Try it, it's for double damage:
PHP Code:
#include <amxmodx>
#include <hamsandwich>
new cvar_dmg;
public plugin_init()
{
RegisterHam(Ham_TakeDamage, "player", "fw_TakeDamage");
cvar_dmg = register_cvar("zp_dmg", "2");
}
public fw_TakeDamage(victim, inflictor, attacker, Float:damage)
{
if (get_user_weapon(attacker) == CSW_M4A1)
{
SetHamParamFloat(4, damage * get_pcvar_float(cvar_dmg))
}
}