External ballistics question about non wind-induced drift....slugs specifically.

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lbc_PSI said:
I assume you’re clicking to the 171yds distance? Do you get the same amount of drift when using hold-over?

Have you tried switching to a different scope or rings? Same results?

Is the scope FFP? Have you tried a SFP scope?

Does the same drift occur with all projectiles you’ve tried?

When you’re clicking to the further distances, is there a point when the drift begins or is it gradual? How close to the max click travel are you? Could the erector tube be coming in contact with something inside the tube?

Grasping at ideas…
Click to expand...

Great ideas.

Scope is a ffp. I own sfp scopes but have not tried to swap out the scopes or the rings.

With how much further pellets get pushed by wind at these distances, it's hard to say if the same thing is happening with pellets. @ 100 yards the slugs ARE already drifting to the right of the impact point of pellets, bout 1 to 1.25"

Drift seems gradual, as distance increases so does drift.

I'm quite a ways from max travel, clicks were between 76 and 80 clicks on the most recent session. I'm away from home to check right now but from memory I've got 100 clicks per revolution with this scope. Seems like I can remember getting about 180 clicks of elevation before it bottoms out (tops out?) but again, I'm away from home and only going off memory.
 
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It is possible that the reticle part's vertical line is not perfectly aligned with the vertical travel direction of the inner scope tube. However if this was the case, there would be equal ammounts of MOA POI shift in the other direction at distances below the zero(ed) distance. Or maybe it moves in a slight curve at extreme click amounts.
A different scope or a jack plate arrangement with the same scope might settle it. Or using holdover.

Actually, you could just check how well the scope zero traces a plumb bob line from 0 to 100 clicks down (with a clamped down rifle or just the scope).
 
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This thought will require a couple of What Ifs" and may not actually be possible..going on a very limited knowledge of scope internals.. That said...

The Athlon scopes have 25Mil of horizontal travel. I assume this is total from left stop to right stop. I assume that the scopes are delivered optically centered, so there would be 12.5Mil Left from center and 12.5Mil Right from center.

What if the rings you used were slightly "off-center" when clamped onto the pic rail. Lets say clamping pulls the optically centered scope to the left. So when you try to align to the bore you have to click adjust the windage to the right. Not sure if the internals work this way, but it would seem the erector tub would be physically clicked to the right.

So now you have the erector tube near the right-most limit of horizontal travel.

You start clicking up for the longer shots and I assume the erector tube elevates so you're zeroed for the longer distances.

You're at or near the right-most travel of horizontal.

What If clicking "up" causes the erector tube to come in contact with the inner scope tube. If the erector tube were to follow the radius of the scope tube, clicking "up" causes the erector tube to be pushed to the left. If the vertical reticle is moving to the left, this would cause the projectile to hit more and more to the right as the erector follows the inner scope tube radius.

The amount of "drift" would follow a curve instead of being linear. This could explain the ability to be "on"..with no left/right deviation up to a certain distance.

And no..I haven't been smoking anything..LOL
 
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lbc_PSI said:
This thought will require a couple of What Ifs" and may not actually be possible..going on a very limited knowledge of scope internals.. That said...

The Athlon scopes have 25Mil of horizontal travel. I assume this is total from left stop to right stop. I assume that the scopes are delivered optically centered, so there would be 12.5Mil Left from center and 12.5Mil Right from center.

What if the rings you used were slightly "off-center" when clamped onto the pic rail. Lets say clamping pulls the optically centered scope to the left. So when you try to align to the bore you have to click adjust the windage to the right. Not sure if the internals work this way, but it would seem the erector tub would be physically clicked to the right.

So now you have the erector tube near the right-most limit of horizontal travel.

You start clicking up for the longer shots and I assume the erector tube elevates so you're zeroed for the longer distances.

You're at or near the right-most travel of horizontal.

What If clicking "up" causes the erector tube to come in contact with the inner scope tube. If the erector tube were to follow the radius of the scope tube, clicking "up" causes the erector tube to be pushed to the left. If the vertical reticle is moving to the left, this would cause the projectile to hit more and more to the right as the erector follows the inner scope tube radius.

The amount of "drift" would follow a curve instead of being linear. This could explain the ability to be "on"..with no left/right deviation up to a certain distance.

And no..I haven't been smoking anything..LOL
Click to expand...

Very interesting thoughts....

I don't know much about the inner workings of scopes.

But Im home now, and I can count. 😁 And maybe somebody who knows scope mechanism can see what they think with this new info.

So it has a scope stop, which I have set at the zero of course so I can't go "down" without undoing the scope stop, but it's got 10.7 mils of vertical before it pegs out (117 clicks @ 1/10th mil per click).

It'll go 4mils "left" before it pegs out, and it'll go 12.5mils "right" before it pegs out. So I'm certainly not optically centered in the horizontal adjustment range.

All of the above click values are counted from where it's zero'd, which is 60-65 yards for the slugs.

And I'm guilty of being a cheap scope rings user. I put UTG on nearly everything, and havent found them to hold me back in field target. These are some of what they were calling their "LE grade" at the time. In the $20-30 range.
 
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If a barrel is very slightly curved then any scope would be slightly to the side of the pellet trajectory, because it is siting some distance behind a muzzle which isn't pointing is the same direction as where the scope is mounted relative to it.
But more likely the turret mechanism isn't pushing optimally on the insides of the scope, than a litteral bottoming out of some sort, IF the line traced by the downward crosshairs movement isn't a line.
 
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Wanted to give an update.....

I've tweaked and tinkered and adjusted the scope no less than 6 times. I have concluded that its current orientation in the rings is the best combination of gun cant/reticle to bore lining up/truly vertical crosshairs.

In the last two weeks I've had lots of shooting with the .20/18.9. More than two boxes of slugs. It was in preparation for a high power/long range field target match, and as such I mapped out actual impact points from 20-100 yards. Some of the shooting sessions were in nearly zero wind conditions, and some were in pretty stiff wind. The wind was blowing both left and right at times and in different sessions.

Spoiler alert, the right drift is still VERY present.

Because the right drift was always experienced in the 870-880fps range prior, I wanted to play around with different speeds to see if it was still occurring at other speeds.

So I first shot at 5 yard increments from 20-100 yards at an average speed of 865. From 20-65 yards there was no right drift. Starting at 70 I started to need to give it 0.1mils of left hold. And by 100 yards it was requiring 0.6mils of left hold.

Next I tried speeds average 912fps (908-915 on the chrono). Same thing, 20-65 yards was dead on, in a left to right sense. But at 70 I started to need to hold 0.1mils to the left. At 100 yards I needed 0.4 mils of left hold at this speed.

I shot a long range/high power field target match this past Saturday, using the .20/18.9 slugs at 908-915fps, using the dope data described above. I scored a 41/48, the second highest overall score (19 shooters). The only higher score was achieved with Altaros slugs with a BC twice that of these slugs and an FPE nearly triple. I was one of only a couple shooters to go 4/4 on the 100 yard shots (and yes, I held 0.4 mils to the left to connect with those kill zones). My misses were operator error (pulled shots/bad trigger breaks, and I even forgot to return the turrets to zero on one shot. Bonehead move).

The right drift is VERY real with this slug/barrel combination. I suppose the next part of this experiment would be to see if the drift continues the trend of becoming less severe with more speed, by going up in speed again and mapping out the trajectory.

I still don't know the why, but it exists. Accounting for it = slug going where I want it to, VERY reliably.
 
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