Buffalo Arms makes the .310, not sure what they use as a parent case as they are out of stock at the moment. The interesting item is they list two different case lenghts, 1.075 and 1.120. When I asked about the difference I was told the shorter cases was for loading the original heel type boollet and the longer case was for "inside lubricated" boolits.
I tried to order some Starline .32-20 cases from Midway USA but even those are out of stock at the moment. I've heard the Starline brand .32-20 has a rim that is closer to the orginal .310 rim.