I know the gun ,it's made be Baikal ,I have a 20 gau O/U
Check inside diamiter of the steel shot tubes .
Dose 1 of them measure close to the lead only MOD tube if so I would try it .
Hevi shot patterns real tight so a Mod tube = full ,IC = a tight mod and Skeet = a mod.
If you over choke Hevi shot the over all pattern will loosen up with a real dense center.
For geese I use #2 reloads that are a little faster than the Rem 3" 1 3/8 oz load
Are you reloading the Hevi shot ?
I use Reloading specialties data ,because their loads are lots faster than Balistic products.
If so I use the 3" Cheddite hull .
I found that the 1 1/2 oz load did not pattern realy any tighter than the 1 3/8 oz load
The BB loads will give you extra range ,but I found that it tore up closer geese . I only load 1 goose load.
For ducks I use 2 3/4 " Fed gold medal hulls with 1 1/4 oz of 6,5,or 4 for ducks ,the 4 load works good for decoying geese
I also load 1 1/8 oz of 7 for teal and decoying ducks