One way to prove Semi's theory is to use a 100 gr charge. That would be easy enough to do. And another thing it would prove is what the most efficient charge, in regard to velocity, would be for the 28" barrel.
Record the differences from 60 - 100 gr. If Semi's theory is correct (and I think it is) there should be even less average velocity difference at 60 gr. But at 100 the 32" barrel should be much faster.