How does scope rail not aligned with barrel affect POI at different distances?

[mchees1]

If my scope rail and barrel are not perfectly aligned, what will that to do the bullet POI at distances other than what it is sighted in at? If the scope is aimed slightly left of barrel and it is sighted in at 200m, where will the bullet hit at 100m, or 400m or 800m? Will it be left or right of the crosshairs? Trying to reason it out but not sure of what will actually happen. Thanks.

 

[Bill Norris]

If my scope rail and barrel are not perfectly aligned, what will that to do the bullet POI at distances other than what it is sighted in at? If the scope is aimed slightly left of barrel and it is sighted in at 200m, where will the bullet hit at 100m, or 400m or 800m? Will it be left or right of the crosshairs? Trying to reason it out but not sure of what will actually happen. Thanks.

If the scope is mounted with the vertical cross-hair mis-aligned straight (perpendicular) with180 degrees then the bullet will go left or right at longer distances whichever way the cross-hair is mis-aligned so I would think the scope being mounted crooked (not aligned with the barrel) may cause the same issue.

A simple test can show exactly where the bullet will go by drawing a vertical line on the target with the scope originally adjusted to zero @ 100 yds. and aiming and firing at that particular spot. Then with the scope elevation adjusted to 8 MOA high or so fire another shot holding for the same POI . The second bullet will hit high and either left or right of the line if the scope is mounted tilted either way.
The same load would have to be used for both shots.

 

[rijndael]

How would the average Joe know where the bore is aligned? And, are we talking about aligned at the muzzle? I'm not sure you can rely on POI to tell you where it's at (exactly). Have you ever had two different loads with different point of impacts?

 

[Bill Norris]

How would the average Joe know where the bore is aligned? And, are we talking about aligned at the muzzle? I'm not sure you can rely on POI to tell you where it's at (exactly). Have you ever had two different loads with different point of impacts?

Yes the same load would have to be used for both shots.

 

[tmwinds]

Sounds like you are a candidate for a set of Burris Signature rings with a handful of various moa inserts, a printout of the Burris instructions and a day at the range. Misalignment can be cured with the Burris setup but it will take time and patience. Read the Burris instructions and watch a few YouTube videos and you can handle it. Good luck.

 

[davidjoe]

Bullets all have some degree of spin drift anyhow once they leave the barrel, but it is correct that a reticle’s fine center above the bore covers exactly one and only one precise point in three dimensional space. The finer the reticle, the smaller the covered point is. (An intersecting point of “first cross” in space, if used, is not actually “covered” by the center of the crosshairs, and any point exactly behind the center will appear to have moved closer to the horizon line, and so is also not covered by the center.)

The center is fixed where you put it and simply cannot track the constantly changing path of the bullet. A scope offset from the centerline because a shooter needs to use the “other” eye will have to be re-zeroed with both knobs instead of one, at every distance. However a scope mounted on a base over slightly skewed holes may be corrected with windage.

The outer tube becomes irrelevant once mounted and if the inner tube can be perfectly aligned with the barrel through windage adjustment, then the scope gets you back to the best case scenario of dealing with spin drift and the other inescapable variables.

 

[mac86951]

If my scope rail and barrel are not perfectly aligned, what will that to do the bullet POI at distances other than what it is sighted in at? If the scope is aimed slightly left of barrel and it is sighted in at 200m, where will the bullet hit at 100m, or 400m or 800m? Will it be left or right of the crosshairs? Trying to reason it out but not sure of what will actually happen. Thanks.

If no scope adjustments are made, POI will only shift due to drop and wind.

I'm assuming that you're asking what POI does if you dial elevation changes, and a misalignment will then mean elevation change will create a needed windage change too, multiplying in magnitude as distance extends. All scopes have this to some extent as bullets don't fly in a straight line, so if this is a deal breaker for you, yes align it; otherwise make a drop / windage table and it'll still work just fine.

M1C/D is a great example. Scope works fine, just needs extra diligence in dialing in elevation. Parallel to bore would be nice; but not required. Above bore is easier for elevation changes since gravity is a constant, but since wind isn't, well maybe it is constant in keeping me guessing...

 

[stevenpoplin]

your asking a legitamate question. If the rear of your pic rail is not directly above the axis of your bore when your action is perfectly plumb, all you have to do is rotate your action on the bore axis untill it is. and if your scope is not pointing the same direction as the bore axis you can change that with the widage adjustment on your scope until it is. For your windage adjustment only moves the front of the errector tube, not the back. The errector tube is the tube inside your scope tube that your looking through. By sighting your bore to a fixed point with a perfectly plumb line on it. then rotating your scope along the bore axis untill the verticle reticle is parallel to the vertical line on your target. regardless if its on the verticle line on the target. then move the vertical line in your reticle to the verticle line on your target and an imaginary countinous line going down from your verticles lines through the center of your bore with your windage adjustment. you are good to go.
Now your action may not be perfectly plumb if your pic rail holes are not prfectly on top of your action when plumb, and your pick rail may not be level at a 90 degree angle to your bore axis either but your scope horizonal retical line is. but you rifle bore and scope will share the same verticle longitudal axis
All that said does'nt mean that a fired bullet from the rifle will hit the verticle axis of the target so you then move the windage adjustment on your scope so it does,
you can then go to an even more accurate test with a tall target to make sure when you dial your scope up and down your cross hairs follow up and down the plumb vertical line on the tall target.if it does'nt you rotate your scope untill it does, then move your vrtical reticle line over to your verticle line on the target.
If you have everything just right and your action is not plumb that's where the Burris rings come in handy.
Now you have to realize yor bullet has spin drift and is blown by wind you'll have to calculate to hit at further distances.
If you

Hope you understand. I'm not so good at explaining

 

[Full Chisel]

If my scope rail and barrel are not perfectly aligned, what will that to do the bullet POI at distances other than what it is sighted in at?

