LB

If that horn is old enough, you may be talking hand hammered, rolled,
and soldered tubes, so wall thickness is variable depending on which way
you measure. You should post to the Musical Instrument Makers Forum,
www.mimf.com . There are a bunch of clever people there, some of whom
make brass winds.

You might also look at Robert Barclay's book, "The Art of the
Trumpet-Maker" which is about making replica 17th and 18th century
Nuremberg style trumpets, but covers a lot of the same processes as were
used making horns.

My guess is once you get out of the bell region down into the tubing
part, the wall thickness will be pretty uniform. I'd do a bunch of
measurements with a long arm caliper at every point you can reach, at
several points around the circumference of each tube you can reach into,
and see if there is a statistically significant variation. You may not
need to make a cast replica (which is always dicey because minor
registration errors will cause huge wall thickness errors).

In point of fact, wall thickness, except in the bell, is insignificant
to the sound of the horn. Even the shape of the air column is
immaterial, which is why you can fold them up and have elliptical
sections near the valves and at the sharp curves. Only the volume counts.

Interesting problem.

Good luck

Jim