There seems to be a misunderstanding of terminology and use regarding 'backpressure' in this thread.
Check out Gordon P. Blair's work on two stroke engine technology "Basic Design of Two-Stroke Engines" (it is HARDLY basic!!) available through the SAE. Look specifically at pressure wave and particle motion in Chapter 2, Gas Flow Through Two-Stroke Engines, with 'Reflection of Pressure Waves in a Pipe.'
'Backpressure' is actually the arrival of the reflected positive pulse from the end of the exhaust system (or some cross-section change) back to the exhaust port, often tuned to stop the sudden outflow of fresh intake charge from the opposite port, thereby increasing efficiency and power. It has a trapping effect. It is HIGHLY tunable, but dependant on multiple variables. In a two-stroke engine, the often peaky powerband is due to the convergence of all variables to make one particular RPM point the most efficient. This will vary with atmosphere, and a host of other issues as well, so with sensitive instruments, will be seen to not be constant.
'Backpressure' is often also used to refer to 'exhaust restriction' but this ignores the true science of pressure waves as gases exit to atmospheric conditions. It's kind of a simplified model.
'Scavenging' is a similar but different animal. This makes use of the NEGATIVE pressure waves to coax burnt gases from the cylinder, and of course can be tuned as well.
Most people without expensive instrumentation are flying by the seat of the pants when they design these systems by hand. Some people do okay at it. Some.....should just be plumbers....!
As to the rifle issue, a complete understanding of combustion is important there too. Research pressure wave analysis of gun shots for internal ballistics. This is the study of what happens in the barrel. External ballistics is the study of what happens when the bullet is in flight, and is what all of us deal with every day to make a good shot.
Have fun fellas!
