So I have a cylinder inside another. How can I make it look like one thick empty cylinder.
Like the barrel on the gun on the last page?
delete the other cylinder keep one and hollow it (delete inner vert). Then extrude and scale by pressing S, then while s is selected hit x y or z to scale along a certain axis. hold ctrl while scaling to lock to the grid
if you dont wanna do that select the faces in groups of two (four total). two on the big cylinder and the smaller version of the same too on the other cylinder, then press f to make a quad face.