RCBS .37-250-FN

This bullet is the most accurate bullet I have for my 38-55s. It casts too small for my use at .378 out of 10WW + 1 50/50 solder. I special ordered a mold which casts a little large at .383 but it is quite usable. THE PROBLEM with both molds is bullet shrinkage and frosting in certain areas. I have cast as low as 600 degrees and have tried alternating the 2 molds and I have tried other alloys and I have cooled the molds under a fan for 15 sec between pours. My success rate is only about 30%.
The molds are Iron and I think too small for the bullet size. I would like to see NOE produce a larger mold in aluminum or possibly brass. I would also suggest 2 different mold sizes one casting at .376 and another at .382. What say you?