I started off with a .311" Lee DC mould. I then tried the .320" buckshot Hornady sells - those balls were quite hard - too hard and difficult to load - yes - I know that sounds impossible coming from me.
I also tried the .310" Hornady commercial balls - again, easy loading with any of the patches I commonly use.
I bought a .319" DC Lee mould and those both came out at .320" perfectly round. They loaded easily with .020", .0225" denim and .0235" mattress ticking patches, due to the balls being dead-soft, X-ray room lead wall sheeting. Such small diameter SOFT balls are easily drawn into short conicals when loaded with thick patches. 2 fingers on the rod pushed them down that bore. That barrel was all bass-ackwards in that the grooves were VERY narrow and the lands very wide, and did not shoot as well as I wanted.
My replacement barrel was supposed to be a .32, however Jason sent a .36 bl. by mistake. Since I already had a .350" DC RCBS mould - no harm done. With those .010"/under balls and the same .0235" mattress ticking patches, I do not even need a short starter, however I use one anyway - it's quicker than shoving the patch and ball into the bore with a choked up rod. No - they cannot be thumb started - not even close.