It's an Athlon Midas Tac 6-24x. Yeah, not sure how that'd be ruled out. Between Mikes recommendation of shooting REALLY close and Bob Sternes (elsewhere) suggestion of using a mirror, I'm going to investigate the possibility of scope cant.
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External ballistics question about non wind-induced drift....slugs specifically.
- Thread starter Franklink
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It's an Athlon Midas Tac 6-24x. Yeah, not sure how that'd be ruled out. Between Mikes recommendation of shooting REALLY close and Bob Sternes (elsewhere) suggestion of using a mirror, I'm going to investigate the possibility of scope cant.
Another thought along the lines of scope cant being the cause of impact point tracking right as distance increases..... is scope height.....does a rather excessive scope height exacerbate the long range negative effects of scope cant? B/c the scope is mighty high on this rig.
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Slugs are still a bit of a mystery to us, but as long as one is accurate and have a known POI....that is really good! Have you tried higher speeds (like 900-1000fps) and lower speeds to see what happens? I have only tried 880-1000....my barrel likes 920-990. Fun thread
. Just read the above posts....I also run an Athlon Ares BTR 2.5-15x50 FFP mounted close to my barrel, less than 1/2".....maybe scope cant.....not sure....
I do know, if my eye is not absolutely close to perpendicular to the reticle as I acquire my sight picture, the NSA 17.5gr's POI will be OFF, the 20.2 are more forgiving. These slugs are just as accurate as the 18gr pellets at close range but way more accurate at longer distances especially with wind. Last edited:
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Coming back confused. I re-read the OP Q and not seeing what gun we talking about.
Scope cant is one thing you shall check (mirror method that a crosshair is in muzzle center) and set the bubble.
But there is also a second possibility ... like the Impact picatinny is not integrated but the rail is floating ... a slight little L-R rotation as much the screw head allows. That needs two POI @ 10 and @ 100 if you can hit the same vertical line, as I mentioned #2. there and back until both distances hit the same line. In archery that was called french tuning I preferred much over walk back tuning.
Scope cant is one thing you shall check (mirror method that a crosshair is in muzzle center) and set the bubble.
But there is also a second possibility ... like the Impact picatinny is not integrated but the rail is floating ... a slight little L-R rotation as much the screw head allows. That needs two POI @ 10 and @ 100 if you can hit the same vertical line, as I mentioned #2. there and back until both distances hit the same line. In archery that was called french tuning I preferred much over walk back tuning.
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A thought on scope alignment..
Assume your hold over for 170yds in MIL is +8 or U80 clicks to click to the same distance. Your super close hold over at ~7yds is +8 MIL or U80 clicks.
So..
If you set a target at 7 yards and use a +8 MIL holdover and take a shot, then if you U80 clicks and take another shot (reticle centered on target)..both shots should go into the same hole. If there's left/right deviation, then there is probably an alignment issue.
Assume your hold over for 170yds in MIL is +8 or U80 clicks to click to the same distance. Your super close hold over at ~7yds is +8 MIL or U80 clicks.
So..
If you set a target at 7 yards and use a +8 MIL holdover and take a shot, then if you U80 clicks and take another shot (reticle centered on target)..both shots should go into the same hole. If there's left/right deviation, then there is probably an alignment issue.
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This will open up a completely new can of worms..LOL
There are a couple of threads on AGN about exactly this issue..hotly debated. ;-)
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That scope height vs. scope cant issue was pretty well resolved on the GTA threads, I thought...:
▪ Scotchmo had a beautiful diagram to compare scope cant to gun cant.
▪ And there was a link of a guy that proved the thing experimentially.
As far as the phenomenon that Franklink has observed... — like mentioned before — it could well be that the elevation turret does not track well at higher mil values.
Matthias
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Just for the record strelock says for a 19 gr nsa 22 cal @ 900 fps with a 1 in 18 twist barrel and a2 mph breeze 90 to the projectile path will push 5.6 “ at 175 yds
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You can safely omit scope height as a potential aggravating factor to the divergent POA/POI problem described in this thread. Cant error is represented by a straightforward mathematical relationship...a relationship that does not involve scope height. For example, the effects of gun cant can be calculated as follows:
- horizontal error X is [drop x sinØ]
- vertical error Y is [drop x (1-cosØ)]
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Spent about 45 minutes last night looking through the scope at a mirror. (Thanks @rsterne)
The mirror trick proved that I had the "A" scenario shown in this graphic (graphic shared by Bob Sterne but I think original source was @Scotchmo)
Lots of guns ago I was in the habit of mounting the scopes using a plumb line to find vertical. I moved away from that when I found that true vertical/horizontal feels cattywampus when I spend any time looking through the scope (shooting)
So for years I've mounted scopes to what "feels" vertical and horizontal to me. Apparently I'm a little off kilter, lol.
