Interesting, thank you for the details of your case. My original though was that something similar to this was going on with my rifle. However what is not clear to me is how constant the error in POI will be from a barrel that is canted in the upper receiver. Let me explain.
Attached File


If there is an angle, of lets say 0.1 or 0.5 MILS (blue and orange lines on the graph) that is caused by the barrel being canted in the upper, I think you would expect the error in POI to be constant. I converted MILS to inches at a given distance to help conceptualize this. So yes the farther you go the more you shift to he right, but as an angle it stays constant. Now what I see with my rifle is the grey line, as the distance to target increases so does the angle in MILs.
What kind of pattern did you see with you upper?

Thoughts aloud....

With that plot already created, how about applying a line of best fit to your data's trend?  The intent is to help rule out small system inconsistencies. (Wind, cartridge, shooter, etc)  With that line, some trig will get you to an approximate face error on your receiver.

Attached File


Done, see above. Running the conversion from inches to mil, I get an error of 1.02 MIL.

Where is the sun positioned with respect to your optics?  Shadow? 

Parallax?

We always have a constant left to right wind, so all of mine will shoot right at extended range.  Both of the ranges locally are in the same orientation, same wind.

Quick search shows the barrel thread to be 1-7/16-18 so I'll use that as the basis for my calc.  1.02mil across 1.4375" = 1.4375 x sin(.05625) = ~.0014"

That's not an outrageous amount.

Scope is not level to the bore.  It only takes a tiny bit to show up past 100 yards.

Thank you. That is alot less then I would have thought. Perhaps this is the case after all.

You're welcome.  I was also surprised at how little runout would equate to a mil.  That's about a third the width of the hair on your head...

I did not take a very scientific approach.  I only shot at 50 to get initial zero and 100 yards so I didn't develop a pattern.  When I moved to 100 yards I was shooting right about 1.5". I assumed it would be liner based on barrel misalignment from an uneven receiver face.  This was after ruling out the scope, ammo, barrel, muzzle device, and scope mount in an effort to improve accuracy which was also not where I wanted it to be.  Since I trued the receiver, I have not noticed a POI shift from 50 to 280 yrds. Also, my accuracy has increased and became more consistent.

Also, I agree with COLE2534's assessment of 0.0014" of runout.  I was lazy and drew it up in CAD, rotated the line representing the receiver face based on 1.02 Mil shifts and got 0.0013" of runout. Let us know if our evaluation is correct if you have someone check runout.

Going back to the scope error issue:

When you dial to your 500 yard dope, you are experiencing this. What happens if you keep your 100 yard yard zero and just hold over with the reticle? Still .5 mil drift?

gene_wi;

I've been watching this thread and scratching my head...  If a bullet doesn't fly straight, then there MUST a force acting on it that is NOT in line with it's flight path.  The only force like this that comes from the rifle is SPIN (if anyone can come with any other forces please chime in).  Your observed deviations seem to be qualitatively like spin drift but they are just too large for that to explain it.  The only other forces that are off the axis of flight (and therefore cause horizontal drift) are wind and "Coriolis" forces.  The magnitude of the corrections for coriolis plus spin drift would cause the type of deviations your are observing but (again) to a significantly smaller degree that you are reporting...  Coriolis drift in the northern hemisphere is to the right so a right hand twist barrel would have a spin drift to the right and the resulting shift would be greater that either alone...  a left hand twist barrel would have a spin drift to the left and combining the 2 would produce a resulting shift smaller than either of the components.

Therefore I am thinking -

1) calculate the spin drift plus coriolis drift for your rifle in your location and plot this deviation vs. distance.  Now the question is - "How much is the predicted drift different from the observed drift? and is that a difference an amount that you can reliably observe - or perhaps, can you shoot with an accuracy greater that the difference?"  I KNOW that I am NOT capable of .1 mil reproducibility with my skill level and equipment on my best day shooting at a 500 yard indoor range.  I would therefore be unable to even notice the deviation your are describing!!!

2) Assuming that your observation and description are accurate, you can stop looking for the origin of this phenomenon at the rifle, it can't be there.  If your observation/description is not accurate, all bets are off!  

<just thinkin' out loud - hoping to provoke thought in others...>

Your premise is wrong. Who says this guy's bullet is NOT flying straight? All the forces that might act on the bullet (except wind) added up aren't going to change the POI by 1 MIL or more at 1,000 yards. Coriolis, spin drift, and so on. Those are not large enough to alter the bullet's trajectory by 1 MIL. And if they were, it would happen to every shooter. My presumption is the bullet is just not headed toward his POA. The bullet is not changing trajectory in flight (by any amount of importance), his barrel and/or scope and/or receiver are simply not aligned. So instead of looking at external forces acting on the bullet, one should be looking at the rifle setup itself. The barrel is bent. The barrel is mounted at a cant. The scope is canted. The scope's reticle is canted. The scope's turrets are not traveling perfectly up/down/left/right. The turrets and reticle aren't aligned. The receiver isn't flush. And so on. Those are all the things I would look at first.

