Legacy weapons are completely different than everything else in the game. You cannot use them as a baseline.

I can, do, and will.

Go look up speed kills with plasma flamers

Oh you mean the plasma flamer, that gets an automatic 50% armor penetration due to a bug?

railway rifles

Oh, you mean abusing sneak damage and VATS crits because you can't actually aim?

In terms of unperked nonlegendary damage, the legacy Dragon (400 range version) has the highest per-shot damage of any gun in the game.

In terms of fully automatic unperked nonlegendary damage, either the 50cal MG or the Gatling Plasma hold that title.

The only reason people are melting bosses is because of ammo glitched B/E/90 dragons. Plasma Flamers aren't shit.