5R is a Boots Obermeyer invention. It uses 5 lands, the idea being an odd number doesn't squeeze opposite sides of the bullet so there is half as much distortion across a given land diameter. I gather it also tapers from groove to land rather than taking an abrupt step, so it fills out with less distortion. Should work great with lead from both standpoints.
The only drawback is measuring your exact bore diameter can no longer be done with just a slug and a micrometer. You need a micrometer and a V-block and a couple of precision ground dowels for calibration. Ideally, for 5 groove, the V would be 108°. In most instances, like S&W barrels, the lands are wide enough to span the 9° error on each side that a common 90° V represents. I haven't had an opportunity to check the 5R land form to see if it can be done this way or not.
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