so which is it? a different power level or a more accurate rifle?
Yes please.
I want a rifle that will hit and kill a big whitetail that kept at 400+ yards.
I realize that will take a lot of practice from me and may never happen but I don't want my gun holding me back.
This is helpful information. So a big Whitetail is the upper limit in weight & 400 yards & some change is the max distance. Bigger animals or
a little longer shooting or a combination of both is where the 300 mags & big 338 shine, but not needed here at all unless you just want one. So, I would eliminate those & I like and shoot a 300 mag from time to time & I am personally fond of them.
The 7mag, 270 Wea. mag, 264, 270WSM are great & I would not fault anyone who made that choice, but keep in mind with a cap at 400 yards they are not required but nice. All of these calibers have good bullets that buck the wind, the 270 has the fewest choices, but with bullets like the 140Acc, you don't need many choices. The 7mag has the most factory choices.
The 308, 7mm-08 & 260 would work if you have time to precisely measure the yardage(not saying you don't need to do that anyway, but more critical with these), but you have a 7-08, might as well go for more cartridge.
The 25-06(only with the better loads), 270, 30-06 & 280 would be fine & you have a huge variety of 30-06 ammo as stated. I would lean more toward the 270 & 280 since you have the Varmit end covered. I have used the 25-06 a bunch for years & it is a jewel, but it was also because I also did long range Varmiting with that round as well. It is a standout for that & the best Antelope round going along with the 257 WEA. & 400 yard Mule Deer are not a problem, but that is about the limit.
MY 270 was used this year in WY & AR to take several Antelope, Mule Deer & Whitetail, 7 so far, from up close to 410 yards & it was devastating with the 140Accubond. It would do your job with ease.
Add the 30-06AI & 280AI to the list if you want to be a serious reloader, I have a 30-06AI I am fond of, but it is used more as a rifle for Deer & Elk. The 280AI would be perfect.
So, for factory ammo I would go with the 270 or 7 mag, 257 Mag if you want to buy the ammo & a 26" version of Wea.
For a serious reloader, still the 270, & 7 mag, fast 25's & add the 280AI, which could be the best choice of all other than resale considerations & the 280AI could use the same bullets as your 7-08 in a pinch.