This is a topic that tests your grasp of several related disciplines,

Question:

Design a gun that has the same offensive capabilities of your regular field firearm, but instead of bullets, it uses water as ammo.

How do you propel water with the same destructive velocity of a bullet?

to avoid leaving solid evidence behind?

This wouldn't be easy to do, sounds easy but not. You need the metal to stay in a very well disperse form as metal is very dense compare to water and tends to sink to the bottom before the water get frozen. To achieve this, you need, extremely fast freezing which is not easily achieve and the process would be costly as you'd need to source out the correct chemicals to do so (something like liquid nitrogen?) and a very good insulating vessels.
The other method can be done is through solid-liquid colloidal suspension, which you need to source the correct surfactant for it and grinding of the metal to very fine dust, which...is more or less as difficult as the former method.
And ...

If the purpose of making such weapon is to eliminate any traces after using it, then adding any substance into the water to enhances it physical resistance to heat and impact would not be ideal as those substances would become uncommon and more likely lead to getting trace back after all plus the process of sourcing out the apparatus and chemicals to do such a weapon makes you even easier to get trace back, which is not ideal at all. Might as well use a normal gun.