You may consider a .408 cheytac for long range target shooting.
Since you are not going to shoot game beyond 300 yards and want an accurate rifle for long range target shooting, just about any high powered rifle rifle should do the job.
I think the .270, 30-06, 308 and even the 6.5 Swedish Mauser would do the job. If you don’t mind a bit more recoil then the 7mm Remington Mag, all the .300 mags, and the 8mm Remington would all produce more than enough power.