Order those 20 rounders from the factory while you can. I am not sure if they are caught up yet, but there was a limit on how many I could order and it was several weeks before they shipped, back when I got some a couple months ago. I think if you want the UPS man to drop fresh Mini-14 mags on your doorstep, time to head over to ShopRuger.com now. That window is effectively closing a lot sooner than October 1.
I have 8 20 rnd factory mags for my new Mini 14. No problems. Tried a couple of after market 30 rnders I picked up in PA. Did not want to chamber 1st rnd. Next 29 worked fine.