Sounds like the mold was not up to temp, the alloy was not up to temp, or the alloy was lacking tin.
That projie shot lights out with 18.5 gr. H110 in a Ruger Super Red Hawk. Right at 1100 fps as I remember.
That is the bottom of recommended loading but it shot the best of the H110 loads, in fact the best loading that revolver shot at 50 yards.
I'm sure you are on the right track and
I have no doubt that it's operator error on my end. I'm pretty much a noob, with only casting less than a thousand or 2 since the mid 90's. The thing is, I've tried other Lee molds and had easy success. This one frustrated me and that's why I picked up the 2 cavity version of the 310.
It's been several years since I last cast that bullet and I made several different attempts. Hotter alloy, cooler alloy. Hot mold, cooler mold, then even hotter mold. I even added some 97/3 solder to the WW alloy in hopes if better fill out.
IIRC, the best bullets came from a very hot mold, but I got a lot of frosted bullets. It seemed like the sweet spot was very narrow, temp-wise.
I did get enough good 310's to load to form an opinion and I agree that they shoot well. I think my load was around 20.5 grains of H110. Seems like I got 1,500-1,600 from my Marlin 1894P's short barrel and about 1,200-1,300 from my 7½" Redhawk.