Step One would be to mount the scope on another rifle that is confirmed to shoot properly. If the rifle stills shoots properly once re-zeroed with the scope swap, you can eliminate the scope and rings as the potential issue.

Step Two would be to ensure the rifle's rail is within proper spec.

Step Three would be to check the barrel to make sure it isn't bent, curved, the bore isn't runout, it's mounted to the receiver properly and straight, and etc.

He did! vis. >>>
48;

The laws of physics are different on your planet compared to mine...  on my planet, if your barrel and optic are out of alignment by some angular measurement, that angular error is a fixed amount (i.e. NOT variable).  Angular measurements are independent of distance...  and, once the optic is zeroed (horizontally), the horizontal angular error has been "dialed" out of the equation and is effectively zero...

I can't speak to the laws of physics on your planet as I am not familiar with it...

Best,

- R -

gene_wi - the thread is coming up on 4 months old...  any additional information/observations/findings/etc.???  Have you solved this one yet???

The problem is internal to the rifle system. It's not some magical, mysterious, outside force acting on the bullet. It's the scope, the mounting system, the barrel, or the the barrel-to-receiver mounting method (i.e. the receiver face isn't true, causing the barrel to be crooked). My first guess is the scope isn't tracking properly due to a canted reticle or the turrets not tracking level. That's why I suggested mounting it on another rifle. But you're the boss.

What, no comment on the bullets not flying straight???  None of your proposed ideas can explain the empirical data - but you're the boss!

yep, different planets...  

<I actually think the first problem is probably one or more errors in observation.  With such small amounts of deviation, it is easy to make errors and I am not certain that the apparent observations are statistically significant when you consider the uncertainty in observation.  Spin drift and Coriolis effect also are factors and account for part of the observations...  You would have to back those out and see what's left...>

Look, anybody with shooting experience should understand what I'm saying, especially regarding the turrets not tracking properly. I don't care enough to explain it a third time. You aren't the one with the issue; the OP is having trouble with his rifle. Any of the things I described can and will cause a bullet to deviate from POA with an angular windage error. If you can't understand that, it's really no skin off my back. Enjoy your planet.

OP, let us know what's up.

If you're dialing in the elevation corrections (which the OP is doing) then the level needs to be level with the reticle adjustments, not the reticle itself.  It's not uncommon to find 2-3 degrees difference between the reticle and the adjustments.  That will show up on a tall target test, and in fact it the primary reasons to run a tall target test.  You can align the reticle with a plumb bob and get true vertical with a level on the rifle.  However it'll still drift to one side of the other on a tall target test if the vertical adjustment is not in the same plane as the vertical cross hair of the reticle.  The longer the tall target the more precisely you can determine if this is an issue.

If on the other hand you are using the reticle for hold overs then (if you have to choose) you want the scope level to be aligned with the reticle, not the adjustments.  You'll still get some elevation vector in the windage adjustments but they tend to be smaller as the windage adjsutments are usually smaller, and the elevation vector from the canted adjustment will usually be masked by the head/tail wind component of the wind.

For example if the adjustments are 3 degrees off from the reticle, then at 500 yards where you've added 11 3/4 MOA of elevation, you'll have .6 MOA of unwanted windage correction (and you'll have added only 11.73 MOA of elevation, which you won't notice the .02 MOA difference).  However, at 1000 yards where you've added perhaps 39 MOA of elevation, you'll now have a full 2 MOA of unwanted windage correction (and the actual elevation adjustment is only 38.9 MOA, and you again won't notice the .1 MOA difference).   Now...will you notice 2 MOA in the real world with real world wind estimation errors?  Maybe, or maybe not.

A 2 degree alignment error creates a windage adjustment error of just .4 MOA at 500 yards and 1.3 MOA at 1000 yards - and that WILL be lost in the noise of wind estimation errors.  That's more or less why most scope manufacturers only worry about a 2 degree tolerance in aligning the adjustments with the reticle.

Spin drift also plays a role here as the spin drift will also produce about 1 MOA of right drift at 1000 yards.  If the adjustments are canted 2 degrees left relative to reticle, then you may get lucky and have the two effects largely cancel each other out.  I'm not aware however of any scope maker designing this in as a feature rather than a bug - and it would be bullet, velocity and rifling specific so I wouldn't count on it happening any time soon.

In any event, given that the OP tried different scopes and got the same error in the same direction, I doubt this is the issue.

-----

In general though...OMG.  Why are some of you working so hard to identify things like mis-aligned barrels, curved barrels and non-true receivers as the cause of this Shooting 101 level problem?  You need to look up Occam's Razor and then apply that concept to your critical thinking skills.

The most likely culprits are a canting error induced by the shooter (repeatable error regardless of scope, mount, etc), and/or spin drift effects, which pretty well explain the OP's observed issues. He's got a non linear right drift error that is consistent with both spin drift  (accounting for maybe 1/4 to 1/3rd of the total problem) from a RH twist barrel and a scope canted to the right (something that is aggravated by high mounted scopes on AR type rifles) that could easily account for the rest of it.  A 6 degree canting error in concert with spin drift accounts for what the OP is reporting in his initial post.

