Inside the scope the range of movement of the pa assembly is in the order of inches. I think it works in reverse to the ao system. In ao the end lens moves and the rest stay in place. In a sidewheel the end lens stays put and the pa lens set inside moves back and forth. I believe it actually turns following a coiled path. The shape of the path can be linear like the coils in normal spring or progressive so the coils are tighter at one end than the other. What keeps the lens on the coiled path I'm not sure of but I wonder if its this area that sees slack or tightness rather than the tube expanding or contracting.
I've got a scope that's shot away so may take it to bits.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep