Mid-long range bullet rise?
I went to the rifle range today to shoot my dads old Parker Hale .308 (I'm not sure what model). It is at least forty years old. He bought it while living in Alaska to use buffalo hunting and really hasn't used it ten times since then. It has a Redfield 3x-9x scope which is equally as old. I started out by shooting at a 1 inch square at 100yds. I was using federal 150grain soft points. I took three shots to establish a group before I decided on the accuracy. It shot right at 5 inches above point of aim. I then held 5 inches low and was able to put three consecutive shots inside the one inch target (about 1/2 inch group). I was pretty happy with that so I moved the target to 200yds. I used the same -5 inch hold and the group was right at 8 inches above point of aim within a 1.5 inch group and about one inch right due to a 2-5 mph wind. With a -8 inch hold it was on target with a 1.5 inch group. (impact was 3inches higher at 200yds than at 100yds)
I am fairly novice when it comes to mid-long range shooting and my dad has used it so little/so long ago that he doesn't remember much about how it was sighted in.
Which leads me to the questions. Could there actually be continual bullet rise at 200yds? Could sighting it in at a further distance be the cause for higher contact points? (I would assume for buffalo it would have been sighted for 200yds plus) And could the height of the scope account for the higher 200 yd impact vs. the 100 yd impact?
Finally, If I want to do 100 to 400yd shots what is the best range/technique to sighting it in? (should I sight at 50, 100, 250 etc.)