You shouldn't shoot at all right b4 opening the slow door, if there is a guard there and you have to, limit it to a single AR33 burst on the way, not too close to the door (see Wouter's run).
The obj B area, all guards from that room should be dead if you placed the mines well (like on my run, when I turn around to look while the slow door is opening you can tell nobody is left), but a couple can always dodge it, plus I would assume it's mostly random whether extra guards from further back in the level will track you into the B room and see you on ur way back from the radio.
Lastly maybe you are shooting at the ZMG guard by the radio too quickly after the door opens? This would get the radio room guards over to your area a little quicker. You can see on my run I wait about 1-1.5 secs after getting through the door b4 I start shooting.