Re: Elevation levels for sighting in a rifle
The interesting word you used was 'much'. Your brother and father in law told you "that the target can be at a much higher or lower angle than the rifle and it will still sight in correctly". As robbobsam and kcebcj said, a slight angle and short range distance shouldn't make much of a difference. However, a 'much higher or lower angle' and the farther the distance will certainly through off the shot.
Essentially, if you zero your rifle at a steep angle and at 500+ yards, as long as you shoot near that same angle, your shots will be on. Once you use that zero for a flat trajectory at similar distance, your shots will be low.
Your best option is to zero at a relatively level trajectory (up to 10 degrees up or down doesn't affect the trajectory) and then aim low if you need to take an angled shot. Once again, a short range and a slight angle will not throw off your shot enough to notice.
Run as fast and as long as you can... my little friend
will catch up and set you free.