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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.

Just love those american .50 cal machineguns, makes german halftrucks cheddar cheese easily :)

Just hate those American HT, they die easily from German HMG :)

This excerpt from the Cmbb Manual confirms that ordinary bullets do have some penetration capability -
page136 - "Armor penetration for bullets (ie 12.7mm and under) reduced slightly [from Cmbo]"

Pic from an actual game, the German MG kills 2 halftracks, here it actually makes the second one blow up (note tracer), must have hit the gas tank!
It took several bursts at each vehicle over a couple of turns before they were knocked out.
Also, they were the weak M5 version, whereas the M5A1 version with a gunners shield usually last much longer..