It appears from the photos that most of the South African shooters favored guns that were relatively large for their caliber.
Heavy gun, small bore, stable platform. This, theoretically, would seem to be the combination most conducive to repeatable accuracy.
Of course, I could be also totally wrong. That's why we need to actually shoot the guns. 
YMMV and all the usual anacronyms...
BTW: DD, did you get my e-mail?
Most shoot a 50 or 54, mostly 54. There barrels are longer-longer sight radius. the Guns are accordingly bigger.
The club gun thatBrooks sells as the SAMCC gun use to be one of the top shooting guns in the club. It has been shot steady for over 30 years. The throat is eroded pretty bad and the gun throws fliers. The bronze gun I brought back was the first gun to shot a score of 50. It was a the top shooting gun in the club for many years, they it went sour.
After I bought the gun I cleaned the barrel and got a ton of lead out of the barrel and was one of the top shooters when I shoot at the next match. Beat the guy I bought it from.
The first time I shoot, I shot Mike Pilgrim's gun. His Napolean is 50 cal, long barrel field pattern gun with oversized swell. The gun shot a patched round ball. I don't remember what the charge was, but most people shot either 40 grains or 90 grains.
You all have seen the this picture before.
That is my first ever shoot at SAMCC. Here is the target from my first shoot.
THe loading drill for the cannon was:
Wet patch wrapped around a bore brush, up and donthe bore a couple of times.
Dry patch around a bore brush up and down the bore.
Vent pick put in vent and left.
Barrel held up right a powder poured down bore
Patch put over muzzle and ball put on top and seated so ball is flush with muzzle and excess patch is trimmed off.
Ball is then seated on the powder.
Gun is put on shooting platform, vent pick is pulled and fast buring powder is pout in vet to charge.
Gun aimed.
The shoot master then moves down the line firing the guns.
