Defensive Edge angle/cosine indicator ring caps for nightforce mounts

Doc M said:
...If the firearm is canted by just a few degrees, it makes sweet f*ck all of a difference.

-M
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Ummmmm, no....It changes your point of impact, as already mentioned.
The further you go (distance or cant, or both), the more it makes a difference.
 
Doc M said:
You're absolutely right - if the RETICLE is canted. If the firearm is canted by just a few degrees, it makes sweet f*ck all of a difference.
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Regardless if it is the reticle or rifle that is canted the two are not in the same vertical plane. Either way you will end up with an arching trajectory to one side or the other because the bore will no longer be in the same vertical plane as the reticle. Your reticle can be perfectly plumb to the world, but if the bore is not in the same vertical plane as the reticle, you're hooped. An extreme example would be to take my 308 (with a +20 MOA rail) with it's reticle in the proper vertical plane as the bore and turn the whole rig precisely 90 degrees. You'll still have a scope that is perfectly plumb (albeit with the windage crosshair in the vertical orientation) but the bore will be out of alignment and the bullet will go WAY off course.

Picture the letter X. Now turn it in it's side and orient one of the legs in the horizontal plane. That is where your reticle is looking. The other leg is an exaggerated example of the inclination of the bore and the trajectory (imperfect, but will serve for the sake of the example). Now rotate the X around the axis of the "reticle" leg of the X. As the "barrel" leg of the X rotates to one side or the other, the corresponding end of the X diverges from the vertical plane of the "scope". At close range there is little effect, but as the distance moves further out the divergence becomes more and more pronounced.

And that, dear friends, is why your reticle must be in the same vertical plane as your rifle bore.

Are we all confused now?:D
 
BigUglyMan, can I copy and print this for use at the range and gun stores??? I have a hell of a time trying to get this point across to people.
 
Rman said:
It's a level. If you can't work one, you need help. It is a valuable tool when shooting longer distances to help eliminate cant from your rifle. Being a long range shooter yourself, judging from the rifle you have for sale, I am suprised that you don't know this.

R.
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Truth be known, I have never cared. I had a very qualified person set up the scope at the time of purchase.:) I am selling the rifle because long range shooting is not as interesting or enjoyable for me as I thought it would be.

Shame. I have a 1300 meter range off the back deck.
 
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This all makes decent sense. It occurs to me that having a true action to barrel alignment would help quite a bit also. Gunsmithing starting to play a bigger and bigger role all the time.
 
Shooter ----/ said:
The level in the top picture as posted by the OP, would rely on the milling of the receiver to measure. The second picture as posted by the OP is mounted to the scope only and can be set exactly inline with the bore.
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no, the top picture shows the level that mounts on top of a nightforce scope ring, not on a receiver. take a closer look.
 
and the scope mounts to the rings, which mounts to the receiver, so what's your point exactly?
 
BigUglyMan said:
And that, dear friends, is why your reticle must be in the same vertical plane as your rifle bore.
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So I did the math on it, and I didn't realize how drastic the POI shift could be - a 3 degree cant can lead to 3 yards at 1000yd based on about 84 thousandths of an inch lateral displacement at 1.6" sight height and a 26" barrel measured from the focal plane of the reticle forward.

I suppose errors in my wind doping have been masking my 2-3º cant, because I'm certainly not missing by 36" or more at 600yd... guess there's something to be said for "shooting by feel" in my case.

That said, having actually crunched the numbers, I stand humbly corrected and will keep this in greater mind in the future. I'm man enough to admit when I'm wrong, and the trigonometry doesn't lie - I was wrong. :redface:

Apologies for the fuss, I'll go self-flagellate now... :slap:

-M
 
And now that I think of it, if one had a receiver with holes drilled slightly off-center casting on or off (the right way to better align the reticle center with the bore axis) would actually improve lateral 'dope' at distance.

Maybe that's why I'm not seeing a grand effect? I'm gonna get the calipers out and have a look when I get a chance.

-M
 
Dan, look forward to your mathematical disortation. Finally, a chance to use your Masters in aeronatical engineering! Might even be helpful.

Kindly,

Peter
 
Slavex, My point was in my earlier post.


Shooter ----/ said:
I was warned to stay away from levels that relied on the milling of the receiver. Could be off even a small amount but will show up progressively worse at longer ranges.

