I tried enlarging a reflector with a hand drill and grinding stone. In no time at all the bulb opening was way off centre and making its way towards the side of the reflector!:sick2:
Controlling the vibration would be a problem without a lathe.
One of the best methods to do by hand would be to take a round wooden ,cover it with sandpaper and slowly scrape the inside of the body. It works but takes a while . Clamp down the flashlight to reduce the vibration.
Ledean
This is not too bad of a non-lathe way to do it. WD-40 is a suitable cutting fluid for aluminum (so don't be cheap with it). Just don't forget to start using finer grit papers well before you reach your final diameter for a smooth finish without blowing the diameter out too much.
Having said that, it's a five-minute job on a lathe. I'm always open to CPFers PMing me for help boring their Surefire lights.