Fortunately, you read it wrong. The range / area of ward off the dam is Self (60 foot cone). Referring to the rules for reading spell ranges, we read that:
Spells that create cones or effect lines that come from you also have a range of self, indicating that the origin of the spell's effect must be you.
You don't throw the ammo into the air and drop it like a cone from above; you are the origin of the cone which spreads directly from you. The scenario you should imagine is that as soon as the gun leaves your hands or the arrow / bolt leaves the bow (etc.), the extra copies conjure up and explode like a shotgun explosion. The requirement to launch or shoot "in the air" is used to indicate that you are not aiming at a specific creature or point (therefore you cannot hit a creature as a real attack nor cast the spell).
You can still be hurt by the spell if you wish; the rules for conical zones state that:
The origin point of a cone is not included in the cone's area of effect, unless you decide otherwise.
But it's entirely your choice, and obviously most launchers deign not to steer clear.