Slip in horn “sag”….

[Flying Dutchman]

Yeah, a 6 jaw lathe chuck is not in my toolbox..unfortunately

[Phono-Phan]

I agree that trying to expand the elbow will risk breaking the seam open. Find a wood dowel that is just under the size of the elbow. Put the dowel in a vice. And then insert the elbow over the dowel and lightly tap on the outside of the elbow where it is out of round with a rubber hammer.

[Flying Dutchman]

Another great idea. Appreciate all your advice.