In my experience with a .308, spin drift accounts for about .5 MOA at 500 yards and about 1 MOA at 1000 yards - it is not a linear function as it becomes more noticeable at longer ranges.   But it's really hard to quantify with any precision as a 1 mph cross wind will result in a .5 MOA adjustment at 1000 yards and that makes it very hard to determine how much is due ton the effects of spin drift and how much is due to less than perfect wind estimation.

Yeah, I thought reticle cant was a likely culprit as it gets (angularly) worse with distance (like spin drift) but I thought that that OP eliminated that as a possibility somewhere in the thead...  but I could be mistaken - it's been a long and winding road...

Still working on this. Any shooting is on hold since its below zero here in WI. I am waiting on a friend with a CM machine, to get some time to check the barrel alignment.

I got in touch with the rifle's manufacture, but as I suspected, they are dodging me and have not even bothered to call me back. I will try them again in January.

Thanx Gene... so it's a little cool in WI is it?...  Well, stay warm and get out and shoot when the weather breaks...  It's cool here too - temps never got out of the 60s today - great shooting weather IMHO...  what a difference 20 degrees of latitude makes!!!  

So the corliolis effect only messes with his AR10? Because he said he is shooting his bolt gun along with his AR10 and it doesn't happen with the bolt gun. I bet this is why old school hunters don't like AR's.

So you can set up a bench and have 4 targets, all at 1k yards, and all 90° apart on a 2k diameter circle, and the bullet will always be to the right on all 4 targets? What's the science on that?

Yes.  Vertical coriolis drift is dependent on the direction you are facing/shooting.  The horizontal coriolis drift is NOT dependent on the direction your are facing/shooting but IS dependent on your latitude!  It's crazy!!!  

See LINK - top of page 2 (and other references)...

I must admit, I did not read through all the lengthy posts here so sorry if this has already been said:
The barrel is aligned properly and the scope is probably setup right. The easy way to test this is to shoot a round and see where it lands. Next, put the scope back on where you originally aimed and adjust it to the point of impact of the round. Now shoot another round and if it's still out by the same or nearly the same amount then most likely the problem is with the OP and his trigger. At 1000 yards, even the slightest pull is going to be amplified and if he's a good shooter that pull will be nearly identical each time. A good shooter knows his own problems as well as he knows the guns problems and compensates for both to overcome them.

That's a great suggestion. I will try this next time I am at the range.

As a 'raw-as-oysters' precision shooter, I just wanted to say how great it is to have this sub-forum be populated by such knowledgeable and helpful people.

Not all the technical forums here are....

It has been a nice long while, but it has taken me time, and life has a flow all of its own.

I think I have this issue resolved. And it was resolved by a trip to the optometrist.

Backstory, after frustration and disappointment with this issue, I moved the Razor to a bolt gun and left the rife that is the subject of this post sitting in the safe. Shooting the bolt gun more, and shooting it in competition, I noticed that sometimes, but not always I would see the same pattern of shooting to the right. And it hit me that to some extent this error depends on the position of my eye relative to the optic, more precisely the position of my glasses. My glasses correct astigmatism. I ponied up the cash and went to an optometrist that has experience with making glasses for shooters. I had a pair of glasses made with larger lenses.

So I mounted an optic on my AR, and had a chance to do some lond distance shooting with both the bolt gun and the AR over this weekend. Shot straight as an arrow with both rifles in all sorts of odd positions, while wearing my new glasses.

Thank you to everyone that participated in this thread. I learned a ton, and ultimately resolved my problem.

-Gene

I've been following since it's been posted, and would have guessed all the mechanical stuff.  Must to nice to finally have the cause and the fix.

Thanks. Yes it is a good feeling. The journey was great, made me learn my gear, my rifle and myself. I got suspicious of my eyes, after a while. I got a kestrel, a ballistic calculator, had vortex mount and level the scope for me but still saw an occasional drift right that I for sure knew was not mechanical.

Anyway I plan on testing several pairs of glasses next to see if I can reproduce the issue.

Have you tried toric contact lenses?  I found my astigmatic error is greater with glasses than with contacts (plus I have better clarity with contacts).

Gene;

SO it was an astigmatic lens (in your glasses) and a "interesting" viewing angle that caused what sounds like a parallax error resulting in the observed deviations...  Excellent, I'm glad you found and fixed the problem...  I love it when a plan comes together!  

Best,

- R -

interesting thread! It got pretty deep there for awhille.

I can't wear contacts because of OSHA rules at work.

Yeah this was an interesting journey. Took a carbine class for Northern Red last summer, one of the instructors was a former military sniper. Interestingly enough one of his lectures was on glasses introducing error in long range shooting. He was talking about non-prescription ballistic glasses creating parallax issues.