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Scope height

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/30/2008 at 19:49
jackG View Drop Down
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I'm relatively new at this scope thing.  I've cranked my rifle ballistic data through a ballistic calculator in order to determine point of impact for the type of hunting I do.  I've been using the point of impact at 100 yards to sight in, based on that calculated trejectory.  I just read an article declaring that the height of the scope above the bore will have a pretty dramatic effect on the trajectory.  A small difference in scope height can move the point of impact by several inches down range.
 
I've mounted a Kahles with a 52 mm objective on a Tikka T3 lite.  The rings recommended by Talley are "medium."  Short of screwing around with a tape measure, I'm not sure how high the sight axis is above the bore axis.  It there a web site that might tell me that?  Thanks.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/30/2008 at 20:58
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I've always been told to get your scope as close to the bore as you can...within reason.  I know that doesn't answer your ?, but I'd be pulling anything more out of my 'lower unit'.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/03/2008 at 15:58
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Originally posted by jackG jackG wrote:

I I just read an article declaring that the height of the scope above the bore will have a pretty dramatic effect on the trajectory.  A small difference in scope height can move the point of impact by several inches down range.
 

I've mounted a Kahles with a 52 mm objective on a Tikka T3 lite.  The rings recommended by Talley are "medium."  Short of screwing around with a tape measure, I'm not sure how high the sight axis is above the bore axis.  It there a web site that might tell me that?  Thanks.


To measure scope center above bore center, which is the correct measurement you need to seek, you must break down (remove the stock from) your rifle. With a caliper, measure outside diameter of objective (lens diameter Lsubd), measure the distance from bottom of the scope(objective) to top of barrel (Offset), measure the barrel diameter directly below where you measured Lsubd(Barrel diameter Bsubd). Height Above Bore (HAB) is Lsubd/2+Offset+Bsubd/2. For my 30-06 the difference between a 1.5in HAB and 2.5in HAB would be approximately 1in at 200yds and 9in at 1000yds. As long as you don't change ring height, your trajectory calculations will be OK. Just know what you are working with. That is why a good ballistics calculation will take HAB into account.
A primary reason for keeping the scope mounted "low to the barrel" is for consistent cheek weld. It is harder for most people to be consistent with a "tall" scope. It can also be uncomfortable causing "craning" of the neck.
If you are not looking for "exact" measurement, you can simply measure(as closely as possible) from the center of the scope to the center of the barrel/bolt. This measurement will serve probably 95% of shooters.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/03/2008 at 16:29
Ed Connelly View Drop Down
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Why don't you just sight the rifle in and see where the bullet holes go at 100, 200, 300 yards, etc. ?   Bandito
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/04/2008 at 10:28
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Ed - you're right.  I've ordered some custom ammo that I'm now waiting for.   The rifle will have to be re-sighted for that.  I'm switching from Fed 140 gr accubond (270 WSM) to Conley 130 polymer tipped barnes TSX.  The velocities are not much different, but with a different bullet I don't expect the point of aim to be identical.
 
As noted, I've gone from standard, or low rings, to medium to accomodate the larger Kahles objective.  I suspect at my hunting ranges, not much over 300 yards, whatever effect the taller rings, it won't be much.   With the Accubonds I sighted in at just shy of 2" at 100 yards.  That produces a point blank range of around 320 yards or so on deer size targets.    I could have stuck with the Accubonds, but trying other stuff is half the fun. 
 
Ballistic calculators are certainly useful.   The trajectory can be tracked at 10 yard intervals all the way out.  I was just not sure what the scope height is, which the calculator requires.
 
Since I was using a 42 mm objective, keeping everything equal, this objective appears to be .5" higher.  It's the difference between 42 and 52mm, divided by 2.  But since I don't know what the 42 mm with low rings scope height was, I don't know what I'm adding the .5" to.  Again, the best way is to trot out to the range, which I'll do anyway, and test it empirically. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/04/2008 at 19:38
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Originally posted by Ed Connelly Ed Connelly wrote:

Why don't you just sight the rifle in and see where the bullet holes go at 100, 200, 300 yards, etc. ?   Bandito


As long as you don't mind expending ammo...
The method described above will save you some shots, but if you are rich, unlike me who is poor, the time spent in calculation is probably not worthwhile. It also helps in knowing all the mechanics of your weapon. Some don't need or want to, which is OK. I sometimes pontificate thinking (needlessly) I am being of assistance.
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