The bullets you shoot in CastleMiner Z will sooner or later drop in distance. Upgrading your weapons gives some of your guns a slight range increase (except for the Shotgun) meaning the bullets will travel further without diving back down again. However, since CastleMiner Z is a very simplistic FPS, you wont see the bullet curve which means you don't have to worry about missing your target at 30 blocks away with a Rifle. A Skeleton has to volley arrows in order to hit it's enemy at long range, but will shoot directly if the player gets too close to it. Bullet drop is also noticable when you're shooting Dragons, as it is very clearly seen arcing when it is shot at the Dragon.
There is no bullet penetration with regular bullets, However laser bullets in CastleMiner Z can destory blocks meaning you can't shoot through solid blocks or Doors with normal bullets but with a different set of bullets. Even skeleton archers can't shoot through solid blocks or doors, Laser Bullets can pentrate and even destory blocks making them useful for Anti-Material Tasks or clearing out a tunnel when a pickaxe is not crafted.