Parrott-Cannon,
I don't have any suggestions, but I do have some observations. First of all my mortar's barrel length is aprox 2 times the bore diameter with fairly thick walls, a 30 degree taper down to a .5" by 1.25" chamber. The barrel is fixed at 45 degrees (see pict below) It was was built along these lines:
http://www.gboreloaded.com/forums//index.php/topic,70859.0.htmlBut turned from solid. The chamber holds about 50 grains of FFFG powder. I fired it with a full chamber at the Montana shoot with an old golfball and DD said he found the ball just over 400yds from the line (I put my name on my balls before I shot them). I fired a dozen balls or so at the thirty yd stake and determined that 2 - .22 long rifle cases of FFFFG, yes 4f, got me pretty close to the stake. Courser powder gave me erratic ignition until I got about 15-20 grains of it in there, then I shot way over the stake. Balls fired with the barrel close to horizontal and a full to half load seemed to hook and slice and generally go in everywhichaway.
If I was going to do another mortar/shoot, assuming the standard 30 yd stake for golfball mortars, I would do the following: I would put a smaller chamber in the mortar, or an arrangement whereby I could insert a smaller chamber and hold it in with a threaded vent so I would end up with a full chamber to prevent erratic ignition. (Maybe 1/4" X 1/2") I would definately screw sabots to the balls to keep them from spinning, once the ball is spinning the dimples do their job, they make the ball fly and if you can't get the spin in exactly the same place each time the balls will not go straight.
I started a test mortar with an offset chamber to try to spin the ball on purpose, in a pridictable way, but never finished it. Sabots would probably work as well and for the pressures involved a disk of plywood sheetrock screwed to the ball would be sufficient.
Since you are getting a 100 yds from 45 grs or so I suspect you have a pretty good sized chamber which would cause erratic ignition, this was covered in the posts on the golfball mortar contest, DD seemed to have the smallest chamber and as the chambers got larger there were more complaints about erratic ignition.