Well actually 20 gauge has a higher average max pressure than a 12 gauge (except 3 1/2"). Granted due to the smaller case head size the 20 doesn't quite give the same thrust on the action as the 12 gauge but it isn't too large a difference. Thus the only real difference in using a 20 gauge sized action comes to chamber wall thickness. Again an area where making your own monoblocks really works out because you can minimize the monoblock wall thicknesses and really give more wall thickness to the barrel itself. The other problem with making the larger calibers on the 20 gauge frame is weight and weight distribution. I've been workign real hard on this on the last couple and by far it is the hardest thing to do properly. I'm trying to do it all with out adding any weight anywhere.

BB