I am printing a cylinder for a pistol (for cosplay). The bottom, print wise, of the cylinder has a ridge running around it. In this section, the % infill works fine- see picture, base layers on the right. Once the printer gets up a few layers and the OD of the piece gets a little smaller again, slicer is turning to solid infill for the rest of the piece- everything between the chambers and the OD is solid, instead of being the %; again, see picture, piece on the left.
Does anyone have an idea why this might be, and where I might find a setting I have missed? Or is this unavoidable?
Apologies for potato-quality photo.

In slic3r in print/infill, there is a setting named 'solid infill threshold area'

Any area below this threshold (expressed in square millimeters) will be solid infilled. It could be this is what is happening to you.

That was it! Thanks!