The weapons in .kkrieger have not been named, so the easiest way to reference them is by the key used to equip the weapon (as well as describe it).
1
The initial weapon you begin with, this rifle-like weapon shoots a slow arcing projectile. Although the firepower of this weapon is terrible, you'll soon find the next weapons to supersede its usage.
2
The upgrade of the basic rifle, you can hold 
down to fire shots rapidly, which is great against the larger enemies in the game. When attacking the basic, four legged walker you should tap the button twice so you don't waste ammo.
3
The third weapon is another rifle, except this time the arc of the projectiles is greater. This deals twice as much damage as the basic rifle, but cannot be fired quickly. It is great for spamming as the ammo is very numerous.
4
The first launcher weapon, it fires a slow arcing explosive projectile.
5
The second launcher weapon, it uses different ammo (although the ammo itself looks the same) than the first launcher, yet it performs just like the other. The main difference between the two is the design of the gun.