[HiBC]

It will take a little work on your part,but I can tell you where to find your answer,without the bias or unfounded opinions.
Do your own ballistic software work.
Thereare others,but on the Hornady site is a useable ballistic calculator,free.
Crunch your own numbers.

Given your situation,put some form of wind flag,sock,or other reference out.
Make up a nice range card .
With a known barn/fields arrangement, a .308 trajectory will work.
Certainly a Jarrett type 280 Ackley bean field rifle would be ideal.
But IMO,if you will be in a barn over a known field,pick an accurate rifle you have,maybe,and spend the money good quality scope with target knobs (and 10X is enough). Get a good laser rangefinder .Use it to build your range card. For some reason the dang things occasionally won't get a read at 300 yds when you are ranging a deer!!

They make some nice wind gauge/flags you could put out at 50 or 100 yds.

Out to 600 yds,25 yd ranging accuracy should do. You can do that with your card detail. Glassing your wind gauge will give you good,quick info.

With a .308 and 168 gr ballistic tips or a 7-08 with 150 gr Ballistic tips out of a1 MOA rifle,hitting the vitals from a bench in a barn should be (reasonably) duck soup.

Of course,flatter shooting cartridges may be an advantage,but I'd rather have good range and wind data with target grade adjustments.

Regardless where you draw the line of sight through a trajectory,I can't think of a cartridge that will not require correction from 400 to 600 yds. They are all accelerating toward earth at that point.
If I have good inputs,twisting in 5 MOA is not so different than twisting in 7.5 MOA.

For myself,in your setting,I'd choose the laser,good adjustable optics,and something for a wind read along with a detailed range card over some brand new high speed 6mm x 378 Weatherby .

But back to answering the question you asked,with many honorable mentions,a 7MM Rem mag with 162 gr Hornady SST's will do about as well as anything.