I would think that a 2 3/4" 20 gauge shell would protrude about 3/4" from a Gagemate or Lil Skeeter shortened for a 12 ga 2" chamber. I do not even know if I can shorten Lil Skeeters to fit a Brit 12 ga 2 1/2" chamber and use regular 28 ga shells. Maybe chamfering the end of the adapter might do, not shortening it in a lathe. I'll try it out and let you know.
Mike