You'd have stood a better chance of attracting someone to your thread to discuss OBT if you had mentioned OBT in the title.
The best place to start is by reading Chris Long's online synopses of OBT (
here and
here). Chris knows a thing or three about OBT because he created it.
Here is a post I wrote a couple of years ago in SH about how I run OBT. In that post I link to three online articles on OBT written by David Wilson. I call David something of an OBT lieutenant to Chris Long. His notes are of a more practical nature than Chris's, and were key to me getting my feet under me, OBT-wise. Unfortunately, David no longer maintains his old web site, so the links in that post are broken, but those pages still are available in through the Wayback Machine. You can find them
here and
here and
here (a PDF). I would highly recommend anyone interested in OBT archive those pages before they're gone for good.
Chris once posted his parameters for tweaking QL for OBT in the now-defunct WSSM forums. I transcribed his notes in another SH post,
here.
Those two posts should give you a running start. One development since the both of them, one of the guys at the old wssmforums.com gave me a formula he swiped from somebody else (maybe Chris, I dunno) for calculating the start pressure to use in QL. The formula is:
( 1420 x bearing surface length ) + 2860. Throw in an additional 600 psi for 215M primers (which probably also goes for any magnum primer).
The bearing surface length is expressed in inches, i.e., 1" = 1.0; 1 1/8"= 1.125.
The guy who gave the start pressure formula used OBT to work up loads for his 243WSSM and his 6mm Dasher, both of which "shoot zeros" (< 0.10 MoA), which speaks to his expertise, both as a hand loader, and with OBT.
OBT is not complicated, once you understand what to change, and when. It is a
scientific method for
calculating a muzzle velocity where an accuracy node
should exist. The muzzle velocity is the shooter's shortcut to measuring bullet dwell or "barrel time," which is so brief an event (~1 ms for a rifle), it otherwise would be beyond our means to measure it with any useful degree of accuracy. But QL correlates MV to BT by calculating a pressure curve based on the components and conditions you enter. And MV the shooter
can measure.
OBT does not guarantee there are not other accuracy nodes outside of its predictions, some of which potentially could be tighter than what OBT predicts. All it alleges to do is to get you on an MV where an accuracy node
should be. The upshot is that it offers to reduce range time and rounds expended in the pursuit of the perfect load. If you trust in OBT, if you utilize it to full effect, you necessarily are ignoring the possibility that there are other charge weights (which OCW or Audette
could identify) that would produce tighter groups. So pick your poison. Since I got read on to it, it is uncommon that I would need more than three range sessions and 18 rounds
total to get me to the tweaking seating depth stage, especially with a rifle that I've done OBT with before.
I only bought a chrono, BTW, because I wanted to do OBT. But since I bought one (and now I have two), I find that its greater value is as QC on my reloading. If I'm shooting paper targets, I shoot over a chrono, period. That's the only thing that lets me keep track of my SDs, which lets me know whether I'm straying from the straight and narrow. And my OBT values. Something could come along to tempt me away from OBT, but my method already is so compact, so economical, and so effective, I'm not holding my breath.
