dropbox.com/s/0ru37dkgyhqvih9/watergunparticles.capx
Here is an old example I had.
More information might be useful. Is your project top down or side view? Are you already using physics?
Depending on your art style and context you are using it in and how large the effect is, it may be better to use an animated sprite. Without knowing more about how you plan on using it, it is hard to be able to say which way is best.
Funny, I already had that .capx file. However, I didn't even notice the idea of applying gravity to the water bullets to create the effect. It was still very helpful and I was able to make adjustments to pretty much get the effect that I wanted. Thanks again.