Just out, Ruger now offers 20 round mags for Mini-30s. I have a BUNCH of 10/20/30 round aftermarket mags for mine and none of them work worth a darn. I just ordered one of the factory mags (Item #90338) $39.95, in the hopes that it actually works!
This is a pick of me about 10 years ago, with one of the aftermarket jobs....: