I recently received a fullsize 45acp P320 with a takedown lever that was almost impossible to turn. An internet search revealed this as a very common problem on the P320 series with various proposed fixes. Upon disassembly I found the lever pin has a camming groove surface that had some machining burrs on it. A little polishing on the surface with 600 grit sandpaper and the lever worked much smoother on the fire control module outside the frame but still bound up when in the frame. Looking at the assembly I could see the position of the camming groove was determined by a molded plastic boss on the frame. A few file strokes on the boss allowed the camming groove to shift inward to correctly align with takedown bar and voila! the takedown lever now works as it should. After seeing how the assembly interacts, this was a relatively easy fix but it wasn't listed as one to be found in my internet search.
If you can't takedown your P320 you may want to try this.