The 75mm gun on the M4 was woefully inadequate to facing either a Tiger or a Panther frontally. So by late June the cry went out for the M4(76)s in the UK to be shipped to France for issue to the medium battalions.
Unfortunately, the new up-gunned Shermans did little to redress the problem. Despite what was shown on Ordnance Department tables, and despite what the tests in the UK had shown, reports from US tank units in the field indicated that the new 76mm guns were no better than the old 75mm guns when it came to facing Panthers and Tigers. Something was wrong.
The experiences of the British, however, seemed to justify their confidence in the 17pdr. Their up-gunned Sherman “Firefly” tanks were indeed apparently capable of handling Tigers and Panthers.
nstrate this, an impromptu test firing was conducted at Balleroy, in Normandy, against a captured Panther tank. US First Army was provided with a British 17pdr, with new APDS (discarding sabot) ammunition. There was no formal report or minutes written from this test firing. However photos of the results were routed through US field commanders in Normandy. The results were summarized in a subsequent report:
… in firing conducted by First U.S. Army at Balleroy on 10 July 44, 5 rounds were fired at the front plate of a Panther tank at 700 yards. Examination of pictures of this firing indicates that the first round struck the mantlet, the second between the track and the nose plate, the third at the junction of the nose and glacis and penetrated. The fourth and fifth were fair hits on the glacis and both penetrated.
In contrast to this, field reports indicated that the 76mm gun was failing to penetrate the Panther’s glacis under any circumstances.
A sense of betrayal quickly developed. US tankers were surprised by the poor performance of the US 75mm gun against the panzers, and even more surprised when the new “hot” 76mm gun did no better.
What was not clearly known at the time was that there was a flaw in the nature of US testing. US Army Ordnance had full belief that ductile armour made safer protection. What this means is that US tanks were made of relatively soft and flexible armour. 240 BHN was the standard for US rolled homogenous plate, while US cast armour, such as found on the M4’s turret and the hull of the M4A1, was often as soft as 210 or 220 BHN. This compares with German armor which ranged from about 260 BHN for their thickest plates, to over 340 BHN for thin armor. The 50mm on the front of the MkIV was face-hardened to 588BHN with 365 homogenous behind it, making it far tougher than the test plate from Shoeburyness.
The benefits of softer more ductile plate are two-fold. First, it is easier to work with in production. Second, when struck the armor tends to bend and flow rather than crack or shatter, and when a round does penetrate there will typically be less spalling and fragmentation carried into the interior of the tank with the projectile. The downside is that the armour will be less resistant to penetration in the first place.
Now, tankers will generally take exception to the notion that a government engineer somewhere is willing to see holes punched in their steeds so long as they don’t get too many “friendly” fragments coming into their crowded office space along with the several pounds of white hot metal travelling at thousands of feet per second that the enemy kindly provides with the new ventilation. Ease of production is even lower on the crewman's priority list. Indeed, the overwhelming view is more that the armour is generally there to keep things out, not to let them in with minimal added disturbance.
In fairness it is worth noting that Ordnance’s concept of minimizing the after-affects of perforation may well have achieved its stated goals, as was demonstrated by the relatively low casualty rates among crews in US tanks which were destroyed in action:
During the period of 6 June through 30 November, 1944, the US First Army suffered a total of 506 tanks knocked-out in combat (counting both those written-off and reparable). Of these 506 cases, in 104 cases there were no casualties associated with the loss of the tank. In 50 cases the casualties were not recorded. Out of the remaining 352 cases there were 129 KIA (0.37 per tank) and 280 WIA (0.80 per tank), for a total average rate of 1.16 casualty per tank lost in combat.
How much this is due to design aspects such as ammunition storage or escape hatches, and how much is due to the ductility of the armor, can not be assessed. And how many more tanks were destroyed in action in the first place, because of their softer armor, is up for anyone’s guess. There is certainly an argument to be said that a Panther's long 7.5cm would go through an M4 even if it had harder armour, so perhaps the engineers in hindsight made the better call, but we could go into what-ifs for ever. I digress.
What can be observed, however, is that US Ordnance testing was done on US Ordnance armour plates. The Germans, being the evil underhanded fiends that they were, did not use US Ordnance plates on their Panther and Tiger tanks. They used German plates, which were notably harder. So despite Ordnance test results and assurances, the weapons available to the US Army forces in Normandy could not reliably penetrate the frontal armor of the heavier German panzers.
The Panther was becoming a real problem for US commanders. Yet it seemed that the British had working solutions.
worldoftanks.com