Greetings, I am a new member and have a question, not really a response to this thread. Can anyone tell me why the US used .50 caliber machine guns to defend the heavy Bombers like the 17 and 24. Why didn't it occur to anyone to use cannons with exploding shells like flak?
Hi, and welcome to the site.
There are a host of considerations, including weight and space and the feasibility of aiming and, as you've mentioned, fusing, as to why a flak style weapon wasn't considered for a defensive armament.
Weight: A fully-powered Sperry ball turret on a B-17 with 2 .50 cals and 250 rounds per gun weighed a little under 1200 lbs, gunner included.
The smallest US flak gun with an exploding shell was the Bofors 40 mm. A single, unpowered mount weighed about 2,440 lbs. A mount with a full power traverse and range finder set-up weighed about 4,200 lbs, not including ammunition or the five to six men it took to properly operate the thing.
Space: A B-17 fuselage is a maximum of 3.5 m (11 1/2 ft) in diameter). A 40 mm Bofors is 3.8 m (12 ft) long, without including the mount. Getting the 5-6 men necessary to operate it into a cramped bomber fuselage is only going to work for 1 gun, maximum.
Then why didnt they simply use grapeshot like old Naval cannons. 500 MPH ME-109's can't outrun huge ammounts of steel ball bearings traveling at 1000 feet per second. Seems to me the Bomber Crews would have been so much more sucessful in self defense if the Gunners simply had better guns firing huge ammounts of grape shot or shrapnel.
A 'grapeshot' round has a very limited effective range (depending on a number of considerations like shell design and firing pattern). Firing a shotgun/canister-like round, even from a large calibre weapon, is going to limit the size of the defensive fire envelope, leaving US crews unable to return fire effectively.
Even a modern 120 mm canister round has an effective range of only 600-700 There is a reason why shotguns are considered a close quarters weapon.
As German fighters got heavier weapons (like the various 30mm cannon) the range they fired from increased. By 1944, the average German fighter pilot began firing at about 900 m out from their bomber target.
1000 fps sounds like a lot, but an M2 round left the barrel at around three times that velocity.