Who Needs Guns?
A gun is a machine that uses chemical energy to accelerate a projectile to (usually) above the speed of sound. When this projectile hits a target, it transfers that energy to the target, damaging it.
In your world, a human can move at supersonic speeds. (Analysis for scenario one: A tennis ball serve is around 120 mph, about 20% faster than a baseball pitch. In a baseball pitch, the pitcher's hand is moving at the ball's exit velocity. A typical bullet is more than 120% the speed of sound, so your scaled human's pitching hand would be supersonic. Analysis for scenario two: Trivial.)
This means that a human can throw a rock with velocity comparable to a bullet, but with an order of magnitude more mass and thus more energy. (A baseball weighs about 145 g, a 45 caliber bullet weighs about 15 g.) Why would you use a gun to accelerate a projectile when you can just throw it?
[Side note: Similarly in close-quarter combat you probably don't need melee weapons, and man-scale armor is probably useless. The sheer amount of energy in a supersonic punch will obliterate a human target absent super strength to go along with super speed and nearly-infinite energy. Carrying a weapon would be overkill, and being even slightly encumbered could only reduce your ability to dodge similarly-fast opponents.]
The question, then, is whether it's worth throwing projectiles at the opponent. It's very cheap to try it, ranging from the cost of carrying a bag of projectiles down to (if there are pebbles around) zero. And your opponents can move fast enough to dodge, but they can't actually hear the projectile; they have to see each individual projectile to take action to avoid it. If you ever see the back of an opponent's head, chuck a rock at it and see it explode in a red mist. If you have three buddies, throw three rocks such that dodging the first puts you in the path of one of the others. If nothing else, why not try? Unless the sun catches it just right they might not notice...