Hi Mark and welcome. This will fire mini shells but not reliably enough to be used in a self defense situation in 20ga. The 12ga model has an adapter available for firing mini shells : budsgunshop.com/product_info.php/products_id/36595/opsol+texas+miniclip+mini-clip+for+mossberg+500+590+12+gauge+1+round+polymer+black+finish

The OPSOL adapter WILL NOT WORK its for a 12 ga not the 20 ga the poster was asking about. mossburg does not make a 590s in 20 ga that being said the only way to know if they will work is to try them some shotguns will cycle them while others of the same model will not.

Hi Mark, this will shoot mini shells with the OpSol adapter budsgunshop.com/product_info.php/products_id/36595/opsol+texas+miniclip+mini-clip+for+mossberg+500+590+12+gauge+1+round+polymer+black+finish ,
It will hold 13 rounds of the mini shells.
Or you can go with the 590S, which needs no adapter to fire mini shells. budsgunshop.com/product_info.php/products_id/140724/mossberg+%26+sons+51602+590s+tactical+12+20+13+1 ,
Stay safe & have a great day!

The 590 will but will need the opsol adapter for minis to cycle properly. Now The 590S was designed around mini shells and needs no adapter.
The 590 will hold roughly 14 mini shells.