J,
Have a look at what 1000 yard competition shooters are using; in my case Fullbore TR we are restricted to only using Military cals so .303, 223/5.56 and 7.62/308 but these cals do very well and are well tested by us that shoot it. I believe the Palma match boys shoot the 155gr Win like we do and pretty sure the Hi-Power boys stick pretty close to Military cals also ...
You may be far more free to play with various cals, projectiles and loads than we are but I would argue that comp shooters have plenty more experience and data using those cals than anybody does with most others ... try 100 years in the case of Fullbore with .303 ... as an aside the .303 round gets BETTER the longer it runs ... so 1200 yards etc ... not kidding.
Quite a few guys are moving to the lighter cals and doing well but we have just had a new projectile introduced into Fullbore that will leave the 5.56mm boys behind.
There's plenty to it but I personally like .303 and .308 for 1000 yard stuff.
Best, TR,