John. What part of the communist state of Maryland are you from? I've got multiple AR-10 "B' rifles. Mine work just fine. Though....I did notice some odd performance with "factory" 10 round mags in the past. I've got plenty of original 20 rounders that we're not allowed to buy in this state any longer. Have you tried a plain jane 20 round mag in the rifle? If not, we can probably hook up to test your rifle with some to see if there's a difference.
Oh.....and welcome from Maryland my man.