At this point, and based on 15 months and lots of field target success with a scope mounted with what seems to be some serious scope cant, I'm of the opinion that scope cant at normal < 100yard airgun ranges is a non-issue, or at least can be accounted for with good dope data. Or rather a non-issue in the critter smacking and EBR targets and field target realm (might be more of a problem in the real benchrest world).
I didn't start this conversation looking for a solution or a fix, but in the interest of learning, I decided to rotate the scope in the rings to make the vertical stadia intersect the bore. In order to do that, the rifle must be canted, like in C. I'm not sure I love how it feels for the scope to seem crooked to me now,, or for the gun to need to be canted but I'm going to do some long range shooting and see if the gradual right tracking of impact points as distance increases thing is minimized by this. If I can't handle feeling like I'm falling over, I'll simply rotate the scope back to how I like it and account for the right drift at the longer ranges like I've been doing up til now.
Per Miles, some right-tracking (spin drift) will still exist. (Thanks Miles)
community.hardairmagazine.com
(I know links to the competitor forum is frowned upon, but hopefully Michael will allow the link to remain intact, for the purposes of this discussion.)
The mirror trick proved that I had the "A" scenario shown in this graphic (graphic shared by Bob Sterne but I think original source was @Scotchmo)
Lots of guns ago I was in the habit of mounting the scopes using a plumb line to find vertical. I moved away from that when I found that true vertical/horizontal feels cattywampus when I spend any time looking through the scope (shooting)
So for years I've mounted scopes to what "feels" vertical and horizontal to me. Apparently I'm a little off kilter, lol.
At this point, and based on 15 months and lots of field target success with a scope mounted with what seems to be some serious scope cant, I'm of the opinion that scope cant at normal < 100yard airgun ranges is a non-issue, or at least can be accounted for with good dope data. Or rather a non-issue in the critter smacking and EBR targets and field target realm (might be more of a problem in the real benchrest world).
I didn't start this conversation looking for a solution or a fix, but in the interest of learning, I decided to rotate the scope in the rings to make the vertical stadia intersect the bore. In order to do that, the rifle must be canted, like in C. I'm not sure I love how it feels for the scope to seem crooked to me now,, or for the gun to need to be canted but I'm going to do some long range shooting and see if the gradual right tracking of impact points as distance increases thing is minimized by this. If I can't handle feeling like I'm falling over, I'll simply rotate the scope back to how I like it and account for the right drift at the longer ranges like I've been doing up til now.
Per Miles, some right-tracking (spin drift) will still exist. (Thanks Miles)
Spin Drift
Following a recent question and discussion on spin drift for slugs, I was asked if I could post this old thread from a UK forum. I wrote it for sub 12FPE guns using pellets, so the speeds are low and slugs are not considered. The general effects are the same at high speeds and for slugs. The...
community.hardairmagazine.com (I know links to the competitor forum is frowned upon, but hopefully Michael will allow the link to remain intact, for the purposes of this discussion.)
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I believe the problem with the mirror and rotation method mentioned above is that the majority of our airgun rails have elevation built into them, 20 moa or more, creating a downward angle to the bore and line of fire. Now that you've canted the gun to the "new level", that rail elevation is no longer in line with your fired projectile, it crosses it. Dialing and holding just vertically is going to introduce some amount of "false windage" and you're going to end up chasing zero at different distances. The method described above only really works with a zero moa rail.
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True, there is a downward angle always necessary to make the line of sight intersect with the trajectory. This angle can be provided by tilting the scope tube via an angled rail or a “dropper” mount, or it can be provided by tilting the erector tube inside the scope via the elevation turret (or some combination thereof). But neither approach compromises the ability to use a mirror to align the reticle to the bore.
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Gotta disagree there. If you use the turret to elevate the scope, that is in the plane that you've aligned with the bore and cant within the rings. That's fine with a zero moa rail. However if your rail has built in elevation, once you cant the scope it is no longer in line with the bore or projectile and you can't fix that. Your turret elevation and holdover will cross that fixed angle instead of being parallel to it.
I think you might see this deviation if you try the mirror alignment at different distances???
I think you might see this deviation if you try the mirror alignment at different distances???
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It may be that I am not fully understanding you or that we are each attempting to describe different things, so let me try approaching it from a different perspective. Let’s say instead of mounting my scope on a 20 MoA angled rail, I mount it on a hinged contraption that allows me to tilt the scope anywhere from 0 MoA to 40 MoA. I set it at 0 MoA and I proceed to orient the scope with the aid of a mirror, clocking it so the reticle simultaneously bisects the bore and objective bell. In so doing, I have placed the reticle and trajectory on a common vertical plane (i.e. have eliminated scope cant). I can then tilt the scope to 20 MoA or 40 MoA, and the line-of-sight and trajectory remain in an overlapping plane, no scope cant is introduced, and there is no windage error at any distance. In other words, no matter how I choose to angle the scope, the trajectory always follows somewhere neatly along the vertical bar of the reticle.
