Mosin Nagant scope height

[earsplittingkarbine]

A while back I mounted one of Kalinka Optic's POSP 4x24 scopes on a 91/30. The scope sits very high above the receiver and there is no cheek weld. It has shot so well at 100 yards that i'm willing to deal with the lack of a proper cheek weld while shooting. What worries me about the setup however is how the scope being offset and about five inches above the bore line might affect POI/POA at distances past 100 yards. Anyone have any experience in this? Would it be wise to sight the scope in so it shoots low and right so the line of sight doesn't cross the bullets trajectory so early?

The setup looks like this:

http://imageshack.us/photo/my-images/577/dragunoivscopemountwith.jpg/

 

[MilJunkie]

Basically, there are two factors at play, one is the declination of the line of sight, the other is parallax.

The increased height increases the declination, or the rate at which your line of sight declines beyond the zero point. This means it takes less MOA of adjustment because the line of sight is already moving down and thus bringing the barrel up. Declination only effects vertical adjustment.

A quick calculation of the typ. 174gr .5BC bullet at 2600fps shows at 500yds with a 1.5in sight height the 'come-up' is 12MOA, while at a 4 inch height it is only 10MOA.



The second factor, parallax, is the degree to which the scope is not 'inline' with the bore, even if offset. This causes an increasing degree of error as the distance increases, and can be seen in horizontal AND vertical deviation.

Parallax can happen no matter the height, but may be increased as height increases.