Scope-rifle angle cant alignment - does it matter?

[adamg]

Here's my thoughts on the matter.

Say I mount my scope like a hack - no special leveling tools - and I mis-align by 2 degrees. When installing the scope I can get it within 2 degrees with some combination of eyeball and crappy 8" levels applied to turret caps and pic rails. A common height over bore for a scope on these guns is, say 6cm at the high end. 2 degrees over 6cm is 0.21cm. Meaning the bore is going to be offset left or right of the vertical axis of the scope by 0.21cm. That's 2.1mm.

When I go to zero my rifle at 100m, my 0.1mil click is 1cm. Meaning I can't even dial one click to compensate. It's worth mentioning that compensating so it is truly zeroed at 100m would be the worst thing one could do, best thing I could do is leave it hitting horizontally off by that same 0.21cm.

Now if I step out to a 300m target my zero will be off by how much? The answer is 0.21cm. So this is where my take that aligning rifle relative to scope isn't critical to accuracy. What _is_ critical is zeroing while the reticle is absolutely as level as you can hold it, and then taking your shots with the reticle absolutely as level as you can hold it.

If the gun is offset sideways of the reticle by 0.21cm because you mounted your scope crooked by 2 degrees, you'll never notice it or be able to measure it on a target.

There it is, what say you? I imagine there are some good articles on the interweb about this topic to confirm or disconfirm my notion.

 

[stampede]

Exactly.

 

[rugbydave]

Yeah, this gets beat to death every once in a while. the scope being absolutely level/plumb to the rifle doesn't really matter. What matters is that when you shoot your reticle is always plumb, hence putting a level on your scope and setting it up so it shows plumb while your reticle is lined up with a plumb bob. Then do a tall target test to both make sure you got it plumb and that your turrets track precisely.

 

[Alpheus]

How would a canted scope base change the math?

 

[Mystic Precision]

Info is spot on.... to add to that, show me a human body that is perfectly aligned?

Look at the Olympic 3 position shooters and tell me how many hold their rifles 'plumb'

As long as you are consistent and the reticle is level to your body AND the target when you break the shot, cant error (reticle vs the verticle boreline) is pretty much a non issue.

Note, if you have a physical handicap which forces the rifle to be way out of line with the reticle axis, then of course adjust for that ... but the vast majority of shooters don't have this issue.

more important is the position of the shooters head and eyes relative to the horizon.... that is going to leave some screwy results if not properly addressed.

Jerry

 

[FirearmsEnthusiast]

I try to mount level, but then shoot a Bryan Litz-style "tall target test" to see if the actual impacts track vertically.

Empirical data trumps everything!

 

[Shooter1971]

Tall target test...one of the more important scope/rifle set up exercises.

 

[skwerl]

The human eye is quite accurate , we can discern small discrepancies
in level and measurement .

That said we should endeavor to start as
close to optimum as possible to make up
for our other inadequacies . I.M.O.

 

[stampede]

To me it is a no brainer.

The reticle should be level when you are shooting it.

Throw away all those gimmicky scope levels.

 

[Alpheus]

Just one point, the error due to cant increases as the distance of your target increases, it isn't always the same. This is due to the drop of the bullet. Using OP's example its 2 deg over 6cm at the muzzle. At 1000yds, its 2 deg over 978cm (for a typical 308 175gr load) This works out to a windage error of 1.4"

JBM ballistic calculator lets you put in the cant as one of the inputs. https://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

 

[adamg]

Just one point, the error due to cant increases as the distance of your target increases, it isn't always the same. This is due to the drop of the bullet. Using OP's example its 2 deg over 6cm at the muzzle. At 1000yds, its 2 deg over 978cm (for a typical 308 175gr load) This works out to a windage error of 1.4"

JBM ballistic calculator lets you put in the cant as one of the inputs. https://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

To be clear, in my OP original example, I said the shot was taken with the scope level with gravity (plumb).

 

[adamg]

To me it is a no brainer.

The reticle should be level when you are shooting it.

Throw away all those gimmicky scope levels.

I don't quite follow your message here. How is one to know if the reticle is level when looking at an arbitrary view that may not include anything that is known straight horizontal or vertical? To me that is where the scope-mounted bubble level is important.

 

[xdmer]

Agreed, and with other posts, would be nice to see a sticky of this somewhere for people to reference.

Info is spot on.... to add to that, show me a human body that is perfectly aligned?

Look at the Olympic 3 position shooters and tell me how many hold their rifles 'plumb'

As long as you are consistent and the reticle is level to your body AND the target when you break the shot, cant error (reticle vs the verticle boreline) is pretty much a non issue.

Note, if you have a physical handicap which forces the rifle to be way out of line with the reticle axis, then of course adjust for that ... but the vast majority of shooters don't have this issue.

more important is the position of the shooters head and eyes relative to the horizon.... that is going to leave some screwy results if not properly addressed.

Jerry

 

[njohnson]

Yeah, this gets beat to death every once in a while. the scope being absolutely level/plumb to the rifle doesn't really matter. What matters is that when you shoot your reticle is always plumb, hence putting a level on your scope and setting it up so it shows plumb while your reticle is lined up with a plumb bob. Then do a tall target test to both make sure you got it plumb and that your turrets track precisely.

^^^ This ^^^

My primary experience with this is on a RPA target rifle with a front aperture sight and rear iron sight base the are adjustable for cant. I have them mounted approx 10 degrees clockwise and hold the rifle canted counter clockwise to fit my body and keep the left wrist straight. The front sight has a bubble level and I have no windage errors out to 900m provided I check my level.

The same can be done with a scope in rings, provided you have a level you’re GTG. Also as stated above keep your bubble level on your optic, even a small error in cant will give you a appreciable windage error once you start to approach 600 plus.