What term describes the outward expansion of gas in an RPG-7 explosion?

The term that accurately describes the outward expansion of gas in an RPG-7 explosion is "blast wave." A blast wave refers to the high-pressure wave generated by an explosion, which expands rapidly outward from the source of the detonation. This wave is characterized by a sudden increase in pressure, followed by a decrease, creating a zone of destruction around the explosion site.

In the context of an RPG-7, the blast wave results from the rapid combustion of the propellant and the subsequent expansion of gases. As this wave travels through the air, it can exert significant force, damaging structures and potentially causing injuries to personnel within its reach.

In contrast, a shock wave typically refers to a pressure wave caused by an object moving through a medium at supersonic speeds, which is not the primary mechanism in this context. A concussion wave is often used interchangeably with blast wave but is more related to the physical impact and trauma caused by the blast rather than the mechanics of the gas expansion itself. A thermal wave would relate to temperature changes and heat distribution, not directly describing the gas expansion from the explosion.