Thanks...I'll watch this one.  
I've been working on an SBE for 10 years now.  It shoots "okay" and I'm not dissatisfied with it, more a project than anything else.  
I shoot a Rhino .673 with 2oz Nitro 4/5/7.  When I fooled with it last week it put right at 200 into 10 inches at 40 yards.  The 3 inch Magblends put about 150 into 10 inches.  Both patterns were very even and both shells shot to the same point of aim.
I noticed on the Nitro site that they recommend the Jellyhead .655 along with a .660 Rhino (I believe that is correct) and I am starting to suspect that my .673 might not be quite as tight as the gun might like.