Honestly, doesn't look big enough for a 20. An Anzio bolt body is 2.5" in diameter. There are four 1.9" diameter lugs. A 50 case swims in the bolt head. I took a pic. The 50 case isn't even against the side - the extractor pushes it over a little. The bolt thrust for a 20mm is a little over 2x the 50 case based on bolt area alone. The next problem is weight. For anything that weighs less than 100#, even with a super efficient brake, the free recoil energy is going to SUCK. Be VERY careful if you scale that bolt head up to a 20. Do lots of math on the strength of the lugs on both the bolt and receiver.
What the free space with the bolt rearward? (How long a round can it feed.)
(Crud, can't insert an image directly. Only from a URL here. Sorry.)