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

[Franklink]

The situation....

Pellets and slugs from the same gun/scope/barrel, at about the same fpe. Scope set up for zero of 60-65 yards with both. Impact points track in a vertical line fairly well until about 80 yards, whereupon the the slugs start to drift to the right. By 100 yards the slugs are hitting about 1-1.25inchs to the right of the pellets. And the slugs only drift more and more to the right as distance increases. This was seen over the summer on long shots on critters, but I always assumed it was a slight wind that I could not see causing the drift. Yesterday evening I finally tried the slugs on paper at 174 yards, right before dark and essentially no wind. @174 yards the slugs are hitting about 12-15 inches to the right of the crosshairs. They'll still group pretty dang good, but that group is off to the right of my crosshairs if I hold dead center.

Were it not for the zeros of both projectiles lining up at 60-65yards I would simply chalk it up to two different projectiles impacting in different places.

The slugs seem to have a golf ball shanking sort of effect, specifically curving off to the right. It is predictable and consistent, and is simply something that needs accounted for, much like the trajectory arc. On all long connecting shots I have to hold to the left of the desired impact point.

I'm vaguely remembering seeing some related comments about slugs behaving opposite of pellets when rifling twist rates are taken into account, but I'm not sure that applies here.

Is this a rifling situation?
Is this something I'd see with pellets or slugs, but the pellets don't hold accuracy well enough at 175ish yards to see it so drastically?
This is happening, and not in just an isolated manner. I'm not trying to prevent it or "fix" it. It just is. But what is it called and what is the "why" behind it?

 

[rsterne]

Slugs and pellets are stabilized in a different way.... Pellets have the CG (center of gravity) ahead of the CP (centre of pressure), and are "flare stabilzed", ie when the pellet yaws, there is a correcting factor applied by the flared skirt.... Slugs have the CG behind the CP (so are naturally unstable) and the correcting force is achieved through the gyroscopic forces due to spin, induced by the rifling.... Slugs are subject to "Spin Drift", which increases with range, and is independant of wind drift, and that spin drift increases with approximately the square of the range (3 times as far, 9 times the drift), which would seem to approximately fit your data, once you take into account that you are already zeroed at 60-65 yards (with pellets).... I am tempted to say that the pellets just aren't seeing any, or very much, spin drift, but I don't know if that is the case or not.... I will ask Miles Morris, UK Ballistician, for his opinion....

It is also possible that the vertical crosshair on your scope does not bisect the boreline, which indicates "scope cant", and results in the trajectory wandering away from the boreline at both closer and farther ranges.... I suggest viewing the rifle in a mirror through the scope to insure the vertical crosshair aligns with the reflection of both the bore and the scope objective lense....

Bob

 

[Franklink]

It is also possible that the vertical crosshair on your scope does not bisect the boreline, which indicates "scope cant", and results in the trajectory wandering away from the boreline at both closer and farther ranges.... I suggest viewing the rifle in a mirror through the scope to insure the vertical crosshair aligns with the reflection of both the bore and the scope objective lense....

Bob

So perhaps simply spin drift. Very interesting. Thank you.

As for the scope cant, on 99% of the long shots I'm dialing the turret, not employing holdover. The way my brain envisions the physics, dialing would make any scope cant a non-issue and holdover would make a scope cant a serious problem, but I'm open to being educated there.

 

[rsterne]

Here is what I mean by checking for Scope Cant (relative to the boreline).... Set up a mirror at half the closest focus distance on your AO, and view the image of the rifle in the mirror.... Here is that image, both what you should see (L) and one where the crosshair does not intersect the boreline....



Here is a drawing showing how this happens, and how to correct it.... A canted scope is in "A", you correct it by twisting the scope in the mount until the vertical crosshair aligns with the bore "B" and then hold the rifle with the crosshairs level as in "C".... Note, the rifle may not be level if the scope is offset to the bore, but it won't affect the results as long as the vertical crosshair intersects the bore and you hold the crosshairs level when shooting....



