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?

 

[bigHUN]

My take on it from experience? Want to shoot pellets and slugs from a same gun setup - is just asking for a trouble.

First is a scope alignment, where all the action shall start actually. POA @ 10 meters and POA at 100 meters with a same projectile... if the POI is not hitting a same vertical line - you shall start there.

Second, you may have not noticed any "wind" but just any slightest air movement (Left-Right or Low-High) will drift the projectiles different - the pellets to one side and slugs to other side.
Depend for which projectile you had your scope sighted in that will determine where will be the POI shift.

Also, if the projectile path is corkscrewing, and that means the ratio is off .... the rifling twist rate to a projectile speed = is off. That is most visible if you want to shoot multiple distances. You sighted in at 60, but at 55 it hits left at 65 it hits right, next again 70 hits left and 75 hits right. A highspeed camera can help, but you can find on yutube some samples what this means.
This one is easier to tune. Let say you have a speed ballpark of 900 fps. You go couple 10 fps incremental steps down from ballpark and couple 10 fps incremental steps up from the ballpark. You will notice that every change will either open the group or tihgten the group. And you will need to do this on couple distances to see where the corcsrewing is minimal.
Forgot to say, every change you document, pencil and a paper and make pictures for every change, so you can get back to your best.

The bottom line forcing shooting pellets and slugs perfectly - Maybe you can get this right if all stars align? but most likely not.

 

[Franklink]

My take on it from experience? Want to shoot pellets and slugs from a same gun setup - is just asking for a trouble.

First is a scope alignment, where all the action shall start actually. POA @ 10 meters and POA at 100 meters with a same projectile... if the POI is not hitting a same vertical line - you shall start there.

Second, you may have not noticed any "wind" but just any slightest air movement (Left-Right or Low-High) will drift the projectiles different - the pellets to one side and slugs to other side.
Depend for which projectile you had your scope sighted in that will determine where will be the POI shift.

Also, the projectile path is corkscrewing, and that means the ratio is off .... the rifling twist rate to a projectile speed = is off. That is most visible if you want to shoot multiple distances. You sighted in at 60, but at 55 it hits left at 65 it hits right, next again 70 hits left and 75 hits right. A highspeed camera can help, but you can find on yutube some samples what this means.

The bottom line forcing shooting pellets and slugs perfectly - Maybe you can get this right if all stars align? but most likely not.

The pellet and the slug from this gun are both examples of the best I've seen from any airgun. Neither are a problem, most would pay serious money for a gun that shoots like this gun does.

Corkscrewing isn't happening, with the pellets or the slugs. I've shot about 2500 of the slugs and about 4500 of the pellets. In all those I've had multiple situations where I could see them both in flight. They're not behaving erratically or corkscrewing. The slugs just curve/drift to the right as the distance increases.

There's nothing to fix here, stars are apparently aligning . Im hoping someone can explain the ballistics of what is happening with the slug in the air.

 

[trlrman]

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 have the same thing . Sat practicing out to 500 with a quartering right to left breeze I was holding 1 to 1.5 mil to the left .

 

[Franklink]

I have the same thing . Sat practicing out to 500 with a quartering right to left breeze I was holding 1 to 1.5 mil to the left .

Bob Sterne replied to the same question elsewhere and his two ideas are either spin drift and/or scope cant.

 

[trlrman]

I would have to guess spin drift . I don't think I am good enough to get both my scopes on my long range guns cant'ed the same ?

 

[Franklink]

I would have to guess spin drift . I don't think I am good enough to get both my scopes on my long range guns cant'ed the same ?

I agree, spin drift seems more plausible. I dial the turrets for nearly all the longest shots. And my tiny brain seems to think scope cant would be a much more serious problem with holdover. I can see how it would maybe be present with dialing turrets, but sure seems like it would be much more problematic with holdover.

 

[Gerry52]

I've been told it's "spin drift". I have to account for it past 110 yards. No big deal because it's consistent.

 

[Franklink]

I've been told it's "spin drift". I have to account for it past 110 yards. No big deal because it's consistent.

I agree 100% "no big deal b/c it's consistent." As I said, not trying to fix it here, just hoping to understand it better.

 

[OldSpook]

I am no expert on this so take the question for what it's worth. There are settings in Strelok and Chairgun which will account for both spin drift and Coriolis. Have you tried those?

Add vertical deflection of crosswind to results.

Crosswind can cause a vertical deflection to the trajectory of the bullet. A 10.6mph crosswind on a 308 Winchester 175gr bullet fired at 2550 fps can add 4.64 inches of vertical deflection at 600 yards. This value is approximately 10% (or slightly less) of the “spindrift” horizontal deflection. Most shooters recommend enabling this.

Add spindrift to results.

The spin of the bullet will add a horizontal deflection to the bullet’s trajectory. A right-hand twist barrel will cause the bullet to drift right, and a left-hand twist to the left. It is recommended that this be enabled.

Add Coriolis effect to results.

The Earth’s rotation can have an effect on the bullet’s flight path. Vertical or horizontal depending on where on the Earth you are and the direction you are shooting. This effect is only really noticeable at distances further than 1,000 yards. If you are shooting that far, then be sure to enable it.

I do know however that Coriolis is dependent upon time of flight NOT range as the author of that asserts. So it might be worth enabling. I had ATNs "professional shooter" give me grief over my disappointment when I discovered that their ballistics calculator (in the X-sight) does not calculate Coriolis. The darn thing has a compass built in... and ignores it (unless something has changed since I last checked).

