Read this:
https://forums.alliedmods.net/showthread.php?t=331064
The base plugin you need (Weapon Handling):
https://forums.alliedmods.net/showthread.php?p=2674761
Code:
#include <sourcemod>
#include <WeaponHandling>
public void WH_OnReloadModifier(int client, int weapon, L4D2WeaponType weapontype, float &speedmodifier)
{
switch (weapontype)
{
case L4D2WeaponType_Pumpshotgun, L4D2WeaponType_PumpshotgunChrome: speedmodifier = speedmodifier * 1.25; // 25% faster reload
case L4D2WeaponType_Autoshotgun, L4D2WeaponType_AutoshotgunSpas: speedmodifier = speedmodifier * 1.25; // 25% faster reload
}
}
Compile this simple code by yourself if you know how to do it. Use weaponhandling.inc included inside the Weapon Handling plugin. Change
1.25; // 25% faster reload to the value you desired.