To this day, there are people who complain that the Army made a serious blunder in not adopting the .276 Pedersen cartridge and the T3E2 Garand rifle, instead choosing to stick with the .30-06 chambering for the M1 rifle. I agree that the Army missed a golden opportunity in the 1930s, but strongly dispute the idea that .276 Pedersen was the path that should have been taken. The .276 round would have offered only a few minor advantages, while incurring some significant drawbacks.
In that time period, there was an existing cartridge which was nearly ideal for the WWII blitzkrieg operations in Europe, and the jungle combat in the Pacific: The .30 Remington.
All the Army would need to have done is shorten the case a bit, load it with a 125-grain spitzer FMJ bullet, and they could've had the world's first "intermediate" rifle cartridge...a decade earlier than the Germans!
Such a .30 Remington Short would also have enabled Garand to design his rifle to be considerably lighter than the .276 and the .30-06 versions. In addition, it would've been far more amenable to use with a straight, 20-round, box magazine than the .276 cartridge, which has such extreme case taper that it would require a more expensive and difficult-to-manufacture curved magazine.
Very little was lost by not adopting .276 Pedersen. The .276 T3E2 Garand was only 12 ounces lighter and 1.5 inches shorter than the .30 M1 Garand. As for clip capacities of 10-rd vs 8-rd, I don't see a difference of two rounds as having any significant impact on combat effectiveness. Additionally, if the .276 Garand had been adopted, the US would've entered WWII with a 3-caliber system: .30-06 machine guns, .276 rifles, and .30 M1 carbines. In contrast, if the Garand had fired .30 Rem Short, its minimal size and weight means there would not have been a perceived need for the M1 carbine, because everyone would've had a short, lightweight assault rifle!
------------------- .30 M1 --- .276 T3E2 --- .30 M1AR
Unloaded Weight --- 9.5 lbs ---- 8.75 lbs ------ 6.5 lbs
Overall Length ----- 43.5 in ---- 42.0 in ------- 36.0 in
Ammo Capacity ----- 8 rds ----- 10 rds -------- 20 rds