Thought I'd post my results here of sizing down the Lee .690" round ball for use in 12 gauge. Like others have posted, I found the .690" ball to be a bit too large for use in shot cups in the 12 gauge, so I decided to size them down using a push through sizer.
I used a 5/8" grade 8 nut, and opened up the hole with a 21/32 reamer. I then opened part of the hole in a taper, using a larger tapered reamer. I polished the hole with emery cloth wrapped around a 1/2" steel rod in my cordless drill.
Using Hornady Unique case lube, resizing air cooled wheel weight balls takes about the same effort as full length sizing 30-06 brass. Water quenched ww balls required a 5 ft cheater pipe on the press handle, so I won't be doing too many of those!
Right now this sizes them down to .670". I was planning to go bigger, to .680", but they shoot pretty good. Accuracy is about the same as unsized round balls from Lyman's .685" mold (I don't have that mold, just some samples, or I wouldn't bother with the sizer idea).
I thought I might have an issue with the balls seating crooked in the shotcups, but they seem to be pretty much self aligning, if I start them in somewhat straight. I'd like to try this in a rifled bore too just for fun..
Any of you guys tried something like this?