I think a gun I had bored lost 2.5-3 ounces by removing .010 from the bore but that was for a 30" gun. It's more complex than a straight weight loss because small barrels loose less and bigger ones loose more. Think about the diameter of the bore. A large diameter barrel with .750 bore will loose more weight, metal reamed out, than a tight bore like .710 for the same amount of reaming. Mike O did mine and he did a very nice job.
Sometimes you just can't get there from where you start at. Easier way to get a 6 pound grouse gun is to drop down to a 16. Heck at most grouse shot ranges an ounce is more than enough shot because my grouse seem to be gone in a flash. A thirty yard grouse shot where we use to hunt would be a once in a year or longer fluke. A 16 with two fairly open chokes holds a lot of appeal for early grouse.
