I have a grooved mandrel which helps (somewhat) for sizing some
U-shaped rings. But what I'm really looking for is something made
from a firm but flexible material (like silicone?) that will
slightly conform to the inside of an unusual shaped ring (like a
finger would) for a more accurate size measurement.
I actually have such a mandrel, made of a black rubber. Commercially
made, but it was given to me by another craftsman, so I don’t know
where to get one or if they’re still made. However, it’s not all that
accurate, since the rubber is somewhat compressable (the mandrel is a
hollow tapered rubber tube, making it a bit flexible), so the size
you measure depends some on how hard you pull the ring onto the
mandrel. Still, it helps, if you couple it with comparisons to a
standard ring sizer pulled similarly hard onto the mandrel.
Then there’s the classic do-it-yourself model. Take a sheet of
ordinary typing/computer paper, and wrap it tightly around a standard
steel mandrel, taping the outer seam shut with scotch tape or the
like. Remove from the steel, and insert it into standard ring sizers
in order to use a pen to mark the sizes on the paper. This works
reasonably well, and when it gets raggedy, is easy enough to make
another. Like my rubber mandrel, us it in comparison to how a
similarly marked standard sizing ring fits the paper mandrel, versus
your ring to be sized, since the paper too, flexes and gives a
little, so how hard you pull the ring onto the mandrel makes a
difference.
And finally, measure your various knuckles to find one that fits the
ring in a somewhat similarly feeling fit to some known standard
sizer. This, in my experience, may be the best method to get the most
accurate size, as it compares how the ring feels on an actual finger,
regardless of how distorted the shape may be, the comparison will
tell you what the ring should be stated as measuring.
Peter