Re: "Out of Box" 30-378 Accumark acurracy?
let me "think out loud" a little--hopefully I'll give you some ideas.
When I look at the equipment listings in the back of Precision Shooting for 1000 yd BR competitions, I see a lot of chamberings like the 300 Wby Improved or the 308 Baer. I don't remember seeing many 30-378s. I too think the 30-378 needs a really long barrel (30+")to use all that case capacity. (The 300 RUM is very overbore as well). I also see where the long range guys use long heavy bullets with high BCs vs lighter faster bullets with low BCs.
Maybe a good choice would be a #6-7 contour, 28" barreled 300 Win, 300 Wby Imp, or 308 Baer. They will have much better barrel life, will allow you to simply rebarrel your Sendero, and are certainly adequate for long range shooting.
My proposed sloutions to long range hunting/shooting (none yet fully sorted out), all on accurized Rem 700 actions:
Varmints: 27.5" SS Pac Nor #5 1x8 twist in 6mm Rem. I plan to shoot 107 grain matchkings of 115 grain VLDs.
Deer: 28" SS Pac Nor #5 in 300 Win mag (180 grain Swift Scirocco)
Large Game: 26" SS Shilen #5 in 338 RUM (225 grain Barnes XLC)
Like I said, all these are being sorted out as we speak. My idea for long range is bullets with BCs in the .5 range, even if they are slow. I can laser range the distance and know exactly, so I know how much to hold over or crank into my elevation knob. However, the wind is always a guess, so I want a bullet that doesn't drift as much in the wind in case I misjudge. This is nothing I came up with, it's what the long range target shooters do.
As a side note, my intial deer load for my 30-06 is a 180 grain Sierra Gameking (BC .5) at a chronographed MV of 2880 fps. I was in a rush to get ready for this season, so I haven't fully sorted out the load. Even so, it is averaging .6 moa at 100 yds. I haven't had time to test it at longer ranges yet, but my expereince with long bullets is that they moa better at 200 and 300 yds than they do at 100. When you look at the ballistics data for a .5 BC bullet with an MV of 2880, it is amazing how flat and wind resistant it is--much more than I thought at fist glance.