Among the more ubiquitous Soviet small arms are Rocket Propelled Grenades. Virtually every conflict in the world has seen the RPG-7 used, often by both sides.
The great strength of the RPG-7 is its simplicity. It takes only a few minutes training to impart a basic competency.
But the problem is, against anything but armored vehicles, its shaped-charge HEAT warhead is really pretty ineffective. Stories of RPGs exploding perilously close to personnel and not injuring them are common. That’s because a HEAT warhead focuses virtually all its power into a very tiny jet in one direction. Avoid the jet, and your chances of injury are quite small.
If you want to punch a hole in something, a HEAT round is the way to go. But a huge number of targets on the battlefield call for something else. Caves, buildings, bunkers and such need a different approach. Ideally, you can fire a high explosive charge through the aperture, and the resulting explosion inside will kill any enemy, and ideally dismantle the target.
And so the Soviets developed the PRO or Flamethrower Projector Rocket. Known in service as the Shmel, the PRO was single shot, disposable rocket propelled grenade. It came in three variants- high explosive (PRO-A), incendiary (PRO-Z), and smoke for screening (PRO-D). Interestingly, all three were designated as flamethrower projectors.
The high explosive variant used a thermobaric warhead. Unlike conventional high explosives that contain all their fuel and oxidizer, thermobaric warheads use the surrounding air for at least a portion of their oxidizer. Whereas a conventional high explosive forms its blast wave from a single point and diminishes in strength from that point, the “burn” of a thermobaric warhead actually increases blast wave pressure as it expands, until all the fuel is consumed. This makes thermobaric warheads nearly ideal for enclosed spaces.
All this is an excuse to post some splodey-
A later development, the PRO-M, increased range and warhead, and is still in use with Russian forces.