Mag Blends from EM aren't doing so hot in the 20ga. Try Straight 7's from Fed, EM or Nitro unless you reload then TSS would be the option. I'm not familar with the HS Undertaker choke. Is it ported? Does it say no steel or no heavy shot? I have put 50-75 tungsten based loads through my 'lead only' RSF choke without a problem.
CB