Mark Walker knows exactly what he is talking about, although the idea that competition barrels can apparently gain velocity as they break in is not really a "new" concept. It has been reported here regularly over quite some time. For most, it seems to happen somewhere between ~50-100 and 200 rounds, although I have read reports of it happening as late as 400-500 fired rounds.
It is also common for quality competition barrels in calibers that aren't barrel-burners such as .223 Rem and .308 Win to shoot with little or no detectable loss of precision for 2000-2500 rounds after they settle in. You will likely have to adjust the load for throat erosion and other time-dependent factors during that period to maintain precision. However, the barrel itself can still be capable of excellent precision with 2500 to 3000 rounds if you have taken proper care of it during it's lifetime. In my hands, the effects of throat erosion and fire-cracking seem to become readily apparent at somewhere between 3000-3500 rounds, but that can easily vary between different barrels and manufacturers.
Doing preliminary load development prior to a barrel having settled in is
not a complete waste of time, especially if you meticulously record solid velocity data. It is usually a matter of minor tweaking to re-adjust the load, rather than completely re-doing a full-blown load development from scratch. It is analogous to working up a load in virgin versus fire-formed brass. It will probably change a bit after fire-forming the brass, but the changes are usually minor adjustments rather than huge ones.
Papa Charlie - I might have missed it, but I didn't notice that you listed your barrel length, freebore length (COAL), or the type of brass you're using. There is a huge amount of information available here on very good loads using Varget and 185 Juggernauts. More often than not, it is actually rather difficult NOT to get that combination to shoot well.
Depending on the amount of freebore the lead/throat has cut, which affects usable case volume, .308 Win loads with Varget and 185 Juggernauts in standard Lapua brass can be expected to tune in from around 2700-2730 fps (~.085 fb/~2.885" COAL) up to around 2770-2800 fps (~0.170 fb/~2.970" COAL) in a 30" barrel. Your load should be expected to tune in somewhere relative to these approximate values dependent on your barrel length, freebore length, and brass type. In my hands, Lapua .308 Win Palma brass usually has from 0.1 to 0.3 gr greater case volume than standard Lapua .308 Win brass, which can also make a difference, as do the large versus small rifle primers.
If you look solely at your seating depth data for 43.0 gr Varget, the only questionable thing I see is the .020" off group. If you possibly might have pulled one of those shots, then your seating depth test at that charge weight is exactly what I'd expect with 185 Juggernauts. That is to say, they generally tune in somewhere around .015" to .020" off the lands and then stay tuned in for several successive seating depth increments, usually as far off as I have tested them, which is about .030" off or so. In all likelihood, your 43.5 gr load is what's often referred to as a scatter node. Although the vertical is mostly tuned out exactly where I'd expect for the Juggernaut at .020" to .025" off the lands, the groups overall just really aren't too great at any seating seating depth. The 44.0 gr load seems to be slightly better and exhibits a similar effect of minimized vertical in the expected seating depth range Juggernauts are known to prefer. However, 44.0 gr may also not be close to where your setup wants to shoot.
My suggestion would be to seat the bullets at .020" off to start. This is where all the charge weights start to show minimized vertical in terms of seating depth. Then go back and test from about 42.8 to 43.8 gr in 0.2 gr increments. If at all possible, record your velocities for every increment. Look
primarily for minimized ES/SD values within that window, with your
secondary focus on group size/shape. The idea is to identify a charge weight window that gives you consistent (minimized) ES/SD, the final charge weight will be the center of that window. Next, carry out a seating depth test in .003" increments from about .012" off to about .027" off, all at the final charge weight you identified previously. Look for at least two
successive seating depth increments that exhibit minimized group spread, ideally a single ragged hole as a couple of your groups show above. You obviously want to avoid groupings that are exaggerated horizontally or vertically. Loading to the longest (i.e. the
shortest jump) of the two or more successive seating depths that exhibit optimal grouping will give you the most room for land erosion before you will need to re-visit seating depth testing.
The above suggestions are merely starting points as I don't know your exact freebore length (~COAL), barrel length, or the type of brass your using. However, you can vary those parameters as necessary based on your specific setup. The key is to optimize charge weight primarily for ES/SD, with group size as a secondary (tie-breaker) readout, then tune in the groups using a fine increment seating depth test. Essentially, that means doing exactly what you already did, except in finer increments for both charge weight and seating depth. The problem with trying to do both at the same time is that the more variables you're trying to analyze and the more groups you have to shoot, even one or two pulled shots can make it extremely difficult to get a good readout on what's happening. Isolate your variables to test one at a time; it can make it much easier to spot the trends you're looking for, even with a bad shot mixed in, than if you're trying to analyze/optimize multiple variables all in the same test.
