How's the wind out there where you are?
Here in eastern Colorado a 5 to 10 mph breeze is a average day.
My 7 saum has an accuracy node about 2950 fps with 160 / 168 g bullets.
This puts me squarely in the 280 AI class of firepower. ( since you brought up 280 )
At 500 yards the 160 bullets get blown about 10 inches or so.
The 100 g bullets from my 25 wssm get blown about twice that far off course at 500.
Just some food for thought.