Probably 1.5" at 100yds. I haven't tried the seating depth test but just found it on this site. Now that I know what it is I will give it a try. Thank you!
Seating depth might shave a couple tenths of an MOA, but its unlikely to take a 1.5 MOA gun to 3/4 MOA. I did a pretty thorough test with the 208 ELDM, and found that there was no correlation between precision and depth. I tested half a dozen lengths between 3.485" (mag length) and 3.600 (touching lands), firing 3 to 4, four shot groups at each length.
The two best groups were 0.33 and 0.36 MOA at 3.485" and 3.590". The two worst groups were 1.16 MOA and 1.18 MOA at 3.485" and 3.600". When plotted, the data shows no meaningful trend. At any given length, there's about a half MOA spread in precision. Even after subtracting the worst "flier" from each group, to reduce shooter/condition influence, the lack of a trend was identical.
Obviously some bullets are more sensitive to depth than others, but I've not seen much that indicates even the most sensitive designs will be improved more than a few percent.
IMO, don't waste your time/money/barrel on a depth test till you can find a bullet that will shoot good.
Also, how "good" are your current reloads? Are you taking steps to reduce other variables like runout, neck tension, etc?
For reference, I took my rifle (Savage LRH 300WM, Shilen 24" select barrel) from about 1.1 MOA down to about 0.8 MOA by doing all the "standard" variable trimming. I first found a bullet it liked (208 ELD-M), then neck turned my brass, weight sorted, trimmed to uniform length, and reduced concentricity to ≤0.002" runout. The only thing that made an immediately obvious change was the bullet choice. Trimming/neck turning seems to have brought it down another 0.1 or so. The rest of my efforts are in the "noise", though I still do them for peace of mind.
...The rifle is for elk only and desired up to 1000yds. I am currently running Burger 215 hybrids with 78.5 gr. Of H1000 at 2930 FPS...
Also, IMO, the 300 WM is marginal for 1000 yd elk shots, even if you do get it shooting good enough to hit a 12" plate 9 times out of 10 at 1000 yds. My calculations show your current load is down to ~1900 fps and ~1700 ftlbs at 1000 yds. Though Berger says their hunting VLD will expand "adequately" down to ~1800 fps, the target line (your 215 included) supposedly has a thicker jacket than the hunting line. Marginal expansion and energy are not ideal for elk.
Others may of course disagree, but I feel that the 2K/2K rule for velocity/energy is probably good to follow on something as gnarly as an elk (about 850 yds for your load).