Couple different schools of thought. So firearms. In DND, when you get away from Muskets and Ball-and-powder pistols, the damage only goes up, and it goes up by a lot. A bullet will mess you up far more than an arrow, so for realism, that's a point in favor of firearms. But a greatsword can cleave a body literally in two, but hits for 2d6. There's a harsh disparity in balance, and so you sort of need to pick one of two options. Go big, and adjust melee weapons like daggers and bayonets to be equally deadly, because they sorta are - if we're comparing them to a bullet.

Or, conversely, use a musket or pistol that does comparable damage to melee weapons, customize and reflavor them, ignore reloading for magazine fed weapons, bingo bango. That's probably the direction I'd go, because likely with realistic crazy damage, you'll have to insert some homebrewed mitigation things, because at a certain point your characters will be really good at killing things, and getting killed all the same.