I realize what I post on this forum may be considered by some bench rest shooters as not of any particular significance to what they do at the range, but the .236" vs .237" bore debate has gone on for a while without any decisive results one way or the other. Rarely on the forums do you see anyone particularly passionate one way or the other on the bore size issue, as you can see with certain other shooting issues. It seems people sometimes have preferences on bore size, but it does not seem to be based upon any form of testing, but more based on what they had in the past, or what some notable shooter uses. I have numerous barrels in each size, chambered in various cartridges, and I cannot honestly say there is a decisively better performance between one vs the other.
The above being said, for certain applications and potential bullet uses (none of them bench rest shooting) I would tend to use a .236" vs a .237" bore.
For a highpower rifle or a prone rifle where there will be a long expected barrel use I would always lean to a .236" bore. From the first shot though the barrel, the bore exhibits wear and opens up, and a tighter bore may be helpful for longevity. With some highpower or prone rifles with the tighter bores, I have noticed it takes 350-500 rounds down some barrels for a rifle to "settle in", but then again those barrels may not be pulled off until after 2500-3000 rounds (whereas for a bench rifle, a barrel may be pulled and done before 1000 rounds).
If you are running commercial bullets, some lots can be "skinny" (slightly undersize in diameter) and they don't always shoot as well as they might unless out of a tighter bore. I remember a couple years back Berger had a certain lot of 6mm 105 VLD's, and Lapua had a lot of 105 Scenars, that were "skinny" and there were shooter complaints about them not shooting well (although they shot well in the .236" bore barrels I had at that time). Palma team shooters have typically tended to run the tight 30 Cal. .298" x .3075" (or tighter) bores, because in the past they "got burned" having to shoot ammo in matches where the supplied ammo had "skinny" projectiles (i.e. the stuff shot poorly in the standard .300" x .308" bore barrels).
The reality is for a 6 PPC bench rest application, it may not really matter whether you run a .236" or .237", as you will probably pull the barrel long before the bore wears and opens up too much and you will likely be using premium custom bullets that are dimensionally well dialed into the barrel and type of shooting you are doing.
Robert Whitley