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I'm a retired artilleryman by trade, so I know a bit about the actual effect of cannon shells. History actually tells us that aircraft cannon used against ground or naval targets were largely inneffective, and the 75mm is particularly puny. An aircraft can't carry the weight of a cannon, or absorb the recoil, to fire a shell large enough to really be effective.
If you want examples of what it takes and the limitations of cannon, read anything about employing submarine guns against naval targets. It sometimes took hundreds of hits by cannon far larger than any aircraft could carry to sink a ship. U.S. subs with 3 or 4 inch naval cannon were replaced by 5 inch cannon late in the war, but the whole idea of naval gunnery for subs was scrapped right after the war, due largely to ineffectivness.
Likewise, as good as 75mm B-25s were supposed to have been, why were they scrapped even before the war was over, and replaced with additional .50 Cal MGs?
The idea of aircraft cannon sounds cool, but it never really panned out. Otherwise, we'd have seen many more cannon armed aircraft.
As far as I'm concerned the cannon did a real fine job the way we used it....It was very effective against shipping....We found that the cannon was more precise against some targets than bombing...
Yes they were abandoned by mid-44...there wasn't really anything left to shoot at with the 75..."Combat sorties confirmed that the cannon armed B-25H offered no particular advantage over specially adapted strafers armed exclusively with machineguns. At this stage in the war, targets specifically suited for cannon attack were few and far between, and many targets that were vulnerable to cannon were also vulnerable to a battery of 0.50-inch machineguns or to bombs. Consequently, the use of heavy cannon was generally abandoned in the South-West Pacific by August of 1944."
Much as there might be arguments about the effectiveness of the 75mm the fact is that a 14lb high velocity high explosive shell is still an effective weapon against many targets; unlike the unguided rockets that had steeply curved trajectories, a flat trajectory 75mm shell could be aimed to hit a target from a distance. The opinions of a veteran who actually used the 75mm armed B-25 in combat should count for something (Jerry Scutts B-25 Mitchell at War, pages 48-51):
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The fact is the 75mm was more effective than muskeg and tomo are alluding to;
the limiting factor, and the reason the 75mm was eventually dropped from the B-25s, was the damage done to the airframe, something which the purpose-built installation of the XA-38 would have helped alleviate.
Attached is a really interesting article from an olde but goode November 1978 Airpower (16 MB):
View attachment 257368
BTW, the Tsetse Mosquitoes stayed operational until VE-Day - they were not rejected.
What this mostly says is that the B-25s had run out of targets, which is surely an indication that the gunships had run themselves out of business because they were so effective: that "many targets that were vulnerable to cannon were also vulnerable to machineguns or to bombs" is a given - how many targets were more vulnerable to 75mm cannon when MGs wouldn't suffice and bombs were too inaccurate?
Don't think I'd like to be on the receiving end...
Yes they were abandoned by mid-44...there wasn't really anything left to shoot at with the 75...
Plenty of stories by the B-25J and B-25H crewmen who appreciated the accuracy and punch of the 75. However, the recoil of the cannon was tearing the ships up.
You can hose a transport all day with .50 cal and all you're going to do is sweep the decks clear of personnel, but hit it at the waterline or the control structure with a 75mm and you've just scored a kill. Also, .50 cal doesn't do much against bridges and hardened structures.
The 75mm cannon used in the B-25G and Hs were adapted from Army ground weapons. It doesn't matter if the cannon is fired from the ground or the air, the same laws of physics apply, particularly if they are firing the same ammunition. Just because a they are fired from an aerial platform, doesn't magically transform a relatively light 75mm cannon shell into a ship killer, not if the ship is of any size. This is barring a lucky shot that may ignite fuel or ammunition. Yes, the 75mm had some valid applications, such as taking out light skinned targets and antipersonnel work, but wouldn't bombs or multiple heavy MGs or 20mm cannon work better?
I think you'd be surprised at how many rounds of ammo are necessary to effectively engage a target, and the larger and more protected a target is, the more and heavier rounds are needed. From the info in the historical links above it seems that at best aircrews might hope to get off 2-3 shots on their target runs, and these shots weren't taken while sitting behind a computer screen.
Final note, and this applies to using an aerial cannon as a bunker buster. Just prior to commencing ground operations in the First Gulf War, the Artillery Center at Ft.Sill, OK ran a series of tests to determine cannon artillery effects against the types of bunkers our forces expected to encounter in Iraq. It quickly became evident that 105mm projectiles (34lb, and exactly what the AC-130 uses) were completely ineffective, no matter how many hits were attained. The 155mm projectile (95 lb) was only marginally effective and only after a large number of hits, sometimes a hundred or more. Only our largest cannon, the 8 inch M110A1 howitzer, firing a 200 pound projectile really could be called a bunker buster, and it would take a C-5 to get this baby in the air.
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When they think of replacing artillery with aircraft, they think about bombs, not cannons.
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