Our firing of projectiles is overdetermined and ill defined: we specify the range, horizontal speed and gravity. However since in a parabolic trajectory the maximum range happens at angle pi/4, we can simple compute the 3rd from the other two. Dropping the speed, and keeping the others. Notice we should not drop the range, since that will make the configuration space difficult (not all combinations of the other two are possible), and range is used for many other things. Dropping gravity is in principle possible, but potentially makes projectiles move in different trajectory forms.
One can also consider adding a "max range angle". Defining at which angle the max range would happen. This is also physical, since the human body is not made for throwing javs at 45deg, usually 30-35deg is best (it varies from person to person). Not sure what an appropriate value for slings would be (archer sand siege would surely be at 45deg).
Reverts the gravity from, while leaving the nicer trajectories rP23144.