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Actually your example is prefect to describe what I'm trying to describe. I can't draw for poop or I would lol.
Imagine your tilting contraption, but at the same time you've made accommodations for scenario "A" above where the scope is offset (but parallel) to the bore. If you rotate the scope to correct for the offset, then the vertical reticle is no longer in alignment with the vertical travel formed by the tilt (same as a fixed 20 moa rail). Now nothing is rotating in the same plane (think of dialing or holding over as the same kind of angular rotation) and the intersection of your angles is going to change with distance.
You'll end up with a similar scenario as if your scope rail is not aligned to the bore, because you've induced that condition. What was once a purely vertical angle (20 moa) is now also a horizontal angle because you have canted the entire rifle. You'll be zero at some distance, but left or right at a different one.
Imagine your tilting contraption, but at the same time you've made accommodations for scenario "A" above where the scope is offset (but parallel) to the bore. If you rotate the scope to correct for the offset, then the vertical reticle is no longer in alignment with the vertical travel formed by the tilt (same as a fixed 20 moa rail). Now nothing is rotating in the same plane (think of dialing or holding over as the same kind of angular rotation) and the intersection of your angles is going to change with distance.
You'll end up with a similar scenario as if your scope rail is not aligned to the bore, because you've induced that condition. What was once a purely vertical angle (20 moa) is now also a horizontal angle because you have canted the entire rifle. You'll be zero at some distance, but left or right at a different one.
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If the barrel is bent sideways it would pass the mirror test, but the projectile axis would still be slightly to the side of the scope axis.
(this probably isn't the case here)
(this probably isn't the case here)
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Okay thanks, yeah I think I understand what you're driving at now. The important distinction for me is the added complication of an offset paired with a tilted mount.
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This imperceivable 2 mph breeze at ground level, plus spin drift, plus time of flight/Coriolis effect, plus his offset scope cant all seem to add up to a 12-15" offset point of impact at 175 yards.
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Gotta agree.
This is a group from the first attempt on paper at 174yards.
9/10 shots in this group.....maybe 2.75" wide and 3.25" tall so bout 1.7 moa or so....
And we're talking .20/18.9grains and 32fpe@ the muzzle, not some powerhouse .25 or .30. Partly why Ive been saying nothing wrong, and there's nothing to "fix" just curious about ideas for the why behind the rightward drift.
Your answer of multiple factors compounding is likely the best response I've seen yet.
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Something I’ve looked into doing that might be related to the issue.
For long range airguns that will never be used inside of 25yds, it’s possible to rotate the scope in the mounts, just enough to almost perfectly compensate for spin drift for a significant distance past the apex.
The drift_compensation must equal:
sin(scope_cant_angle) x drop_compensation
For my 7mm slug gun, it would be:
zero at the apex
reticle held level
cant scope CCW 1.6 degrees
results in gun cant CW 1.6 degrees
I get these opposing numbers that balance to the point where spin drift is not noticed.
100m spin drift = 0.3moa
100m cant error = 0.2moa
200m spin drift = 0.8moa
200m cant error = 0.7moa
300m spin drift = 1.3moa
300m cant error = 1.3moa
400m spin drift = 1.8moa
400m cant error = 1.9moa
500m spin drift = 2.4moa
500m cant error = 2.67moa
That’s less than one click off all the way to 400m.
In the end, I decided to leave the reticle in line with the bore, and just hold off for spin drift.
For long range airguns that will never be used inside of 25yds, it’s possible to rotate the scope in the mounts, just enough to almost perfectly compensate for spin drift for a significant distance past the apex.
The drift_compensation must equal:
sin(scope_cant_angle) x drop_compensation
For my 7mm slug gun, it would be:
zero at the apex
reticle held level
cant scope CCW 1.6 degrees
results in gun cant CW 1.6 degrees
I get these opposing numbers that balance to the point where spin drift is not noticed.
100m spin drift = 0.3moa
100m cant error = 0.2moa
200m spin drift = 0.8moa
200m cant error = 0.7moa
300m spin drift = 1.3moa
300m cant error = 1.3moa
400m spin drift = 1.8moa
400m cant error = 1.9moa
500m spin drift = 2.4moa
500m cant error = 2.67moa
That’s less than one click off all the way to 400m.
In the end, I decided to leave the reticle in line with the bore, and just hold off for spin drift.
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