Going to add some useful information for those who are participating in this: Those familiar with physics will know what this is, I will add a TL:DR at the bottom.
To maximize the range (distance traveled) of a projectile given some initial velocity, theta, muzzle speed initial and final s_y dimension (height), you maximize range by evaluating the longest time of travel (meaning, the object is in air for the longest time possible).
For this to occur, it depends on those above mentioned values, but let's simplify that a bit and make an easy assumption for those not too keen on the subject,
If you have a flat line and shoot at the given angles below
You can see that the one traveling the furthest distance is not fired too high, or too low, it's fired just right. This is defined as maximum range of a projectile (google if needed).
Simply put: Firing on a flat plane at a 45 degree angle will give you maximum range assuming everything else is constant (what can vary is how fast the grenades are shot out of the gun, if there's any changes in your height, the final height (maybe the zombie is up a hill)).
The mathematical proof of this is below
TLDR: Try to aim at a 45 degree angle from the ground, you will get the furthest range. Hope this helps, see you at the finish line.