There is wear on the cylinder but without knowing ring clearance it is impossible to ascertain how much wear on cylinder or rings.Most handheld 2 strokes have locating pins in the in the grooves to keep the rings from turning to where a ring end is in a port (bad). And the ends are tapered to fit the pins. I have never had to file rings. OEM or AM I just put new rings on the piston and install. Never had a problem. Not sure if the FS400 uses a plated cylinder but if it is not scored it is probably good to go.
Take the old rings after removing from the piston, push down into the cylinder with the piston to make even. Take your feeler gauge and determine gap. Repeat process with new rings. This will give you a comparison at least.I am not being informed if there are replies to my post, even though I have notification on. Is this normal?
My badFS 400 is a Stihl strimmer