I think I see where the confusion is coming from. OK, so .276 Pedersen immediately post-war basically did happen IRL, but it was .30 cal and they ended up adopting it as 7.62mm NATO. Here's an excerpt from my unpublished fifth installment of the Light Rifle series:
"In 1931, he (Studler) returned to the United States and was appointed Assistant Officer in Charge of Engineering Division of the Small Arms Ammunition Department at Frankford Arsenal, where the U.S. Army's new experimental .276 caliber ammunition was being developed. Studler therefore was present for the last year or two of development of the .276 caliber, including design and testing of .276 caliber armor piercing and tracer type bullets. The conclusions drawn from this program provide an important piece of the puzzle: Testing had shown that while the .276 caliber was an excellent fit for infantry rifles, it could not replace the .30-06 caliber in the machine gun, as it could not match the .30 caliber's performance with armor-piercing projectiles and other ammunition types. In fact, while the .276 produced excellent performance with lead-cored bullets, its low case capacity became a hindrance with larger, lower-density armor piercing projectiles, and performance was also reduced when loaded with certain kinds of commercial propellants, a serious downside in the event of commercial contract production during wartime. As a result, Frankford Arsenal began development in 1929 of a new round, called the .276 T2, which used a larger diameter (0.470" vs. 0.450"), longer case, which would alleviate these issues. By the time Studler arrived at Frankford, development of the T2 was thoroughly underway, and it continued until the .276 project's cancellation in the Summer of 1932. Largely due to the need to retain the .30 caliber at the machine gun level, the .276 was abandoned and the .30 caliber Garand rifle was adopted instead of its .276 caliber counterpart."
"Studler remained at Frankford until 1935, and continued developing improved armor-piercing ammunition for .30 caliber and .50 caliber weapons. During this time, he graduated from Assistant to Officer in Charge of the department, and presided over the development of the .30 cal. M1 Armor Piercing cartridge, a higher-velocity, higher-performance (but more complex to produce) predecessor to the M2 Armor Piercing round used in World War II. For his efforts in this, and .50 caliber armor-piercing development, Studler received a commendation."
It was Col. Studler who oversaw the end of the development of the .276 Pedersen in the early 1930s, which means he was intimately familiar with the program. He was also the manager for the .30 caliber armor piercing program of the same period, which was a type of projectile that the .276 had some issues with. By the time .276 development had finished, it no longer offered much of an advantage in ammunition capacity versus .30-06, as it shared the same case base. By 1943, Studler, now CSA for Ordnance, would notify the industry that the Army was investigating the .300 Savage as the basis for a new round. The .300 Savage, of course, was the commercial round most similar in size to the .276 T2 round, and in .30 caliber which would ease the development of AP and other styles of bullet. Notably, the case length and diameter specifications set for the .30 Light Rifle were virtually identical to that of the .276 T2 - this is likely not a coincidence.
I have very little doubt that Studler considered the .30 Light Rifle to be a "perfected" version of the .276 Pedersen he worked on in the early 1930s. It would also explain why he so quickly dismissed the British .280 proposal - in his mind, the Army had already "been there, done that".
"In 1931, he (Studler) returned to the United States and was appointed Assistant Officer in Charge of Engineering Division of the Small Arms Ammunition Department at Frankford Arsenal, where the U.S. Army's new experimental .276 caliber ammunition was being developed. Studler therefore was present for the last year or two of development of the .276 caliber, including design and testing of .276 caliber armor piercing and tracer type bullets. The conclusions drawn from this program provide an important piece of the puzzle: Testing had shown that while the .276 caliber was an excellent fit for infantry rifles, it could not replace the .30-06 caliber in the machine gun, as it could not match the .30 caliber's performance with armor-piercing projectiles and other ammunition types. In fact, while the .276 produced excellent performance with lead-cored bullets, its low case capacity became a hindrance with larger, lower-density armor piercing projectiles, and performance was also reduced when loaded with certain kinds of commercial propellants, a serious downside in the event of commercial contract production during wartime. As a result, Frankford Arsenal began development in 1929 of a new round, called the .276 T2, which used a larger diameter (0.470" vs. 0.450"), longer case, which would alleviate these issues. By the time Studler arrived at Frankford, development of the T2 was thoroughly underway, and it continued until the .276 project's cancellation in the Summer of 1932. Largely due to the need to retain the .30 caliber at the machine gun level, the .276 was abandoned and the .30 caliber Garand rifle was adopted instead of its .276 caliber counterpart."
"Studler remained at Frankford until 1935, and continued developing improved armor-piercing ammunition for .30 caliber and .50 caliber weapons. During this time, he graduated from Assistant to Officer in Charge of the department, and presided over the development of the .30 cal. M1 Armor Piercing cartridge, a higher-velocity, higher-performance (but more complex to produce) predecessor to the M2 Armor Piercing round used in World War II. For his efforts in this, and .50 caliber armor-piercing development, Studler received a commendation."
It was Col. Studler who oversaw the end of the development of the .276 Pedersen in the early 1930s, which means he was intimately familiar with the program. He was also the manager for the .30 caliber armor piercing program of the same period, which was a type of projectile that the .276 had some issues with. By the time .276 development had finished, it no longer offered much of an advantage in ammunition capacity versus .30-06, as it shared the same case base. By 1943, Studler, now CSA for Ordnance, would notify the industry that the Army was investigating the .300 Savage as the basis for a new round. The .300 Savage, of course, was the commercial round most similar in size to the .276 T2 round, and in .30 caliber which would ease the development of AP and other styles of bullet. Notably, the case length and diameter specifications set for the .30 Light Rifle were virtually identical to that of the .276 T2 - this is likely not a coincidence.
I have very little doubt that Studler considered the .30 Light Rifle to be a "perfected" version of the .276 Pedersen he worked on in the early 1930s. It would also explain why he so quickly dismissed the British .280 proposal - in his mind, the Army had already "been there, done that".