Originally Posted by The_Eric
The M1 was a really good rifle no doubt, but was originally spec'ed with a 20rd box mag. The army didn't want that because the troops would have wasted ammo.
No, it actually was spec'ed with a 10 round internal magazine of .276 Pedersen. A shorter 7mm round, much shorter than the lengthy .30-06.
The U.S. Army Chief of Staff Gen. Douglas MacArthur rejected the .276 Pedersen Garand in 1932 once he found out that it was technically feasible to build the M1 around the .30-06, which the Army already had in stock. The loss of two rounds of capacity was not considered important when viewed against the logistics/cost of two different kinds of ammunition during a time when the US Army had little money.
Internal magazines were thought to be less susceptible to the introduction of dirt/debris in the action than external box magazines.
The concern for wasting ammunition was legitimate when the Army first bought the .30-40 Krag-Jorgensen in 1892. The Krag was the transition from large-bore, black-powder calibers to modern smokeless ammunition and was the first Army rifle to have a magazine. Its predecessor, the Model 1884 Springfield, itself an improvement on the 1873 Springfield of the same caliber, known as the "Trap-door" was a breech-loaded, single shot rifle in .45-70 (.45 caliber over 70 grains of black powder).
At that time, most Army units were re-supplied by horses and mules and it took months for re-supply. The Krag was equipped with a magazine cut-off that allowed the rifle to operate as a single shot* bolt action and local commanders could choose whether the magazine cut off was to be placed in the on (bolt loads from the magazine) or off (bolt functions as a single shot). It was thought that a soldier in a firefight (e.g. Little Bighorn, fought just after the introduction of the 1873 Springfield) could always flip the switch and access the full 5 round capacity of the magazine.
That concern, and that feature, were carried over into the Model 1903 Springfield.
So, at the time of the Garand's conception, re-supply by rail was demonstrated in the battlefields of WW I, and while the Army was still concerned about wasting ammo, it was far less of a concern than it had been with the previous generations of infantry weapons. With no first hand experience using box magazines, the Army stuck with the tried-and-true internal magazine for the M1 Garand.
*One of the reasons that the Krag-Jorgensen was adopted over competing designs is that its box magazine could be topped off without pulling the bolt to the rear. This was thought to have a practical/tactical advantage in combat, as reloading would not leave a soldier unable to fire immediately, as it would when the bolt was drawn to the rear to top off a magazine.