Ideal scope height?

[bandg]

Point Blank Range is a specific distance from the shooter where your projectile will hit exactly on point of aim.  There can be either ONE or TWO PBR's for a given rifle/pistol depending on how one chooses to zero that gun and this is related to height of the sight above the bore.  If you choose to zero your gun so that the projectile NEVER rises above line of sight then you will have ONE PBR-the zero distance. 
If you zero your gun so that the projectile DOES go above line of sight and then falls back across the line of sight at some further distance then you will have TWO PBR's.  The projectile crosses line of sight at the near PBR, arcs above line of sight for some distance, then gravity pulls it down and it crosses line of sight again at the far PBR and continues dropping below LOS.

MPBR is a span of distance usually stated from shooter out to a maximum yardage where the projectile remains an "acceptable" distance ABOVE or BELOW line of sight.  It is related to sight height above bore, performance of the projectile, AND TARGET SIZE.  MPBR and distant PBR may be related but are not necessarily the same thing.

For an extreme example, think of zeroing your super ultra magnum rifle at 20 yards.  20 yards would be the near PBR but the projectile would rise so far above LOS that it would not be practical.  At some extreme range (assume 1000 yards for this example) the projectile would fall to cross the LOS again at the distant PBR.  The distant PBR would be a really long range out but the SPAN OF DISTANCE from the 20 yard near PBR in this example to the distant PBR would really be unusable in terms of aiming and hitting any normal sized target.  You would have 2 PBR's (points where the projectile hits exactly where aimed) but really an unusable rifle at any distance other than those two specific distances.  Calling the distant PBR point the MPBR is wrong in this case

Consider a 10" diameter as your common target.  You can zero your gun to rise up to 5" above LOS where you will hit the top edge of the target if aiming at its center.  The projectile then arcs (gravity) moving back down to center on the target and continues to fall until it hits the bottom edge of the target.  That distance from where the projectile hits the bottom of the target back to the shooter is the MPBR for that specific setup and is determined by projectile performance, sight height above bore, and specific target size.  Within that MPBR you can aim at the middle of the target and expect to hit it.   Now assume that you need to hit a 4" target with the exact same gun and sight while still aiming at the center.  You would have to change your zero distance so that the projectile would not rise more than 2" above the LOS in order to aim at center and be sure of hitting on the top edge.  This would move the near PBR out some distance but it would also lessen the overall MPBR since you have less projectile "arc" to utilize. 

In hunting terms, consider setting up a rifle for a deer size kill zone where you want to aim at the center of the zone with no holdover or scope "clicks" and be confident in a clean hit.  You would have an MPBR for that specific setup of rifle/scope/bullet, a range out to which you can aim dead center and not worry about rise or drop.  That MPBR would extend from bore to the range at which the bullet fell below the kill zone.  Now take the same rifle out to shoot squirrels in the head (not practical but informative).  In that case, your MPBR would be a very short span of distance centered around the near PBR due to the smaller target size.  A short part of the distance from bore to squirrel head might not allow center aim since the projectile must approach LOS from below (scope height) into the acceptable size range of the smaller target and just beyond that near PBR the bullet would miss high for that smaller target and would continue to impact high until the shooting distance approached the distant PBR.  If you knew the squirrel was at the distant PBR distance you could hit it but you've lost the MPBR concept of knowing that your aim will hit a specific size target from bore to a given distance. 

[Wolverineshooter]

There is an easier method. Uninstall Chairgun, install Java, then reinstall CG.

Good to know. Will do that for the next computer. Thanks

[shooter r]

You certainly don't want it too low.

Too high, just to provide some trajectory compensation would also be a mistake as your chin will float around on your stock.

Another consideration is: are you setting up a high power PCP, or an under 12 fpe springer with a loopy trajectory? I'd go "lower" with a PCP.

[Wolverineshooter]

Something that hasn't been clarified here is that as you increase the scope height, although it extends the ranges at which the PBR occurs....  it SHORTENS the difference between the two (known as the Maximum Point Blank Range, or MPBR).... The PBR is where you can hold "dead on" and have your shots fall within the Kill Zone (KZ).... As an example, for my Hatsan AT-44 Long in .25 cal., shooting a 34 gr. pellet (BC 0.050) at 955 fps.... with a 1/2" KZ, using ChairGun....

1.5" Scope height.... Near PBR = 13.5 yds.... Optimum Zero = 38.6 yds.... Far PBR= 42.9 yds.... MPBR Range (difference) = 29.4 yds (68.5% of Far PBR)....

3.0" Scope height.... Near PBR = 23.4 yds.... Optimum Zero = 47.8 yds.... Far PBR = 51.9 yds.... MPBR Range (difference) = 28.5 yds (54.9% of Far PBR)....

Just something else to consider....

Bob

Thanks Bob, good points.

For the airguns, the "high" and "low" in my mind are more like 1.75 vs 1.25, so I will use my Hatsan AT44S10 as an example. I use H&N Baracuda, 31gr, BC=0.042, speed 720fs. Killzone=0.5, This is un-tuned factory setting.
with scope height 1.25,  optimal far zero at 28.3, PBR"   8.8 to 31.7 yards (diff=22.9)
with scope height 1.75,  optimal far zero at 31.3, PBR" 11.9 to 34.6 yards (diff=22.7)

If I tune the gun to have more power, speed=850fs, then
with scope height 1.25,  optimal far zero at 32.9, PBR" 10.2 to 36.8 yards (diff=26.6)
with scope height 1.75,  optimal far zero at 36.3, PBR" 13.8 to 40.1 yards (diff=26.3)

Indeed the higher mount will have a shorter distance between the PBRs, but only slightly. I tend to think you may encounter something a bit far than a bit close so I would give the far end more weight, therefore I may prefer 1.75 over 1.25 where I may struggle for the mount sometimes.

[Wolverineshooter]

You certainly don't want it too low.

Too high, just to provide some trajectory compensation would also be a mistake as your chin will float around on your stock.

Another consideration is: are you setting up a high power PCP, or an under 12 fpe springer with a loopy trajectory? I'd go "lower" with a PCP.

That is another reason I am thinking to make the mount higher. I struggled with some guns in getting clear pictures. So after all the calculations I think I do not lose much if I stop struggling to always have the lowest mount. I think the thing to look for is the comfortableness when shooting and not worried about the mount being half inch higher.

[TF89]

Interesting points raised here.  To add to what Bob has already stated.  I have always gone as low as I can, most of the air rifles I have used have either no comb or very low ones.  I often have to pad the stock to get enough cheek height other wise I find myself raising my head to get a clear site window.  I don't shoot long distances, for me most shots are inside 35 yards.  Most hunting shots are between 12 and 25 yards.  I zero with a .5 kill zone which works great for my needs.  Outside of that I use the mil-dots.

The various scenarios for using low or high mounts is something i'm go keep in mind going forward.   Really great information and I admit is challenging my steadfast belief in always going low.

Dave