Soren
one could not count that pilot 's back armour in 109G keep .5 API out, one case when the API went through the back armour and killed the pilot which immediately come into mind is that of L. Otto Fönnekold, 109G-6, WNr. 441931, 31.8.44 at Budak. Only one case but I don't have info on very many cases in which 109's back armour was hit by .5 Browning bullets. Other cases of hits which immediately came into mind are one in which the back armour withstood a hit by a Soviet 20mm shell and one in which the stack of aluminium sheets with combined thickness of 22mm, one version of 109G back armour, slowed 2 Soviet 20mm shells or their fragments enough that they stopped inside the fuel tank.
Juha
Hmm, test show that you could pretty much rely on that back armor in the 109F/G. The composition was a 8mm back plate, and a 10mm head plate (later variants 10mm head plate with a 60mm armored glass in it). Under the pilots butt there was two pieces of 4mm plates.
Behind the pilot there was the fuel tank, through which the bullet has to pass through, and slow somewhat further down, and behind the fuel tank there was a large 30 layer dural plate of ca. 22mm thickness as you describe.
This dural layer was effective in stopping incendinary rounds on its own:
Aircraft Evaluation Report, GERMAN MESSERSCHMITT-109 F, prepered by Materiel Command, Engineering Division, Wright Field, Dayton, Ohio, summarized the results of British firing trials on the effectiveness of the Leichtmetallpanzerung :
"Additional armor has been found on "109-F" airplanes recently inspected. This is in the form of a laminated dural bulkhead placed approximately 6 inches [152 mm] behind the tank. Total thickness of the thirty laminations used is about 7/8 inches [22,25mm].
The British have carried out tests on one of these bulkheads with the following results : from 100 yards range dead astern, .303 inch and .5 inch incendiary ammunition will not penetrate the dural bulkhead. At this range 20 mm HE/I(high explosive / incendiary ) is still effective in penetrating the bulkhead and ingniting the tank. At 200 yards range, the effect of the interposition of the dural bulkhead is that at 5 degrees off dead astern, .303 A.P. (armor piercing) is completely ineffective against the pilot, but .5 AP in about 30 percent of cases will pass through the bulkhead and will penetrate the 8mm pilot`s armor, even if it has to pass through the tank below fuel level.* Twenty mm A.P. will still be effective in penetrating the pilot`s armor. Subsequent tests reveal that U.S. .50 caliber M1 incendinary ammunition is capable of penetrating the dural plate effectively."
* This is IIRC a mistranscript from the British report, which stated that in 30% of the cases of hits where the the projectile passed above the fuel line in the test setup to .50 AP could penetrate, but in 70% it couldn't, neither could hits that passed through the fuel tank.
The layered "Leichtmetallpanzerung" was appearantly removed though on late Gs and Ks because space was needed for the 115 liter MW50 booster tank (though some drawings show its still present..), though the light alloy tank and the liquid in it must have offered some ballistic protection as well.
Overall, I would say the pilot was reasonable safe in the Bf 109G from attacks with .50 API from astern at typical combat ranges.