards (arcane magic) that's primary function is to deal radiant damage (sunbeam).
As for radiant damage dealt by a mechanical apparatus, I like to think of it as something akin to a laser or plasma weapon (burning, but not fire).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.