Testing POI shift from gun cant

[Ghostranger169]

For this little test, I used a .177 Brocock Bantam with Atlas Bipod and Athlon Midas Tac 6-24 x 50(with bubble level) shooting JSB Monsters @ 820 fps.
I place a target at 28 yards(zero distance) with a dot in the center. I used this dot as a holdpoint on all shots fired.
I leveled the gun and fired a shot at the dot, hitting it in the center. I then canted the gun all the way to the left(15°) and fired a shot, hitting an inch to the left of the holdpoint dot. I then canted all the way to the right (15°) and fired a shot, hitting an inch to the right of the holdpoint dot.
I then moved the target to 50 yards and repeated this procedure.

 

[uladz]

Yep, seems consistent with what would be expected . That’s why we use a bubble level on a scope or rail . Thank you testing and sharing this experiment results!

 

[Bedrock Bob]

It's my understanding that the higher your crosshairs are above the bore the more pronounced it is.

Some guy convinced me he knew all about it and told me that. I believed him because it seemed to make sense. I may be wrong.

 

[Airgun-hobbyist]

Great post. This is precisely why a bubble level permanently attacked to the scope tube is beneficial to hole in hole accuracy.

 

[uladz]

Great post. This is precisely why a bubble level permanently attacked to the scope tube is beneficial to hole in hole accuracy.

Yep. Must have.

 

[Ghostranger169]

It's my understanding that the higher your crosshairs are above the bore the more pronounced it is.

Some guy convinced me he knew all about it and told me that. I believed him because it seemed to make sense. I may be wrong.

The scope height of the Athlon is 2.25". I guess I'll have to do that test next

 

[Airgun-hobbyist]

@Ghostranger169 try your test at close range using the correct holdover hash mark. You'll see a pronounced movement there as well based on your 50 yard zero and the arc of the projectile.

 

[Ghostranger169]

@Ghostranger169 try your test at close range using the correct holdover hash mark. You'll see a pronounced movement there as well based on your 50 yard zero and the arc of the projectile.

Yes, thanks for pointing that out. In hindsight, I should have done a "closer than zero" round of shots at about 10 yards to go along with the zero(28yds) and "farther than zero"(50yds), but I was worried about cluttering up the target. I will do one of those in the future.

 

[Airgun-hobbyist]

Yes, thanks for pointing that out. In hindsight, I should have done a "closer than zero" round of shots at about 10 yards to go along with the zero(28yds) and "farther than zero"(50yds), but I was worried about cluttering up the target. I will do one of those in the future.

I typically use a 40 yard zero and shooting close range with a holdover hash mark and not paying attention to the cant and bubble level has a noticable difference in POI.

 

[Bedrock Bob]

The scope height Athlon is 2.25". I guess I'll have to do that test next

If your barrel is arcing pellets up to the POA at 28 it's arcing them up toward the crosshair.

At 15 degrees it's still arcing them toward the crosshair but it's not plumb with the crosshair. So It's going right or left. And gravity is pulling it sideways instead of down against the curve.


It makes sense if your crosshair is higher off the bore the more angle is involved to get POI and POA to intersect. The pellet rises more to hit zero to intersect.

If you can't the rifle your error will be greater because of the greater difference in scope height.

If you are looking straight down the barrel at the target it would make no difference which way it rotated. The pellet would fall with gravity below the line of sight. The angle between the barrel and the sights enable elevation adjustment. But the more angle the more error when things are a half bubble off.

That's kinda the way it was explained to me.

Can't right and you hit right and low as the upward tilt of the barrel is not arcing the bullet straight against gravity. It isn't plumb under the crosshair.

The farther out the range the more it matters. The higher the scope the more it matters. That's how I think I understand it. It's exactly the same when your scope rotates in the mount and you plumb the vertical crosshair.

If you rise 2.5 inches to hit at 25 yards you will be greater than 2.5 inches left and 2.5 inches low if you lay the gun on its side. Your barrel is pointing at a spot where the arc defeats gravity in a level position. Lay it 90 degrees and there is no elevation at all. It's just starts dropping when it leaves the barrel. All your elevation is now just going left or right (and not pulling against gravity)..

The direction that used to be "up" when the gun was level is now left or right from the axis of the bore.

Does that make sense to anyone but me?

 

[nervoustrig]

It's my understanding that the higher your crosshairs are above the bore the more pronounced it is.

 

[nervoustrig]

Scope height can be mistakenly perceived to affect cant error, but only in circumstances where the distance is judged incorrectly, where as a consequence the shooter is using the wrong aim point on the reticle (mildot). Some more detailed discussion on the topic can be found in the followin thread.

