By those measurements, a turned rim and extractor groove would be a 1 minute job on a lathe, per case.

When I needed to do this to a .450 Alaskan case (originally a .348 Winch.), turning off the rims to .532" and cutting an extractor groove for my .458 Alaskan ctg. to fit a model 70 Winchester rifle, I ground a cutter to make both cuts in a single plunge. That might have taken 15 seconds per case on my old worn-out lathe.

Here is a picture that should show some of what is needed to convert a .303 case to the .303 Velox. For the .303 Rimmed conversion to rimless, there will be less material removed than what I had to do, due to the .303 rimless rim's diameter being closer to the ctg. base diameter.

I had to turn the rim down to a rebated rim (smaller than the case head) to fit a standard magnum bolt face. A standard magnum bolt is considerably smaller in diameter than the .348 case head diameter.

Any machinist or die grinder can grind the cutting tool, given a picture and measurements to go by. If I can do it, anyone can.

An alternative is to use a single tool and cut each case in two steps.