MG's can kill armour

[PoorOldSpike]

It's difficult to pin down in live-fire tests which MG's can kill armour and which can't, but in most cases the gun crew will open fire on a tank without you having to order them to, because they know better than us whether their weapon can hurt it.
Needless to say they'll only fire at light armour, although they'll also spray heavier tanks if the commander is exposed in the hope of nailing him.
You can issue fire orders against buttoned armour yourself, but the crew will usually cease fire after just one or two bursts if they know their gun can't penetrate.