My "ol reliable" is an SX3 12, 26" bbl, with a Carlson's Longbeard XR tube (.665 as mentioned above) and I can tell you, after patterning 5 different shells through it, XRs are all I've shot out of it the last 3-4 years. Now, that being said, I would definitely consider the Carlson Longbeard XR .568 Invector Plus for your 20. I've shot that tube out of 2 Affinity 20 gauges and it patterned the #5 XRs great so I am sure there would be no problems with the #6s! I have had several brands of chokes and patterned countless commercially available loads and I don't know if it's because they are all made to be together or if I just got lucky, but my SX3 12, Longbeard 5s or 6s in either 3" or 3.5", and that Carlson's XR choke is all I ever need. Anywhere to 60 yards I'm fully confident!