This sounds counter intuitive. Please can you confirm its correct. You have to wait longer if Master arm is already on?
Before employing the AIM-7E Sparrow, a delay of four seconds should be given if the Master Arm switch is set in the ON position prior to radar lock on, or a delay of two seconds if the Master Arm switch is set to ON after radar lock on. This is due to the set-in period of the missile speed-gate. With the AIM-7F, this delay is reduced to two seconds with Master Arm switch On prior to lock, or immediately after selecting Master Arm switch to On if lock on was achieved first. Should these delays not be adhered to, the missile may fail to track because of improper target doppler injection.