Maybe this has been covered before, but I am a little confused.

Per the doctrine of the Armor Review board tanks were to attack bunkers and enemy fortifications and support infantry. They were not to battle other tanks. Tank destroyers were to battle other tanks.

The M4 medium tank weighed 35 tons, had a medium velocity 75mm gun suitable to engage the anticipated enemy fortifications, medium armor thickness and generally good mobility with a top speed of approx. 30 mph. Yes, at the time it was introduced it was as good as anything else out there, except the T34.

Tank destroyers were supposed to be lighter and faster, but more heavily armed. The M10 had a 3" gun (shortened for weight or balance problems or both, reducing the velocity of the round fired) with a higher velocity than the M4's 75mm gun, weighed 33 tons and had a top speed of approx. 30 mph. It was based on the M4A2 and used a diesel engine. So it was slightly better armed than the M4, but had much less armor protection. Speed and mobility appear to be the same. This is all based on information I found online.

In 1943, the Army assigned the M10 to the Pacific theater specifically so that the 3" gun, with it's greater penetrating power, could better engage Japanese pillboxes and other fortifications. That would mean the 75mm gun of the M4 was inadequate as an assault gun. The M10 was employed in this manner at Kwajalien, Bougainville, and other locations. This was stated in the US Army in WWII, Pacific Campaign.

The Armor Review Board knew the M4 and the 75mm gun were inadequate after 1942. They knew that their tank destroyer plan was flawed as well. General Devers claimed he was trying to resolve the problem but the Armor Development board and the ordinance board were not cooperating. Why did this happen? Is the same thing going on today with new projects?

One of the main problems was the Chief of the Army Ground Forces, Gen. McNair. He was an artilleryman and fully accepted the age old stance that Artillery is "King of Battle". He wanted artillery (i.e. anti-tank artillery) to have the responsibilty for destroying tanks.

There was plenty of inter branch rivalry. It was just a few years earlier where only infantry could have tanks. Cavalry could only have "combat cars", which were just the same tanks the infantry had but machine gun equipped only.

The 3" gun was also known as McNair's Folly. I believe they tried to adapt a naval 3" gun to be used as an anti-tank gun. There is even a towed version of it and it's successor, the 90mm gun that was eventually mounted to the Pershing. McNair stuck to his guns that towed artillery was the king of battle. It is inherently more accurate, but in terms of manuever, he was still a WW1 general fighting WW2's mobile battlefields.

There is very little inter branch rivalry that goes on today that isn't just good natured ribbing. There is and always has been some interservice rivalry, mainly between the Army and the Air Force. It usually involves developing some sort of recon or logistics fixed wing ability the Army needs and wants, but the Air Force usually puts a stop to it since they are the fixed wing proponent for the services.

The Air Force normally states that their (add aircraft name here) can do everything the Army thinks it needs a fixed wing aircraft to do. However, the Air Force normally gives low priority to Army support and the Army doesn't get the responsiveness from the Air Force that they would get from an Army owned fixed wing platform.

The reasons were many and varied. One was the ruthless concentration on mass production. Only changes deemed vital were alllowed to be made. Another was that the Army brass were not paying attention to the gun/armor race on the eastern front. The British were a little more realistic, hence the 17 pounder gun. As late as D day, Army brass thought the 75mm Sherman could handle anything on the battlefield except the German heavy tanks. The brass thought the Panther was just another low-production heavy like the Tiger. McNair never liked the M10, he settled for it since it could be put into production quickly and all the other choices at that time were failures. You could say it was the lesser of two weevils.

well ill take a jab at it, the m4 sherman 75mm variant was believed to have a more explosive gun that the 76mm on the 76 mm sherman and the m10s, but it had less Armor piercing ability compared to the 76mm. So they tended to put in a mix of 75mm and 76mm together.In my opinion there was too little cooperation , you can only please so much people. in one of the sherman books ive read, it said that by the time the panther tank was encountered in italy, it was too late to have a impact on the tank program or something like that. wasnt mcnair that guy who kept pushing the m18 hellcat as the preffred tank destroyer?

To add to the previous information, the M10 cost only two thirds as much as an M4, so there was an institutional desire to keep it in production even after it was found wanting (the M18 was much nearer to what the theorists had had in mind for a tank destroyer, but by the time it appeared, its gun was already outclassed, and its armor was even weaker than the M10's).
In practice, the tank destroyer doctrine was nonsense. In Europe, each infantry division was assigned a tank battalion to support the infantry, and a tank destroyer battalion to counter any German armored thrusts. However, the divisional commander could hardly be expected to hold half of his armor assets permanently in reserve against a hypothetical German armored strike, so the tank destroyers and tanks ended up being used identically to support the infantry in their advance. In this role, the M10 was more vulnerable to infantry attack, as it had no machine gun under armor, and no roof on the turret (the commander had to dismount from the turret and stand unprotected on the engine deck to fire the Browning .50" machine gun, unless they repositioned the pintle to the front of the turret). Since the Allies had air superiority, the Germans rarely concentrated their armor in the final year of the war, so a tank or tank destroyer battalion was equally likely to encounter panzers, but they were rarely seen in more than platoon strength.

The HE round for the 75 was better than the 76mm/3" HE, and there was a WP round for the 75.
The big difference was in training. M4 crews were trained for infantry support, M10 (and M18 and M36 crews) were trained to kill tanks. Read "The Tank Killers" by Harry Yeide, it covers the entire history of the U.S. TD Force.


Sending the M10s and M18s to the Pacific was silly. The Japanese had no tanks worthy of them. Hell, the Stuart was better than most Japanese tanks! Since the TDs were usually used for support fire there, why not replace the 3" gun with a 105? Perfect for the mission they had in the Pacific.


JMO

Although the M4 series of tanks was used for everything from "Infantry Support" (infantry support tanks usually are designed wth very heavy frontal armor, we all know most Shermans did not have this feature) to "Tank on Tank", it was designed as a "Break Out" or "Exploitation" tank. Once the enemy line had been breached by artillary and infantry, it was the Shermans that were to "exploit' the hole and race for Berlin hence the emphasis on reliability and mobility in the Sherman design (can't exploit very well if you are broken down). The Sherman shined in this role as proven by Patton's 3rd Army. The 75, as mentioned earlier had an excellent HE round and some units refused to give up their 75mm armed tanks when the 76mm armed tank became available. They knew their chances of meeting dug in infantry and AT weapons were far greater than meeting up with "Tiger" tanks. The 75mm gun was also capable of handling the most prevailent German armor, Stugs and Pz. IVs. Alot of units had a mix of both 76mm and 75mm armed tanks. It has often been said the Sherman was a "flawed" tank but it was anything but when used as intended. It was when it was used for roles it wasn't designed for such as taking on Tigers and Panthers (which coincedently were designed after the Sherman) that it was found wanting. Just my 2 cents.

Roy

Thanks for the replies and information. With the breakthrough concept, that would, I guess, explain why there were plans to produce so many of the 105mm gun version.