Gatling Gun focuses on long range shot with high rate of fire, good for attacking a single enemy. Explosive bullet explode at the end of its lifetime, gives damage to nearby enemy. Spray Gun has a short range but devastating damage to nearby enemy.
I also put an upgrade lines on the weapons. At level 10, gatling gun shoots multiple bullet while explosives explode twice.
I also redefine some effects on the scene. I use a line based effect for the explosion. It looks prettier than the point based but it doesn't match the theme of the game. It looks weird. But I'll decide later.
The next step would be converting the windowing system to SLD instead of GLUT. I had a few troubles with GLUT limitations and it's impossible to use this library for the next step.