If the scope and/or rifle is canted, here are the possible results.... Since your slugs are hitting right at long range (and assuming you are holding the crosshairs level), your most likely situation is "G"....



If you look at the drawing "G", and imagine dialing the scope up and down, you can see that it is still offset relative to the trajectory of the slug.... Of course spin drift could be the problem, not scope cant....

Bob

 

[Franklink]

@rsterne, If it's cant, I've sure had a lot of success with a canted scope with this gun in the last 15 months! Although all of the competitive success have been < 100 yards, and the long range pest connections have needed that hold left of impact point thing. So if it is scope cant, effects must become more and more exaggerated as distance increases.

Thank you, I'm going to do the mirror trick the next chance I get.

 

[tec]

Ok, that was more difficult than I thought it would be...had to do that prone from one bedroom down the hall to the other bedroom.....did my pushups for the day + some.....the low lighting and tight eye box on the Element Helix made it even more difficult...but unfortunately the rifle-scope are not canted...so that means all the misses are 100% me...haha.

 

[Franklink]

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

 

[Franklink]

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 (the larger A) shown in @rsterne s first graphic a few posts ago (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 this gun 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 (the larger 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.

 

[rsterne]

I had the same issue with a Hatsan I had.... off-center scope rail.... It was enough the gun felt awkward for while until I just concentrated on keeping the horizontal crosshair level, and didn't worry about how the gun felt....

Bob

 

[tec]

I had the same issue with a Hatsan I had.... off-center scope rail.... It was enough the gun felt awkward for while until I just concentrated on keeping the horizontal crosshair level, and didn't worry about how the gun felt....

Bob

I literally spent, cumulatively, well over a couple hours mounting my scope on my Revere ….making sure the rifle was perfectly level, that the scope was perfectly “square” to the rifle, and then later attaching a level to the rifle.
I attached the level because my “instinctual” hold resulted in the crosshairs slightly rotated to the right, which made me go back and recheck level multiple times…finally resulting in me adding the attached level so that I could square up and learn to adjust my hold.
For the Ghost, first thing I did was attach a level….then the scope..
it never occurred to me that the scope could be canted …. Luckily neither are based on the mirror test from what I can see, but I have a big game, long range hunter friend who has scope mounting jig… going to have him dbl ck
greatly appreciate the knowledge transfer!
tc

 

[Franklink]

Stretched it back out yesterday evening (171 yards this time).

This is after the mirror trick, and the shooting close trick, and the plumb bomb trick, and the etc etc etc trick. Readjusted the bubble level on the gun so that I know when the bubble level is good, my vertical crosshair is truly vertical.

So, first shot @ 171 drifted right again, off the paper. Maybe 12-15inches? and only basing that on the fact that it hit that woven wire fence (like it did the first attempt at 170+ a few weeks ago) and threw some shrapnel back into my cardboard. I did see the woven wire vibrate when hit, but hard to pinpoint exactly where hit. Adjusted it to the left so that I could shoot my groups. Wind was pretty dang minimal. Very slight headwind, and only on some of the shots. I'd guess most shots were no wind, but I could feel a gentle push straight in from the target on some of the shots, couldn't have been more than 2-3mph at it's peak, and it wasn't peaking very often.

I think spin drift.

 

[Hare]

Sounds to me like they are destabilizing and at their maximum range.
I shoot at night with a spotlight beam to verify destabization along the flight path.

Edit: The easiest way to test for cant is to shoot a string of shots and move the windage or elevation with each shot and see if the shots stay level.

 

[Danman]

I have to go down this scope alignment rabbit hole I think for my S510. So if I have this right....Grab a small mirror ..6inch say? Put it at 5yards away if your min parralax is 10Y. Set parralax to 10 and look through scope at mirror. Reflection of scope should be centered in your crosshairs on the mirror? Or am I wrong? Like Franklinks ghost this gun is super accurate but scope has always seemed a bit crooked. Have never shoot it past 60Y yet either to verify what it's doing long range! Thanks.