Why height over bore does not matter

[Sharpie]

As long as you can get a consistent and comfortable position with your cheek/jaw then that's good.

Only issue with a high scope mount is when you have a close zero distance and then are required to take further away shots as now the the point blank range is either much less or you need a bigger target size to ensure hits without aiming off.

It's not quite as simple as going for the lowest scope height possible. Depending what you want to do there may be an optimum combination of scope height, zero distance, "maximum point blank range" etc. for you particular setup and acceptable "kill zone" dimensions. Of course those who use laser rangefinders and dial in with ballistic turrets can ignore this to an extent, providing they have calculated or better tested their drops over the range of interest.

It can become an issue with some bullpup designs where the 'scope has to be mounted substantially above the boreline of the barrel.

 

[Mike L]

Cheek weld' my backside lol.

Is that a medical procedure?

 

[Smellydog]

By the way the child is shooting a 9422.

 

[HandB]

Mark and Sam have delved into this topic. They found that higher mounted scopes allow the shooter's eyes and neck to be square and level behind the scope rather than tilted down to the right as happens with a low mounted scope for a right eyed shooter. That is for prone long range shooting so they basically get down behind the rifle and mount the scope at whatever height naturally allows their eyes and neck to be level. Of course they have an adjustable cheek piece that can go high enough to match this. I do this for my long range rifles but for my short range running boar rifle I use a small LPVO scope mounted low as that suits shotgun style snap shooting usage.

 

[muddy42]

For me rifle shooting and shotgun shooting have a lot of similarities. With a rifle, you often startle deer and have to raise the rifle and act quickly. Walked up rough shotgun shooting also involves a degree of stalking, as the birds are wild and few and far between. Familiarity and being able to acquire the target quickly is a real benefit. I want my rifle to feel as similar to my shotgun as possible. A massive high scope might "work" fine if you are just punching paper, but completely impractical in the real world. I use the same reasoning against trigger grips, thumb hole stocks, long barrels etc..

 

[Sharpie]

Interestingly my mental picture of this comes from the other direction.

The vertical trajectory plane is the constant. Thus, when you cant the rifle you just move the line of sight to the side relative to the trajectory rather than shifting the point of impact....

Alan

If I have understood you, I don't think that's correct. Though it amounts to much the same. What concerns us is point of aim vs. point of impact. What matters is the point of impact. In reality what we have control of is the point of aim.

My mental image is that the sightline of the 'scope should be absolutely vertically above, and also horizontally in line, with the bore axis of the barrel.

Cant alters this, resulting in horizontal stringing. As also does a 'scope mounting which is not absolutely in line with the bore axis, which is where good quality mounts, rails etc. matter. If they are only slightly horizontally misaligned similar things will happen. E.g. get a perfect zero, preferably at the second, not the first one (e.g. say you have the first zero at 100 yards and the second at 200), and except at precisely 200 yards the POI will be off to one side or the other.

Of course there are the old school types who consider that zeroing say 1.5" high at 100 yards is a universal method. I don't have much time for that. Enough to get you on the paper at further distances perhaps.

In the example I quoted, 1.5" scope height (that is optical centre of scope above centre of bore), using a zero of 200 yards. The primary zero will then be at between 25 and 50 yards. At 100 yards it will actually be 1.81" high. Quite how you could do that anyway with even a genuine 1 MOA rifle and determine where the centre of a group is to within tenths of an inch, I don't know. Much better to complete your zeroing at the secondary zero, even if it means you have to walk 400 yards every time you look at the target. or, better, use a spotting scope.

