Actual Process

First, everything is set up:  tent, tables, chronographs, chop saw, generator, et cetera.  We shoot on private land out in the country, where there is no shelter or power.  We use as a backstop an old cabin made from railroad ties.  Once everything is set up, and the ammo to be tested laid out, a series of three test shots is made using a 6" Python .357, shooting .38 special ball ammo from one batch.  This gives us a benchmark which would indicate data drift due to atmospheric change or a problem with one or both chronographs.  Time and temperature are noted, and we would begin.

Three shots are made of a given ammo.  If we get consistent readings, then we swab the barrel once with a Boresnake of the appropriate size, and the next ammo in that caliber is tested.  Once all the ammo in a given caliber is tested, the barrel is removed and a new barrel in another caliber mounted.  After removal, the barrel is chopped one inch, using a metal cut-off saw.  Then the outside of the barrel is dressed using a flat metal file, and the inside of the barrel dressed using first a rat-tail file and then a small buffing stone.  Once chopped, the barrel is cleaned using three passes of a Boresnake of the appropriate size.

Typically, there are three of us involved at any given time.  One person organizes the ammo and records the data.  One person handles the gun and does the actual shooting.  One person aids with swabbing the gun, chopping barrels, et cetera.  We rotate positions, so no one person gets tired, unless we run into a problem and one member of the team is having better success at one particular position.  This usually comes with shooting the Thompson when the barrels are very short or firing a problematic 'real world' gun.

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Other Resources

BBTI is not the end-all of ballistics testing, just one more component available for the common good.  In addition to extensive discussion about ballistics to be found at many gun forums, here are some other great resources pertaining to ballistics testing you should check out.  (And if you would like to recommend a site to list here, please send an email.)
  • BrassFetcher:  excellent resource, with an emphasis on bullet performance in ballistic gelatin
  • The Box O' Truth:  testing ammo penetration through various barriers
  • Terminal Ballistics Research:  Specializes in the research of cartridge & projectile performance, using hard data gathered from 20 years of hunting game.

Acknowledgements

We'd like to personally and specifically thank Pat Childs at Fin & Feather in Iowa City, as he not only helped get most of our ammunition and other supplies, he was the brilliant gunsmith who worked with us to make this insane project much more practical.  Without his help all of this would have been much more difficult and perhaps impossible.  Anyone who uses our data owes him a debt of gratitude.

And thanks to our spouses, who were not just tolerant but enthusiastically supportive of this rather nutty project.

Disclaimer

This project, and all of its results, is only our fault.  We (well, Jim K, mostly) paid for everything ourselves, and we did not receive any kind of sponsorship or remuneration from anyone.  We did all the work.  We used products we were either familiar with, or because they were what was available, and mentioning them by name does not constitute an endorsement of any kind.  Furthermore, the data is provided purely for entertainment purposes - to better facilitate arguments over what ammo or caliber or gun is "best."  How you use the data is entirely up to you.  And if you think you could do better, feel free to spend the money and do the work and publish your own results.  Or not.  Your choice.