I shoot .312" copper plate bullets out of my CZ carbine. It is the fit in the chamber throat that matters most to pressures, not bore size. If you have adequate clearance in the chamber throat your oversize bullets will swage down that couple thousandths of an inch without pushing up pressures much. But if your chamber throat is tight, the pressures can go dangerously high very easily. When loading larger diameter than "normal" cast bullets, I usually just do the simple check you referred to - if a bullet slips easily into the fired / not sized brass , there will not be a problem with excess pressure if normal loading precautions are followed of loading a starting load and working up.