 

[ChesterP]

The reason I suspect most precision shooters like the scope as low as possible is because it reduces any cant error. The higher the scope mount, then the included angle between bore and line of is is rotated, acting a bit like windage error but with reduced elevation of shot. The vertical compensation for trajectory is lowered and the projectile flies off left or right from POI depending on cant angle. The fashion for larger diameter objective lenses and tubes exacerbates this problem as optical centres have to be higher above the barrel, so this amplifies cant error in the field. The error is also increased with distance. That's why it's a good idea to limit the height above the barrel because it does matter. It minimises error. The main point is not to get too wrapped up in the geometry and just understand, especially when in the field, that some way of ensuring a vertical hold is important, more so at distance. You can buy little flick out cant levels for scopes but they're fiddly at best to use as you position for the shot. I have fitted a picatinny rail one in front of my scopes anyway. Having a reliable cheek weld position is essential for consistent shot placement and one way of practicing this set up is once you are satisfied that your scope is correctly mounted, you can place a plumb line over a vertical board at say 50 yds, then look through the scope and see where your natural shooting position has the cross hairs in relation to the plumb. It's a simple way of trying to optimise your set up and fit.

The subject's been done to death over the years and any you tuber claiming it doesn't mater clearly didn't do very well at geometry lessons at school. One article worth reading which sums up the issues neatly was written a few years ago by a guy called Mike Salcedo, and he published his summary which can be found HERE

 

[Klenchblaize]

K

 

[Alantoo]

If I have understood you, I don't think that's correct. Though it amounts to much the same. What concerns us is point of aim vs. point of impact. What matters is the point of impact. In reality what we have control of is the point of aim.

My mental image is that the sightline of the 'scope should be absolutely vertically above, and also horizontally in line, with the bore axis of the barrel.

I don't think you did understand me.

My concept of the relationship with the trajectory plane being the constant was confirmed to me when I was offered a few rounds with an old military rifle which had an offset scope.

The scope was mounted some 20mm to the left of vertically above the bore when the rifle was held without cant. The rifle had been zeroed by recognising the horizontal offset of 20mm between the trajectory plane and line of sight. With the reticle of the scope vertical when the rifle was held without cant, the 20mm offset line of sight was parallel and therefore constant throughout the entire trajectory.

Alan

 

[Sharpie]

You can then calculate the effect of say five degrees of cant. Which is drop x (cosine 5 degrees). The same calculation as for the "rifleman's rule" for when shooting up or down hill.

So at 100 yards five degrees of cant moves the POI sideways by 2.31"

At 200 yards the same 5 degrees of cant moves it sideways by 9.72"

Sorry, I messed that up I now realise (it did seem rather a lot.

Thinking about it again, it should be drop x (tangent 5 degrees).

So at 100 yards, 2.32" drop, 5 degrees of cant moves it by 0.20"
At 200 yards 9.76" drop, gives 0.85"

That seems much more likely

But say you had 15 degrees of cant. Probably more realistic even for a good shooter.

100 yards > 0.62"
200yards > 2.615"

 

[Muir]

I really couldn't give a hoot about whether it matters or not but the question begs one more: Why do ballistics programs care about accurately measuring height above the bore? Never have seen a thorough ballistics program that doesn't. ~Muir

 

[Alantoo]

Why do ballistics programs care about accurately measuring height above the bore?

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

 

[Sharpie]

I don't think you did understand me.

My concept of the relationship and the trajectory plane being the constant was confirmed to me when I was offered a few rounds with an old military rifle which had an offset scope.

The scope was mounted some 20mm to the left of vertically above the bore when the rifle was held without cant. The rifle had been zeroed by recognising the horizontal offset of 20mm between the trajectory plane and line of sight. With the reticle of the scope vertical when the rifle was held without cant, the 20mm offset line of sight was parallel and therefore constant throughout the entire trajectory.

Alan

That is a different issue. You are correct, if your scope is offset say 2cm to the left, and you zero it 2cm to the right, i.e. your POI is 2cm to the right of the POA, that will be maintained at any distance. However you have to "aim off" by 2cm at whatever distance.

I daresay that's what WW2 snipers used to do with the offset scopes on their Mosins, and Enfields. The German snipers had the advantage of a centrally mounted scope on their KAR98s

 

[BenBhoy]

Here it is…

Ow shït, that'll spoil that pretty face.....

