This is a comment that is often made about 'barrel over cylinder' pistols, but I wonder how true it is? A sudden change of direction certainly causes measurable energy losses in the movement of solids, but gases and liquids are a whole different ball game and fall under the heading of hydraulics. As an example, the rate of flow of water under high pressure through a hose pipe is not perceptibly changed by introducing as many loops or U-bends in the hose as you like, as long as the tube diameter is kept constant throughout. Just as well really, as our central heating and car breaking systems would be in trouble. Gases, being even less viscous than liquids, should be even less affected by path that the tube takes.
I supect that if you could set up a sensitive experiment where all the parameters were equal, the muzzle energy difference between and in-line air rifle and a barrel-over- the -cylinder air rifle would be immeasurable. But I could be wrong