The .30-06 cartridge continued to evolve in U.S. military use after the period described by minnow's excellent writeup above. When it was first introduced into military service, the cartridge was designated the M1906. This served up until the mid 1920s. At that point, several U.S. machine guns had been deployed which used the .30-06; however, it was found that the range of the M1906, fired from machine guns, was short of most other nations' frontline machine guns. As a result, a new variant of the cartridge was developed which had a different bullet. The new round, designated M1 Ball, had a boat tailed spitzer bullet of 174 grains, which fired at approximately 2,700 feet per second. The improved aerodynamics and increased weight of the bullet pushed the maximum range of the round out to approximately 5,000 yards. However, the rapid firing of the new round in machine guns also exposed the fact that the jacket metal, which was the same as on the M1906, a mixture of nickel and copper, was found to cause bore fouling. Accordingly, a new 'gilding metal' was used to jacket the bullet - a mixture of copper and zinc - which reduced fouling significantly.
As new stocks of M1 Ball were built up, older M1906 rounds were preferentially used for military training. Eventually, in the 1930s, M1 Ball began to be issued for training purposes, and it was discovered that the M1 Ball's long range was in fact too long for use on most training ranges. As training ammunition consumption began to rise with the introduction of the semi-automatic M1 Garand, a new round was ordered.
The M2 Ball round lowered the bullet weight to 152 grains (almost back to the 150 of the original M1906) and reverted to a flat base bullet rather than the boat tail. Although it had a higher muzzle velocity of approximately 2,810 feet per second, these changes reduced the maximum range of the bullet from 5,000 to approximately 3,500 yards, which was deemed an improvement.
World War II was fought with the M2 Ball round and its variants (tracer, armor piercing, incendiary). Surplus ammunition from this period can still be found and purchased on the open market, although for shooting, it is better to seek out later production years of M2 Ball (after approximately 1952) as the military switched from corrosive primers to non-corrosive chemicals around that time, and it is easier on your rifle. To determine the headstamps of 'safe' ammunition, see this guide at the Civilian Marksmanship Program.
Sources:
Thompson, Leroy. The M1 Garand. Osprey Publishing: Oxford UK, 2012.
Johnson, Melvin Maynard and Haven, Charles Tower. Ammunition: its history, development and use, 1600 to 1943. William Morrow: New York, 1943. p. 109.