An interesting alternative is rounds fired to achieve a claim (destroyed, probably destroyed, damaged) on an enemy aircraft. The 303rd Bombardment Group flew 364 missions, so a reduced sample of 25 will have to suffice here.
| 303rd BG Mission No. | Aircraft Claimed | Rounds Fired | Rounds per Claim |
|---|---|---|---|
| 1 | 0 | 0 | 0 |
| 2 | 1-4-3 | 12,138 | 1,517.25 |
| 3 | 0 | 2,395 | 0 |
| 4 | 2-1-2 | 5,574 | 1,114.8 |
| 5 | 0 | 6,456 | 0 |
| 6 | 2-2-1 | 10,359 | 2,071.8 |
| 7 | 15-15-0 | 20,701 | 690 |
| 8 | 4-1-0 | 8,525 | 1,705 |
| 9 | 12-6-2 | 6,552 | 327.6 |
| 10 | 0 | 11,667 | 0 |
| 11 | 3-5-3 | 26,817 | 2,437.9 |
| 12 | 1-0-1 | 14,142 | 7,071 |
| 13 | 0 | 0 | 0 |
| 14 | 8-1-2 | 20,991 | 1,908 |
| 15 | 1-0-1 | 8,748 | 4,374 |
| 16 | 4-1-1 | 30,119 | 5,019 |
| 17 | 0 | 20,500 | 0 |
| 18 | 0 | 9,440 | 0 |
| 19 | 0 | 13,621 | 0 |
| 20 | 0 | 10,650 | 0 |
| 21 | 4-3-1 | 23,243 | 2,905.4 |
| 22 | 0 | 1,096 | 0 |
| 23 | 0 | 5,535 | 0 |
| 24 | 8-5-3 | 45,000 | 2,812.5 |
| 25 | 3-0-0 | 38,367 | 12,789 |
The ammunition capacity of each of the B-17's .50 caliber AN/M2 machine guns and their higher rate of fire (750-850 rounds per minute) when compared with the ground-based version of the weapon (450-600 rounds per minute) only allowed about a minute or so of firing in total for missions that could routinely last upwards of six hours. Extra ammunition in heavy wooden boxes was carried sparingly and carefully due to the need to maintain proper weight distribution in the aircraft during flight. As an alternative, ammunition was often shifted between guns during lulls in enemy attacks.
B-17F:
| Turret | Ammunition carried |
|---|---|
| Nose (as built; single .30 caliber machine gun) | 300 rounds |
| Dorsal | 800 rounds (400 per gun) |
| Radio Room | 250 rounds |
| Ball | 1,000 rounds (500 per gun) |
| Left Waist | 300 rounds |
| Right Waist | 300 rounds |
| Tail | 1,130 rounds (565 per gun) |
B-17G:
| Turret | Ammunition carried |
|---|---|
| Chin | 730 rounds (365 per gun) |
| Left Nose | 305 rounds |
| Right Nose | 305 rounds |
| Dorsal | 800 rounds (400 per gun) |
| Radio Room (later deleted) | 250 rounds |
| Ball | 1,000 rounds (500 per gun) |
| Left Waist | 600 rounds |
| Right Waist | 600 rounds |
| Tail | 1,130 rounds (565 per gun) |
Claims by B-17 gunners were often overstated due to multiple gunners claiming the same aircraft, or claiming an aircraft as destroyed or probably destroyed when it was only just damaged. During the Schweinfurt-Regensburg mission of August 17, 1943, American and British fighter and bomber crews claimed 319 total German aircraft destroyed, when the Luftwaffe reported that only 26-27 of their own aircraft had actually been lost.
Even if defensive gunners on American heavy bombers weren't particularly accurate, they did create a hailstorm of fire that influenced German anti-bomber tactics. A German nickname for the B-17 was Fliegendes Stachelschwein, or "Flying Porcupine." Rifle-caliber machine guns and light cannons gave way to unguided air-to-air rockets and larger, longer-ranged cannons as German fighters attempted to avoid exposing themselves to American bomber formations as much as possible.
Sources:
Green, William. Famous Bombers of the Second World War, Volume One. New York: Doubleday, 1959.
Zaloga, Steven. Operation Pointblank 1944: Defeating the Luftwaffe. Oxford: Osprey Publishing, 2011.