Is there such a thing?
Have the book. Have not cracked it yet. Have some take offs. Watching for more.I don't know of any thing called a "Practice" reamer, But there are practice chambers.
I recommend finding a used or take off barrel (Preferably stainless) and setting it up in the lathe.
find a reamer (A good one is recommended so you can see the results when you do it right).
A good chamber doesn't need polishing so cut some of the shank (One thread at a time) and cut a new chamber and repeat this process until you have a chamber that has no defects.
It just takes practice and craftsmanship
Hear is a link to a good gun smithing book That will help.
http://www.mansonreamers.com/Current catalog/May 2013 Catalog.pdf (Page 25).
J E CUSTOM