Final answer:
The ammunition used for the US service rifle designed during WWI was the .30-06 round. The .30-06 became popular among civilians due to postwar surplus. The significance of automatic weapons like machine guns grew as the war progressed.
Step-by-step explanation:
The ammunition of the Service Carbine, specifically for the rifle that was developed to replace the 1903 Springfield rifle during the time of America's involvement in World War I, was the .30-06 round. This new service rifle, which incorporated elements of the British Enfield rifle, featured a five-round magazine. The widespread adoption of the .30-06 cartridge among hunters and sportsmen was facilitated by the government allowing veterans to keep their rifles postwar, creating a surplus that influenced civilian markets.
The impact of automatic weapons, such as the machine gun developed by an American named Hiram Maxim, was reconsidered during the war. Initially, military leaders criticized the wastefulness of these weapons, but the static nature of trench warfare quickly highlighted their efficacy, leading to a dramatic increase in their use. By the war's end, nations like France had gone from having a few thousand machine guns to amassing over 300,000, indicative of the shift in military tactics and recognition of the power of automatic weaponry on World War I battlefields.