The chambers on Lee Enfields were cut somewhat generously
That's an understatement. SMLE's are notorious for case head separation with reloads. Chambers have plenty of clearance to work when filthy. I currently load for 2 Enfields. Each has its own dedicated batch of brass fireformed with the O-ring method above. Mine shoots Prvi and my daughter's uses R-P, that way I have no trouble keeping them separated. I use a collet die to ensure that I only neck size. I don't shoot these all that often so I'm only up to a max of 4 reloads to a case. Annealed the necks after 2. I've only lost a couple of cases.