
 > (1)  What calibre M-16 was used during the early 1970's by the US Army;

5.56 mm, also known as the .223 Remington cartridge.

 > (2) is/was there a 7.62 caliber M-16?

For the full-auto M-16, I'm not sure. But for the AR-15 semi-auto and its
clones, yes there is a version chambered for 7.62x39, and a 9mm, and a .22.

7.62x39 is also the AK-47 and SKS round.

There are serveral different cartridges with the 7.62 designation.
- 7.62x39 is the AK and SKS round.
- 7.62 Nato is also called 7.62x51 and the civilian designation
  is .308 Winchester. A fine hunting cartridge.
- 7.62 Russian is also known as 7.62x54R, and is a rimmed cartridge.
  Many rifles chambered for this caliber are about 100 years old but
  are still popular. The bolt-action Russian Mosin-Nagant in this
  caliber was the North Vietnamese sniper rifle. The Russians are
  currently selling off their stockpiles of this weapon, and they
  can be had very cheap.
- 7.62x63mm is another designation of the .30-'06 (thirty-ought-six).

Although all of these 7.62's are basically .30 caliber bullets,
or .308" actually (well, the 7.62x39 is .311") the cartridges
containing the bullets are different for each caliber, with
the size, shape, length, width of the casing depending on the
exact cartridge designation.

