Why height over bore does not matter

PSE Composites Limited - The next level Rifle Stocks
caberslash said:
Met a guy who shot rabbits on an island, he had the scope mounted on his .22 semi so the reticle was an ' X ' instead of the proper ' + '... asked him why, his reply: 'They're called cross-hairs!' :rofl:
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Although unconventional, that would work. Zeroing it would be interesting, but once zeroed it would be OK.

The problem occurs when the reticle is just slightly out of true, and your eye causes you to instinctively rectify it, which of course you do by canting the rifle slightly without even being aware that you've done it. The higher the scope is above the bore the more pronounced the cant will be.
 
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Sharpie said:
OK, here is a handwaving example, taken to the extreme, about how cant can affect both horizontal and vertical.

Consider a rifle sighted in correctly at say 200 yards. In my example the scope might be angled so that is is compensating for say 10" of vertical drop from the bore axis, due to gravity. Ignoring the influence of scope optical axis separation from bore axis.

Now cant the rifle by 90 degrees and shoot it "gangsta style"

Simplistically you could now expect the bullets to hit 10" to the side, and 10" low.

Cant it instead by 180 degrees (shoot it upside down), and expect it to shoot 20" low, but horizontally in line.
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I was discussing exactly this at a shoot a year or so back. The comment was made that this would never be necessary. A knowledgeable chap who had spent a few years in the sandbox reported that you never need it …….until you have to engage a target from under a car with a long magazine in your rifle!! Apparently the Israelis practice this a lot.
 
srvet said:
I was discussing exactly this at a shoot a year or so back. The comment was made that this would never be necessary. A knowledgeable chap who had spent a few years in the sandbox reported that you never need it …….until you have to engage a target from under a car with a long magazine in your rifle!! Apparently the Israelis practice this a lot.
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Did he describe how they went about the calculation? Presumably if the scope was mounted on a 20MOA base they could not adjust windage and elevation to compensate / reset to a previously zeroed setting with 90˚ cant.....So 3D geometry or rule of thumb?

I would be inclined (pun intended) to take the magazine off and shoot singles... :)

Alan
 
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Alantoo said:
Did he describe how they went about the calculation? Presumably if the scope was mounted on a 20MOA base they could not adjust windage and elevation to compensate / reset to a previously zeroed setting with 90˚ cant.....So 3D geometry or rule of thumb?

I would be inclined (pun intended) to take the magazine off and shoot singles... :)

Alan
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Yes we discussed this at length. At 90 degrees of rotation the elevation becomes windage and vice versa. Due to the inclined barrel in relation to line of sight this means that the bullet point of impact will hit progressively further to the left if you rotate the rifle anti-clockwise and right if you rotate the rifle clockwise. The vertical point of impact of a rotated rifle behaves differently and will always be lower than the point of aim. I haven’t had the need to try it for real yet….
So being a right handed shooter who will typically always rotate the rifle anti-clockwise, the next time I have to address a rogue deer from under my car I will expect the point of impact to be low and left of my point of aim!
I seem to recall the magnitude of left POI is about 4-5 inches at 100. Considering the situations the Israelis use this in I think removing the magazine might be a risky thing to do!!
 
Alantoo said:
Is it not because the higher the scope is mounted the greater the distance and the more curved the section of trajectory the line of sight is tangential to?

Minimal difference in centre fire hunting rifles but maybe more significant in 12ftlb Air rifles and subsonic 22LR?

Alan
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Minimal you say. Well lets look at my .243 with scope sat 2" inches above bore.
Using my preferred load aiming dead on the rifle is spot on at 47 yards and 2" low at 243 yards and at some point its exactly 2" above the line of sight. So in the hunting field I am never more than 2" above or below point of aim out to 243 yards.
Raise the scope 1/2" brings the bang on too 35yards? 2 inches low at 255yards and some 5" high at some point in-between, it alters the trajectory curve somewhat! So I would say it make a substantial difference to all rifles.
 
levigsp said:
Minimal you say. Well lets look at my .243 with scope sat 2" inches above bore.
Using my preferred load aiming dead on the rifle is spot on at 47 yards and 2" low at 243 yards and at some point its exactly 2" above the line of sight. So in the hunting field I am never more than 2" above or below point of aim out to 243 yards.
Raise the scope 1/2" brings the bang on too 35yards? 2 inches low at 255yards and some 5" high at some point in-between, it alters the trajectory curve somewhat! So I would say it make a substantial difference to all rifles.
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Well raising the scope mounting doesn't alter the trajectory curve of course. You just interact with a different, tighter curved section of it.

The higher the scope mounting the further the point in trajectory that is tangential to line of sight. The further that tangential point is along the trajectory, the tighter the trajectory curve. Because of the tighter curve you will have less distance between near and far PB extremes. Although that PB range will be further away from you.

Your example of a 5" high trajectory peak above the line of sight is misleading...it no longer is a 2" above and 2" below PBR.

If you keep your maximum height of the trajectory at 2" above the line of sight with the higher mounted scope, ie setting your ballistic tables to maintain your original maximum trajectory height at 2" above line of sight, you will have a PBR which is shorter but peaks further out along the trajectory.