Just passing on info that was given to me.
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I guess I should add that the milling of the base also should have been included in that same first post. My bad.

If you use a scope mounted level they are infinitely adjustable so milling on the receiver does not play the same factor.
 
WAIT a minute... so I was just thinking about this, and realized that I've already dialed in the proper lateral correction when I've zeroed at 200yd.

So that must reduce the overall error... I'm going to crunch the numbers again here. My previous calculations were looking at error with respect to the bore axis, but I just realized that what I'd done is calculated for a 26" "adjacent" side on the tangent - which means I'd have zeroed my scope to the end of my barrel... this obviously isn't the case.

Re-calculating now.

-M
 
Okay, here's what I've got for proper numbers now:

ASSUMING :
-26" barrel, measured from focal plane of reticle to muzzle
-Sight height of 1.6" from bore centerline to reticle centerline
-Cant angle of 3º with respect to BORE centerline, but RETICLE perfectly vertical
-Initial zero at 200yd from muzzle (thus, 200yd + 26" from focal plane)
-No other variables influencing lateral displacement of the bullet (i.e. wind, earth's rotation, etc.)

THEN :

200yd + 26" = 7,226" total distance

3º cant, at 1.6" sight height, gives lateral displacement with respect to bore centerline of approximately 0.084".

tan(theta) = 0.084"/7226" = 1.162x10^-5

(If desired, solve for theta and you get an angle of 6.66x10^-4 degrees; this is unnecessary though - just for interest).

Using the relationship tan(theta)=opposite/adjacent,

and using 1,000yd as 'adjacent', with 'opposite' being the lateral displacement error at said distance when 'theta' is our calculated angular error, we have:

1.162x10^-5=lateral error/36,026"

Solving for lateral error, we get approximately 0.418" at 1,000yd + 26" for the focal plane-muzzle distance.

Thus, a 3º cant produces an aiming error of less than half an inch at 1000yd assuming the shot is taken with the reticle perfectly vertical.

So it seems this is why I'm not missing 600yd shots by a yard or more... and I apologize or my premature apology! :p

That is, of course, until rnbra-shooter/Daniel comes in and makes a fool out of me... ? I'm dreading that I may have missed something?

-M
 
I think the amount of error will directly relate to the amount that the scope cants towards the barrel and what distance you sight in for. Most long range shooters install a base with say 20MOA cant to allow for more scope adjustment, but we can not forget that the scope already cants toward the barrel. This is necessary, if it didn’t the bullet would never cross paths with the scope’s line of sight.

That being said if we twist the gun 3 degree’s to one side it will change a small amount of the vertical scope cant (which we need) into a lateral cant of the barrel (which we don’t need). That means if we twist the gun and the scope moves to the right, we are also effectively pointing the barrel to the right, even if the scope reticule is level.

Think about it this way, the bullet path must cross the line of sight, preferably in the vertical axis. If we twist the scope to the right, the necessary scope to barrel cant will still point toward the scope, and now that cant points to the right as well. This means that when the bullet leaves the barrel it travels to the right to cross the line of sight, and continues to travel to the right. Where the bullet crosses the line of sight will determine how fast it travels to the right. Meaning the distance you sight in at has a direct effect on the amount of error.

How much does this affect the windage at 1000yards? I suck at math so I’m not going to try but I suspect very little, as long as your not twisting the gun a noticeable amount.

Also since most shooters sight in at a minimum of 100yards, most of the lateral cant effect is minimized right there. The further your sight in distance, the less the effect of a canted rifle, at least I think that’s how it works.

I probably have it all wrong, but that’s how I see it.

Dave
 
Dave42 said:
How much does this affect the windage at 1000yards? I suck at math so I’m not going to try but I suspect very little, as long as your not twisting the gun a noticeable amount.

Also since most shooters sight in at a minimum of 100yards, most of the lateral cant effect is minimized right there. The further your sight in distance, the less the effect of a canted rifle, at least I think that’s how it works.

I probably have it all wrong, but that’s how I see it.

Dave
Click to expand...

Dave -

this is basically what I've shown mathematically in post #37 directly above.

It also matches what I've seen empirically in the field, so I'm reasonably certain that I haven't gone on too big of a fishing trip... hopefully.

-M
 
While I am sure the Defensive Edge is a fine product, the one from Horus Vision (that is mounted on my .243) is easy to see and simple to use.

HV-ASLI.jpg
 
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