Robert,
Your new/used barrel may be one of the most accurate you own.
I primarily use bolt, canon breech, or falling block actions, but when I have a custom barrel made for my Contender, Mike is at the top of my list, specifically because of his exceptional tolerances that he holds.
Many times, when I build a custom XP I am paying for 1/2 of a new reamer so that I can build/design a reamer around my brass. The beauty of it with Mike is that you can send him your brass and your dies and he can build it to give the best possible fit. Recently there was a very good article about Mike Sirios on 6BR.com and they quality of work he does.
There are a number of guys that do good work on or with TC products, and Mike is one of those guys.
FWIW you could have someone send you a couple of fired JDJ rounds and do some comparisons. There will even be some discrepencies with JD's chambers simply becasue of the wearing of the reamer over time.