Why cant matters to holdover shooters (Super basic)

This will be super obvious to 99% of the people here, but I wanted to put my thoughts out there for the 1% that maybe haven't thought this all the way through. First let's define what we are talking about with cant. Cant is where you twist the gun a little bit. Sometimes you can have scope...

www.airgunnation.com

 

[Bedrock Bob]

Scope height can be mistakenly perceived to affect cant error, but only in circumstances where the distance is judged incorrectly, where as a consequence the shooter is using the wrong aim point on the reticle (mildot). Some more detailed discussion on the topic can be found in the followin thread.

Why cant matters to holdover shooters (Super basic)

This will be super obvious to 99% of the people here, but I wanted to put my thoughts out there for the 1% that maybe haven't thought this all the way through. First let's define what we are talking about with cant. Cant is where you twist the gun a little bit. Sometimes you can have scope...

www.airgunnation.com

I respectfully disagree.

All of this is great when you are theorizing with metal rods. But add gravity to the equation (the reason we need elevation and rod "b" is bent) and things are much different.

A scope aligned with the bore and held plumb shoots the arc of the bullet to resist gravity. Cant the gun and the geometry completely changes. Elevation becomes windage as the gun rotates.

If it didn't you could hit your mark at range with a canted rifle. Or put a scope on out of axis and not get a windage error when changing range.

It matters a bunch at long ranges. Holding a rifle level is a huge factor if your not doing it. If you are doing it, it matters very little.

As far as scope height goes it has to be a factor. The angle between rod "a" and rod "b" is the basis for the equation. If it changes then so does the end result. Differences in scope height may affect very little. But it does to some degree.

I'm not afraid to be wrong and I may be.
I'm still setting up plumb and centered and holding my rifle in line with gravity. And I'm mounting my scope as low as possible. Even if rod "b" says it does not matter. If I'm wrong then I'll still do it because I'm stubborn and old.

 

[nervoustrig]

Believe me, I get your skepticism. For a long time, I was completely convinced scope height mattered. Then I read this guy's experiment, read up quite a bit on scotchmo's explanations, and mounted two scopes on a rifle and observed it for myself.

 

[uladz]

Believe me, I get your skepticism. For a long time, I was completely convinced scope height mattered. Then I read this guy's experiment, read up quite a bit on scotchmo's explanations, and mounted two scopes on a rifle and observed it for myself.
View attachment 482953

Waaaaaaaaaat, two scopes, wait a minute....

 

[uladz]

That's 12 pages of explanation printed on Letter pages, WOW. Okay, today I'm too tired to read it all, but I'm gonna take it as my bedtime reading and go through it in the next few days. I'm seriously curious about this experiment, explanation, and outcome. Thank you @nervoustrig for the info and the link!!! Yeah, that's something you don't see every day...

 

[delooper]

Generally yes you'll get a "cant effect" and it will depend on the scope height, plus many other things. For most practical purposes it doesn't have much effect until your trajectory is "significantly parabolic", but if you're aiming for tiny things then it'll matter more. If you're shooting at 10 yards away and only aim by bore-sighting, then you've got nothing to worry about.

 

[cavedweller]

For this little test, I used a .177 Brocock Bantam with Atlas Bipod and Athlon Midas Tac 6-24 x 50(with bubble level) shooting JSB Monsters @ 820 fps.
I place a target at 28 yards(zero distance) with a dot in the center. I used this dot as a holdpoint on all shots fired.
I leveled the gun and fired a shot at the dot, hitting it in the center. I then canted the gun all the way to the left(15°) and fired a shot, hitting an inch to the left of the holdpoint dot. I then canted all the way to the right (15°) and fired a shot, hitting an inch to the right of the holdpoint dot.
I then moved the target to 50 yards and repeated this procedure.

View attachment 482904

View attachment 482905

Nice example!

 

[bigHUN]

If you want to test it right next time use paper with fine square/cross-hairs.
You may discover more types of errors, scope cant (reticle crosshair and bubble are separate issues) and scope misalignment (to the bore centerline) are different things.
mount the paper with scross hair pattern to a backface using plumb line,
shoot with walk back or you need to mount multiple papers (walk back tuning)
shoot groups at close and long range without scope reticle tracking
shoot groups at close and long range with scope reticle tracking
You will see the pattern is different

 

[WNYBill]

There is a reason the BR guys use a level. Since the POI isn't the same distance for each side tells me your scope in not centered on your rifle. No big thing if you can keep the gun level.