From my experiences flying the CFS3 Mosquito FB.VI in a number of different campaign mission types,
it seems like .303 machineguns are completely useless against ground targets of any sort; Whilst I'd expect
them to be useless against armoured vehicles, even light ones (though maybe open-topped halftracks would be
vulnerable to attack from above), surely "soft" targets like trucks and staff cars, fuel tanks, tents, supply crates etc would be
vulnerable? However the only target of this type I've destroyed is a tent, after hitting it with several hundred
rounds (most of which definitely hit). Fuel tanks seem totally immune to .30 cal fire.
.50 cal MGs seem to act more or less like you'd expect - they're very effective against soft targets, can take out
light armour with concentrated fire, but tend to just bounce off tanks.

On a more positive note the effectiveness of cannons vs. armoured targets seems to be modelled very well. Tanks are
virtually immune to 20mm fire unless you hit them from behind or above with a concentrated barrage, and naval ships
can shrug off a huge amount of damage when hit in the heavily-armoured sides, even with 57 or 75mm cannons, but a lucky
shot through the deck can sink them with one or two hits.