I'll take a que from Oskar Boesch, an Austrian, who volunteered for a Stormboken gruppe. He wanted to destroy bombers because of what he saw them do to the population below after experiencing the Bombing of Hamlin in Germany as a civilian would. He had the outer wing guns replaced by 30mm Mk 108, steel armour on both sides, bottom of his cockpit. Bullet proof glass on the sides of the canopy and thicker bulled proof glass on the 3 wind shield panels. Armour also protected the guns ammunition. The front of the cowling of the BMW801D2 was made of 6mm steal and essentially bullet proof. The toroidal oil cooler was safely behind this and pressure fed from the cooling fan. Armour was upgraded to 10mm on the BMW801TS more due to C of G reasons.
The usual approach was from the rear. The 13.2mm and 20mm guns would be fired first. The rear gunner position of a B-17/B24/B-26 etc was not protected from this calibre of guns and often fell silent during this part of the approach though it generally didn't matter as the twin 50 calibre guns lacked the power to stop the Fw 190 in time. Like the P-47 the Fw 190 often returned with several cylinder blown off and the Fw 190 had the additional protection of the cowling armour protecting the cylinder heads. It is of course at one level horrific: men dying, being burned, shot (one both sides) and other horrors below.
The 30mm guns would then be then be fired, these weapons on average could being down a 4 engine bomber with only 3-4 hits. Gun cam footage of the result of this gun shows tailplanes, main undercarriage and even compete engines falling of the wing with the engine still spinning.
For a while, upon signing up, (they were volunteers) these men were required to ram bombers should an attack fail, which was unlikely unless their 30mm guns jammed. The pilots resented this, not because they didn't want to ram a bomber but because they resented it being an order rather than something they wouldn't do naturally.
As the aircraft flew through the formation the armour protected the pilots life and limb from cross fire.
Had the B-29 been introduced into Europe they had better have put the 20mm guns back.
The weight of the armour, while not exorbitant, was enough to make these modified 190s vulnerable to escorts and they received their own escorts. As USAAF air superiority grew these types of aircraft had to be abandoned.
Otherwise they were extremely effective and I think its fair to say would have lead to prohibitive losses very quickly. Head on attacks were more likely to be conducted by unarmoured fighter variants of the Fw 190A or the Me 109.
Oskars story is told in one of the aviation magazines.