I have a couple of these and they are neat; wish Savage or somebody else still made them. In general these kind of guage adapters pattern better the closer the "parent" barrel is in diameter to a .410: a .410 adapter in a 20 patterns closer to the parent 20 barrel than it would in a 12, for example.
But you are introducing so many variables when you go from a .410 to ANY other guage this way that the only way to figure out how they are going to pattern in your gun is to try yours with a number of different loads. For example, it's going to pattern a .410 3" load of 4s VERY differently than a 2 1/2" load of 9s, and even that will vary with how tight the "parent" barrel choke is. "Fit and try" is all you can do, but I've found that the "Four-Tenners" I have in my Savage/Valmet 20 guage O/U which is choked IC and C, make very nice winghooting patterns with 3" .410 7 1/2s, and adequate snake/possum/jackrabbit/cottontail/squirrel patterns with 3" 4s.
One of the main reasons people bought "Four-Tenners" back when they were relatively cheap is to get recoil reduction in 12 or 16 guage single shot shotguns, which "back up" pretty hard with high base shells. The lighter .410 loads plus the extra weight of the "Four-Tenner" tames 'em down nicely. Good "kid starter"as long as you aren't shooting flying targets; that can be discouraging with ANY .410!
