An "assault weapon" as defined by the Army in it's film series "The History of Small Arms Development" is; a select fire rifle chambered for an intermediate size cartridge.
The Germans are generally credited with development of the first assault rifle, the STG-44. This was a select fire weapon built around the 8mm kurz (short) cartridge. The Russians were next with the AK-47. The British had a bullpup design. Even the FN-FAL was designed around an intermediate caliber. It was the US that pushed the 7.62x51 round as the NATO standard. We then deployed the M16, our first true assault rifle after insisting that NATO adopt the larger 7.62x51 round as standard. It's interesting to note that from the time the US Army standardized the M16A1 as standard issue about 1973 until 1985 when the 5.56x45 round was adopted as the NATO standard we didn't field a rifle that had ammo compatibility with our NATO allies.