Wall thickness at the muzzles is not as important from a safety standpoint as wall thickness at 3", 6", and 9" from the breech. Those measurements should be taken by someone who knows their business. Get a small penlight and slide it down the barrel so you can check for pits, if there are any. (You may have already done this.) If it's a 14 gauge use 13 gauge wads. I always bump up a notch with a ML. Is it a percussion or flint?