the fundamental of abstract class. When are you use abstract class or use interface.

TheGunShoots.cs

            TheGunListener theGunListener = new TheGunListener();
            //shotGun is take out one projectiles per shoot
            Gun shotGun = new ShotGun("S12K", "Brown");
            shotGun.addProjectile(3);
            shotGun.setOnShootListener(theGunListener);

            //Deagle is take out two projectiles in one shoot
            Gun deagle = new Deagle("Deagle", "Red");
            deagle.addProjectile(3);
            deagle.setOnShootListener(theGunListener);


            Snipper snipper = new Snipper();

            snipper.setGun(shotGun);
            string info1 = snipper.getGun().getType(); // S12K
            Console.WriteLine(info1);
            string info2 = snipper.getGun().getColor(); // Brown
            Console.WriteLine(info2);
            snipper.shoot(); // Completed. The projectile was 2 
            snipper.shoot(); // Completed. The projectile was 1 
            snipper.shoot(); // Failed. Please reload 

            Console.WriteLine("================================= next gun >>>>>");

            snipper.setGun(deagle);
            string info3 = snipper.getGun().getType(); // Deagle
            Console.WriteLine(info3);
            string info4 = snipper.getGun().getColor(); // Red
            Console.WriteLine(info2);
            snipper.shoot(); // Completed. The projectile was 1 
            snipper.shoot(); // Failed. Please reload 
            snipper.shoot(); // Failed. Please reload 

https://gist.github.com/herisulistiyanto/48878ac7832119e3edadb54bd27d808e (in Kotlin)

Swift, https://gist.github.com/dhiyaulhaqZA/618d5b06d558b58bb7aa011508c85ec0