I'm trying to form 7.62x25 from .223. I measured and cut the .223, then ran it through the die. The cases won't chamber completely (I removed the barrel to test). .223 headspaces on the shoulder (right?) and that appears to be the problem - the shoulder isn't set back enough. I tried two different presses, two different shell holders, shimmed under the brass in the holder. Could it be the dies?