I have bought and used two different ring filers, one mechanical that you rotate with one hand while holding the ring against the grinding wheel and the other one is battery powered that I hook the charger up to while using it and way easer and better to use.
I gap my rings way wider now than the minimums due to seeing the rings end butt and hurt the cylinder walls and pistons when they are driven on the streets in slow traffic in the summer times in SO CA up scope
On 440 motors I shoot for .020+ and more on the second ring like .024+ up wrench

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)