Ammo slot ranged item restricts range slot ranged item
 

if the item in your Ammo slot has a lower range than the item in your Range slot, it restricts the range of the item in your Range slot when trying to throw it

example:

Forged Javelins (205 range) in Range slot
Summoned: Shuriken of Quellious (100 range) in Ammo slot

when trying to throw your Forged Javelins, they only throw within 100 range

it doesn't work the opposite way from what i've tested; you can't take a lower ranged item and exploit it to throw at a longer range