I also personally think that if we had also used the .280 Pederson round we would have been even better off.
I've heard it repeated more than once that the Garand would've been a better rifle had it been chambered in the .276 Pedersen round that it was designed for originally.
Forgive me if this is a stupid question, but why?
Look at the dimensions of the .276 Pedersen cartridge. It's 7x51mm. Due to powder limitations of the day, this cartridge was
not compact like the current crop of six-point-whatevers. It was very nearly the same external dimensions of a .308 Winchester round.
The weight difference, therefore, would've been insignificant, in my opinion.
Ballistically, it was a 150 grain slug at about 2,400 feet per second. That's better than 7.62x39, and is very nearly identical to the .30-30WCF round. .30-30 isn't a barn burner, but it's no slouch either. (That muzzle velocity is also about what you'll get, with a 150 grain bullet, from a .308 with a 12-13" barrel.)
Now, the .276 Garand would've had two more rounds in the magazine, so that's a plus. It also would've had less recoil, also a plus. But the rifle itself wouldn't have been any shorter, lighter, or handier. The ammunition would've been lighter, but probably not enough to notice (if you can tell the weight difference between a box of .30-06 and a box of .308 without looking, then you'd notice the difference here).
On the flipside, the rifle would've had less range, penetration, and wouldn't have had the commonality of sharing the ammunition with the machine guns, which would've complicated things on the logistics end (though that's not necessarily a deal-breaker either).
Personally, I'm unconvinced that the .276 Garand would've been a significant improvement over the one that was issued. Am I wrong?