As you note, this is a problem with bows and crossbows in D&D. Really, it's the nature of hit points. Either a weapon can kill something in one shot, in which case it generally will kill it in two, or it simply cannot kill it in one shot and needs two, three, four, or whatever number of shots, plus or minus one.
No, the problem is thinking that bows, arrow, or guns need different rules from other weapons in a HP system. HP are just an amalgam of actual wound-taking ability and your ability as an experienced combatant to avoid taking a quality hit.
Because most weapons can kill in one blow or wound in several. If I grabbed a paring knife out of my kitchen and slashed your femoral artery or jugular vein, you'd likely be dead before the first responders got to you. And that's one strike from a 2" blade.
What makes guns so deadly is a combination of ease of use, range, RoF, reliability, range and compactness and the properties of the rounds they shoot... But those didn't all come together at once. It took hundreds of years to get the formula right.
So you could, fo purposes of your game, have guns that combine those elements in various combinations, just don't give any one weapon ALL of them.