Thank you for your nice and useful feedback!! I think your idea to change the bullet sprite to a diamond is really good so I changed it.
Is this what you meant? 
I also took a look at your work and I have to say it's amazing so I followed you :)
Viewing post in Triangulator
It’s not quite what I had in mind, but I think it would work as well!
I probably should’ve said “rhombus” instead of diamond, I was thinking about some of the bullet graphics in this game since the strong colored outlines had me thinking that way.