I started sighting(going for a 250 zero)in with federal fusion 150's.......I got on at 100 ...dead on but 2.5" high within 6 shots with the brand new set up !! got it on at 200 within a few more shots . This was all with a sand bag !! Shot it at 300 and they all hit lower than I expected and missed the target(some below and some a tad right)!!
How much lower did you expect them to hit at 300 yards, and how far lower did they hit? If you are dead on at 200 yards, I would adjust the elevation up 3 minutes to get you close to where you should be at 300.
Was running low on Fusions so I shot a few Fed.Accubonds.......nothing. Went back to 100 and was 2" high........went to 200 and was 2" low and right !!
If you sight in with one brand of ammo then switch brands, or even bullets or bullet weights with the same brand, you are going to be chasing your tail. They will all hit in different places unless the stars, moon, and the planet venus are perfectly aligned for you.