Lawrence, I didn't get where your question asked about mini-shells, but could have missed something. May have been asking about Mossberg mini-shell adapter. Only in 12 ga. Mossberg does have the shockwave in 20 ga and also the persuader with a regular pistol grip in 20 ga.
An adapter would not be needed for a break action or pump shotgun to fire the Aguila mini shells Lawrence. I have not heard of nor seen any 20ga Aguila mini Shells by the way.
sorry there is not as far as I know the mini shells are only made in 12 ga