You need a modded .html or .unpacked file. Do not use the vanilla Eaglercraft launcher. Search for "EaglercraftX 1.8 prepackaged with guns" (or download the modded client from community Discord servers like Eaglercraft Modding Hub ).
Developed primarily for the Eagler Forge 2.3.3 ecosystem, this script is the closest thing to a traditional Java gun mod. eaglercraft gun mod top
Eaglercraft is a web-based version of Minecraft 1.5.2 (and later 1.8.8) that runs via Javascript and TeaVM. Because it is not standard Java Minecraft, it cannot use traditional Forge or Fabric mods. "Gun mods" in Eaglercraft are typically or pre-configured custom clients that include weapons. You need a modded