public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid)
{
    new Float:HP;
    GetPlayerHealth(playerid, HP);
    if(weaponid == /*weapon id*/) SetPlayerHealth(playerid, /*the player's HP - normally it follows "HP - x", where x is the ammount of damage taken by each shot*/);
    return 1;
}