[Berky]

Code:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate = PLAYER_STATE_DRIVER || newstate == PLAYER_STATE_PASSENGER)
    {
          if(GetPlayerWeapon(playerid)) != 29) SetPlayerArmedWeapon(playerid, 0);
    }
return 1;
}

Better fix if you want to make the MP5's allowed in a vehicle as passenger.