The equations are pretty straightfoward until you add drag in.
For the non-drag situation the point mass travels as in a parabola, and mass doesn't even enter into the equations:
x=(v_0 cos (theta))t
y=(v_0 sin (theta))t -0.5(g)(t)^2
x= distance traveled along the ground
v_0= initial velocity
theta= initial firing angle
t= time the projectile is in the air
g= acceleration due to gravity
You can iterate through this, or solve for the t intercepts in the y equation to see how long it will take the mass to land.
For the drag situation the point mass travels approximately in a parabola, but the math gets a little stickier, and you have to iterate and work in two dimensions. Any first year college physics text will have an example of this iteration.
In general, you can get a decent estimate of distance with a low profile item (like a Nerf dart) doing the calc without drag.
Now, you are asking why do my heavier darts seem to fly further if mass doesn't enter into the equation? It has to do with maintaining the center of mass forward of the center of pressure to eliminate the net torque on the body and keep the low drag nose facing forward. By adding mass you are bringing the center of gravity closer to the tip and further forward of the center of pressure. When the mass is too far back, the dart could spin out, or otherwise not keep the low drag nose pointed forward.
These pages do a much better job explaining it than I can:http://en.wikipedia....ter_of_pressurehttp://en.wikipedia....al_significancehttp://www.grc.nasa....lane/rktcp.html