Matters not what some chart says, or even if it is stamped on the barrel. Choke is a measure of constriction at a specified distance. Use whatever ammo you intend and draw a 30" circle at 40 yards (25 for .410), and see what type of pattern you get. I have had SK chokes that are closer to LM and a F that was more like a M with the ammo I was intending to use. Older guns especially were meant to originally use felt wads, so choke designations from that time frame with that ammo are different than using modern ammo with shot cups.