 

[User00040]

That is a different issue. You are correct, if your scope is offset say 2cm to the left, and you zero it 2cm to the right, i.e. your POI is 2cm to the right of the POA, that will be maintained at any distance. However you have to "aim off" by 2cm at whatever distance.

I daresay that's what WW2 snipers used to do with the offset scopes on their Mosins, and Enfields. The German snipers had the advantage of a centrally mounted scope on their KAR98s

Do you own a collimator?

You'll find that a lot of scopes are 'zeroed' with the windage slightly off center due to stock design (scope zeroed slightly to the left for right handed shooters and vice versa for lefties).

It's not just a cast on/off issue either...

 

[Klenchblaize]

No but I’ve got a set of feeler gauges and a sharp knife to trim a Butler Creek objective cover.

K

 

[Sharpie]

I really couldn't give a hoot about whether it matters or not but the question begs one more: Why do ballistics programs care about accurately measuring height above the bore? Never have seen a thorough ballistics program that doesn't. ~Muir

Maybe it does matter a tiny little bit when it comes to calculating drops at hunting ranges. But not by a lot. However it does make a difference for long range shooting though. Where having your scope mounted as low as possible is probably not the best idea.

E.g. on my example, with a scope centreline height of 1" (that might be a really closely fitting 32 mm objective scope) and a 200 yard secondary zero.



If instead the scope was mounted at 2.5" high (that might be say a 56mm objective scope, not quite so closely fitted), that would flatten out the curve a little bit.


Now, taking it to the extreme, if it was possible to have a scope height of zero, the ballistics would look like this: Significantly less flat-shooting. So it is not so clear cut that you should be striving to mount your scope as low as possible, aesthetically pleasing though it might be.

 

[Alantoo]

That is a different issue.

It is the issue I was musing on prompted by your comment that the POI shifts sideways.

Any cant moves the point of impact sideways, depending on the distance you are shooting to.

I agree that effectively the POI does shift, and not just sideways, elevation is also effected by cant as well. Hence my comment that the line of sight is diagonal to the trajectory plane...the dreaded edit limit closed before I could add "when viewed from above"

The vertical plane of the trajectory is the only constant, and why the gist of my comment was that I find that is the most useful datum. Everything else, mounting of scope whether offset or rotated, cant of rifle, POI, POA and etc. are all variables in relation to it.

Alan

 

[Sharpie]

It is the issue I was musing on prompted by your comment that the POI shifts sideways.



I agree that effectively the POI does shift, and not just sideways, elevation is also effected by cant as well. Hence my comment that the line of sight is diagonal to the trajectory plane...the dreaded edit limit closed before I could add "when viewed from above"

The vertical plane of the trajectory is the only constant, and why the gist of my comment was that I find that is the most useful datum. Everything else, mounting of scope whether offset or rotated, cant of rifle, POI, POA and etc. are all variables in relation to it.

Alan

Yes I was simplifying things. Cant does not purely shift the POI horizontally, but in an arc of a circle, of radius equal to the drop. So there is a small contribution to vertical shift as well. I was trying not to get too complicated.

The point I was trying to make is that the effect of cant is purely related to the basic "raw" vertical drop from the muzzle which is a constant. Purely affected by the muzzle velocity of the bullet, together with how it slows down in flight whilst gravity works on it (BC related).

Shooting angle also come into it. E.g. if you shoot a bullet vertically into the sky it will theoretically drop back down to where it was fired from. Cant would have no effect.

Sight height, zeroing distance etc. are not factors.

 

[Alantoo]

E.g. if you shoot a bullet vertically into the sky it will theoretically drop back down to where it was fired from. Cant would have no effect.

how could you tell?

I was also interested in your comment that a higher mounted scope would [effectively] flatten out the curve a little bit...my take was that a lower mounted scope would be tangential to a point on a flatter curved section of the trajectory parabola and thus give you a greater point blank range. The PB range would start and finish closer but should extend over a greater distance than the high mounted scope who's PB range would start and finish further away but actually extend over a shorter distance given the tightening curve of the parabola?

Alan