I think it is a gas problem. No one is reporting any difference in accuracy between Lee C312-155-25 and Lee C312-160-2R. They are identical except for grooves GC designs. The recent .25-20 project involved bullets identical except one is GC and one is PB.

If a GC TL bullet can be pushed accurately past 2000 fps, the difference has to be be in gas cutting. Only thing that has been changed is the seal.