Your query about barrel tuners go me on a goose chase. Not sure if Beggs tuner is still available, or the one that mounts into the stock that you mention. Interesting concept though.

From I what I was able to dig up today, the biggest advantage to have a tuner is that you can load your ammo before your shooting event, and then use the tuner to adjust the load to the density altitude conditions the day of the shoot. Or even as the day warms during the shoot, you can adjust your barrel to account for the less dense air.

The numbers I read today were you REDUCE your load by .3 grain of powder for every 10 degrees F the temp rises (ref: Gene Beggs makes a big mistake, Benchrest Central, 4-23-10) and vice versa as temps drop OR atmospheric pressure increases such as after a cold front passes. This is in reference to that day your load was developed, of course.

So as I read it, you develop your load and record the temp and pressure of the day you shoot. Then adjust your load accordingly, or use a barrel tuner as shooting conditions differ.