Question about AR's and scopes

[pecospuppy]

I've had my DB15 for about a year.  I was running a red dot and saw a 50% off deal on a 1.25 - 4 x 20mm Leupold AR scope so I bought it.  Today I sighted it in.  I think I did ok for an old guy with old eyeballs.  It's not yet 1 MOA.  This is from 120 Yds.

Question about AR's and scopes

My question is why does the bullet's trajectory rise as you move from 50 to 100+ yards?  I first got on paper at 50 Yds.  Then I moved to 120 and the rifle was shooting high.  About 5 inches high.  If I had to guess I'd say it's because of the height the scope is set above the barrel.  In order to be on the same point at 50 yards the scope and the barrel make an acute angle.  When you move to 120 yards, the bullet has more time to rise in comparison to what the scope is seeing, which hasn't changed. 

If I am right, which distance should I use to sight it in?  The obvious answer is "whatever distance I am going to be shooting", but if I am expecting to shoot both, which one do I choose?  And, do I then just use Kentucky windage when I am at the other distance?  If I was taking a 400 Yd shot I'd have to so I suspect I would just do that for a 50yd shot if I leave it zeroed at 120 yds.  If there is a way to get both I'd like to know what it is.

[superbeau]

Check out this video on YouTube. It's a pretty good intro to the bigger question you're asking. These folks have a lot of good videos on YT and are worth following.

[pecospuppy]

Thanks, I was just watching these videos looking for the answer.  I hadn't seen this one yet.  I understood everything he was showing but I still am not sure of my answer. 

In his example the bullet trajectory was always dropping after the zero distance of 100 Yds, so everything was adjusted to raise the impact point.  In my example the bullet is still rising after the zero point.  I am not sure where it is going to begin falling after reaching it's apex.  So again I need to know if I should leave it zero'd at 120 Yds, or some other distance.  One of the military guys was saying a 25 yd zero will also be zero'd at 300 Yds, and a 50 Yd Zero will be zero'd at 200 Yds.  One showed that a 36 Yd zero will be the closest for all ranges up to 300 Yds with the rise and drop being withing a DVD disc size. 

Maybe that is good enough.  I would have to hold above the target for closer in stuff because of the distance the scope is above the barrel, but anything farther out than 36 Yds I can just hold on the target and be within a DVD disk out to 300 Yds.  I think Ill try that.

I just found this guy who explains what I am seeing.  My numbers are a little different from his.  I think my ammo is a little hotter than his.

[superbeau]

Maybe play around with Nikon's SpotOn website. I have found it accurate for my setups. It'll show you trajectory and POI at varying distances.

http://spoton.nikonsportoptics.com/spoton/spoton.html#Index:4