My "minimal" comment was that we do not tend to use the steeply dropping part of the trajectory for hunting with centre fire...but we do push towards it with 12ftlb and 22LR

Alan
 
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Height of scope above the bore does make quite a difference to how need to adjust for drop etc. Have a play with a Ballistic Calculator with different sight lines above the bore.
 
Well it's all guff! Anyone with a wit of sense will agree that the most sensible reason for choosing from a low mount, a medium mount of a high mount is sixfold.

1) To ensure that the 'scope can be mounted so that the bolt handle when manipulated it easily past the rear end of the telescopic sight.
2) To ensure (if iron sights are fitted to the gun) that the 'scope is high enough to not pick up a ghost image of the front sight.
3) To give the maximum "point blank range" (see explanation below) according to the actual "target" that it is intended to kill.
4) To try and expose the least amount of the very visible pink flesh of the users face to that which is being shot at.
5) If relevant to achieve a compromise between when the gun is used prone and when the gun is used standing up, off sticks or sitting or kneeling.
6) If the rifle has iron sights fitted and is intended to be used with the 'scope off to achieve a compromise of a comfortable head position for both uses.

Note 1)
Better as if as sometimes has to be done a crescent shape has to machined from the upper surface of the bolt handle at least that's kept minimal.

Note 3)
On a military rifle fitted with a fixed power 'scope the "point blank range" will usually be four inches above and four inches below the aiming point on the picket post (or crosshairs) on the 'scope. That is so that a bullet aimed with no elevation adjustment at the centre of a human face will strike somewhere on the head of the enemy. It's why the scoring circle on the British Figure 14 "Hun Head" target is where it is. On the Pattern 1919 sniper rifle that was built on the Enfield Pattern '14 .303 that was a very usable 475 yards. So if you aimed at the the centre of the face of an enemy at any range from 0 to 475 yards you'd hit him in the head.

On a deer we are trying to kill cleanly and instantly not to maim or to wound. So our killing zone is less. I'd say that two inches (or even one and a half inches) above and below the point aimed at is the most many would be confident with of killing instantly with with a heart or lung shot. So for a sporting rifle for use on deer our mount after we've taken account of 1) and 2) can be lower especially with high velocity cartridges over 2,600 fps.

Note 4)
Less important for stalkers and hunters but may be relevant for military use.

Note 6)
There's never a "best of both worlds" in truth and "one size fits all" doesn't work. Many older rifles will have been stocked for use with iron sights. The reason why Enfield No4(T) had to use a screwed on cheek piece. Take the cheek piece off and try and use the 'scope on a No4(T)! So it's best to choose the height above bore that works best with what, 'scope or iron sights, is going to be used most often especially where rapid acquisition is needed such as driven boar or even running squirrels.

FWIW with my rifles (they all still have their iron sights) I preferred 'scopes that are a maximum of 36mm diameter at the front end...or a worst 42mm...so that I can service 1) and 2) and do this with a maximum height of centre of 'scope above bore of one and a half inches to two inches. That's why I always preferred Zeiss Diavari-C 3-9x36 'scopes.

In "old school" stalking rifles the set up height and factory integral mounting system Zeiss-Jena factory mounts and 'scope on iron sight BRNO ZKK rifles were sublime perfection! On modern rifles Ruger's system if you want a detachable 'scope (but yet retain and use the rifle's factory mounted iron sights) runs it close. Or on Mauser '98 actioned rifles where money is truly no object Holland's side mounting system.
 
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The antithesis:
www.thestalkingdirectory.co.uk

Sold: - Leupold VXL 4.5-14 x 50 and Tally scope rings

I have this on my LH Anschutz (which I am also selling separately) Both have never been in the field, only at the range at Bisley half a dozen times Comes with a pair of Tally scope rings which are excellent Selling as I am residing abroad and will not be able to bring these over...
www.thestalkingdirectory.co.uk www.thestalkingdirectory.co.uk
I wish I'd bought it!
 
VSS said:
The antithesis:
www.thestalkingdirectory.co.uk

Sold: - Leupold VXL 4.5-14 x 50 and Tally scope rings

I have this on my LH Anschutz (which I am also selling separately) Both have never been in the field, only at the range at Bisley half a dozen times Comes with a pair of Tally scope rings which are excellent Selling as I am residing abroad and will not be able to bring these over...
www.thestalkingdirectory.co.uk www.thestalkingdirectory.co.uk
I wish I'd bought it!
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Me too as it would have spared yours truly the cost of PRS milling a slot in the chamber section of barrel and re-proofing!

K
 
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Of course we are all assuming a barreled action is perfectly true and no slight angle from true is inclining the scope at all!
 
Smellydog said:
Of course we are all assuming a barreled action is perfectly true and no slight angle from true is inclining the scope at all!
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Or more importantly, that the bore is true and barrel clocked to the centreline of the action...

Very rarely is, and I suspect this will be out on even some ridiculously priced 'custom rifles'!

Truth is, these things don't matter so as long as they remain constant.
 
It does matter, (preparing for incoming) not in a huge technical way but in being comfortable and being able to have a accurate repetion of head position between each shot.

David.
 
And in a technical way it is pretty important too, in a "the other guy may well shoot back" situation, having the 'scope close to the barrel means less showing to the slightly annoyed onlooker.

David.
 
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