Thank you so much!
Actually you can know if an enemy is out of your range or not. Hold "right-click" to aim, and then look at an enemy, if crosshair turns to red that means you can shot the enemy. But this system is hard to understand, so I'll update the game in a few hours to make it more easier. After the update, crosshair will always turn to red if an enemy is in range. Also the range of the weapons will be larger.
By the way, thanks for the projectile idea. I'll consider it.