 

[trlrman]

I am no expert on this so take the question for what it's worth. There are settings in Strelok and Chairgun which will account for both spin drift and Coriolis. Have you tried those?

Yes sir and it is correct .

 

[Ranchibi]

Like you Franklink, I shoot both pellets and slugs in .22 from my RAW tuned to shoot the JSB 18gr @ 970fps and the NSA 17.5gr @ 950fps & 20.2gr @ 930fps all on the same tune. @ 100yds shooting the slugs slower with less spin, they do do drift right but @ this tune no drift. I do not know my non choked poly barrels twist rate.....I attributed the drift to spin/speed combo like you. But, as long as the POI is consistent, that's a very good thing! Very lucky to be able to do this with one tune.

 

[thomasair]

For what it's worth...if my slug gun is on at 50....it's on at 100 or 200 too. It's probably scope cant.

Try dialing your scope for a super close shot like 5 yards or whatever is near the same clicks as your long shots. If it's scope cant the super short ones will be off too....and you can be sure it's not wind at super short distances.

Mike

 

[Franklink]

For what it's worth...if my slug gun is on at 50....it's on at 100 or 200 too. It's probably scope cant.

Try dialing your scope for a super close shot like 5 yards or whatever is near the same clicks as your long shots. If it's scope cant the super short ones will be off too....and you can be sure it's not wind at super short distances.

Mike

If it's scope cant I've sure had a lot of success with a canted scope in the last 15 months! That includes 10 yard shots on 3/8" kill zones in field target, but not closer than that. But I am willing to investigate to see if that's a problem.

 

[Franklink]

Like you Franklink, I shoot both pellets and slugs in .22 from my RAW tuned to shoot the JSB 18gr @ 970fps and the NSA 17.5gr @ 950fps & 20.2gr @ 930fps all on the same tune. @ 100yds shooting the slugs slower with less spin, they do do drift right but @ this tune no drift. I do not know my non choked poly barrels twist rate.....I attributed the drift to spin/speed combo like you. But, as long as the POI is consistent, that's a very good thing! Very lucky to be able to do this with one tune.

So you saw drift to the right with slower speeds but not faster speeds?

Interesting because these slugs are only going. 870-880.

 

[Franklink]

I am no expert on this so take the question for what it's worth. There are settings in Strelok and Chairgun which will account for both spin drift and Coriolis. Have you tried those?

I use Strelok but have not played with settings for spin drift.

Bob Sterne and Mike Nische are thinking scope cant. Can't argue with their knowledge base and with the #s being cited for spin drift, well, spin drift exists but seems to be much less than what I'm seeing.

I'm going to investigate the scope cant thing a bit. Kinda wild if the scope is canted so drastically, considering the success I've had with this gun and that the scope has been in the same place for over a year. . Makes me think < 100 yards makes scope cant a pretty minimal problem, but as distance increases, it becomes much more apparent.

 

[lbc_PSI]

For what it's worth...if my slug gun is on at 50....it's on at 100 or 200 too. It's probably scope cant.

Try dialing your scope for a super close shot like 5 yards or whatever is near the same clicks as your long shots. If it's scope cant the super short ones will be off too....and you can be sure it's not wind at super short distances.

Mike

This suggestion is brilliant.

@Franklink
If it isn't scope cant, could it possibly be the scope internals not vertically tracking at the outer limits of click travel? Not sure how you'd rule this out though. Does the right drift occur if you use hold over and don't click?

 

[Ranchibi]

So you saw drift to the right with slower speeds but not faster speeds?

Interesting because these slugs are only going. 870-880.

Yes. On my current tune, the NSA 17.5gr are going 950fps and 20.2 @ 930fps....I am wondering though if they are drifting pass lets says 130yds....h'mmm....will have to see the next time I get a chance at longer distances again! Just thinking the faster speeds are keeping them stabilized longer than slower speeds....

 

[Franklink]

Yes. On my current tune, the NSA 17.5gr are going 950fps and 20.2 @ 930fps....I am wondering though if they are drifting pass lets says 130yds....h'mmm....will have to see the next time I get a chance at longer distances again! Just thinking the faster speeds are keeping them stabilized longer than slower speeds....

The drift doesn't seem to be a stability issue, because they're consistently drifting. Ie I can make em go where I want, if I account for that drift like I account for trajectory arc. Seems if they were losing stability the impact points wouldn't be reliable.

But your underlying implication that slower may drift more is interesting.

 

[OldSpook]

I use Strelok but have not played with settings for spin drift.

Bob Sterne and Mike Nische are thinking scope cant. Can't argue with their knowledge base and with the #s being cited for spin drift, well, spin drift exists but seems to be much less than what I'm seeing.

I'm going to investigate the scope cant thing a bit. Kinda wild if the scope is canted so drastically, considering the success I've had with this gun and that the scope has been in the same place for over a year. . Makes me think < 100 yards makes scope cant a pretty minimal problem, but as distance increases, it becomes much more apparent.

Well I can't argue with those guys but you could input the data in strelok and see if it correlates with what you have measured. AG time of flight makes Coriolis a real thing for AGs at much shorter ranges than PBs.

Then again I think Coriolis swaps left to right and right to left when you face north or south... not really sure and too old to care.
I hope you get it sorted. Happy New Year!