The simple answer is nothing for typical top mounting - meaning on top of receiver, not intentionally off to the side like someone might do on a lever action. Let's say the rear of rail is perfectly over bore axis, but front of rail is .030" off of bore axis, and scope rail is 6". If you do the math, the scope will be aimed 17 MOA off bull at all distances. Now you adjust you're scope windage to get 200 zero, and in doing so the erector tube had to move to accomplish this. Now look at worst case, which is the center of the erector tube can't possibly be more than .030" off the bore axis. That means that rifle bullet can't possibly start more than .030" off scope axis. At 200 yd it's perfectly in line with scope axis, at 400 yd it's .030" off of scope axis the other way, at 600 yd it's .060" off, and 1,000 yd it's .120" off. If you're going for a world record, then it matters, otherwise it's not anything to think about. But as someone mentioned, Burris makes signature rings that help so you don't have to do it all with the scope windage adjustment.

 

[NZ_Fclass]

How would the average Joe know where the bore is aligned? And, are we talking about aligned at the muzzle? I'm not sure you can rely on POI to tell you where it's at (exactly). Have you ever had two different loads with different point of impacts?

A bore sighter will usually give you the clues. Zero the scope out and look at the alignment on the vertical axis.

 

[rijndael]

A bore sighter will usually give you the clues. Zero the scope out and look at the alignment on the vertical axis.

I thought this thread was about theory, not practical/real-life use cases. I read the OPs post differently than others, I guess.

 

[NZ_Fclass]

I thought this thread was about theory, not practical/real-life use cases. I read the OPs post differently than others, I guess.

Well, yes, but, if the scope is intended to be parallel but high ( the way we normally mount scopes) the alignment to the bore can be checked for left/right deviation, assuming that the barrel is fitted correctly. If we are talking about, say Russian military "sniper" guns, where the scope is offset, it will need some sort of cleverness to ascertain that the scope remains parallel, and the POI remains at the offset distance (So the bullet always hits by the offset at all ranges) or converges at a given range. Best of luck having to correct for elevation and windage simultaneously.

 

[Knotwild]

I don't know the answer to your question, but here is what happened to me Friday. I put a Sightron S-TAC 4x20x50 on a CZ452 using Burris Signature Zee rings adjusted for 20 MOA elevation and a DIP 0 MOA rail. Scope has 40 MOA of windage. I couldn't get it zeroed at 50 yards because I ran out of windage - 8 moa short. I looked at my hasty mount job and had one ring tightening left and one right. I turned one so they both tightened from the left side - problem solved. Scope zeroed close to center of 40 MOA range.

 

[mikecr]

Is the rail offset from bore, or skewed w/resp to the bore?
Is either the front or the back of the rail at centerline?

 

[Jeff Porter]

Ok a bit more information is needed from the OP. Is this a theoretical question or is your rail crooked ?

I have a old Mauser that was sporterized to a .243 years ago and the holes for the Weaver rail were drilled and tapped off line with the axis of the bore but the rail was mounted straight with the bore with the holes drilled crooked in the rail to match the receiver. Worked fine for a couple of decades and killed a lot of coyotes. Finally shot the pencil barrel out and spun a fat .308 barrel on it and added a new Piccatinny rail from EWG. My gunsmith did a great job boring they holes in the Pic rail to the exact spacing but in the middle of the rail so that when I mounted it it was pointing to the left.
When I mounted a Millet LRS on it I ran out of windage trying to get it to the center. I bought some Burris Signature rings and used the 20 MOA inserts to get the scope bore sighted with the windage knob centered. Shoots fine and the scope tracks true out to 600 now.( longest I have used it yet )

back to the OP question. If you can crank the windage to hit point of aim and still have a reasonable amount of adjustment or you plan to hold off … it will track as intended.

 

[4xforfun]

I don't understand.....Unless you mount a PERFECTLY centered scope on a rifle and it exactly zeroed at xyz distance......EVERY one is miss aligned. That is what the elevation and windage knobs are for.

 

[jelenko]

I don't understand.....Unless you mount a PERFECTLY centered scope on a rifle and it exactly zeroed at xyz distance......EVERY one is miss aligned. That is what the elevation and windage knobs are for.

Exactly. If you can zero the scope, it is aligned.

 

[Ggmac]

Exactly. If you can zero the scope, it is aligned.

No , not exactly. Say the scope is offset to the left 1” ( exaggerated) and you sight in at 100 , poa = poi then at 200 you will be to the right of poa . And the poi will continue to be to the right even more the farther you go .
Now if your poi ( still 1” offset scope ) is 1” to the left of poa itll stay that way ( assuming no wind or spin drift )

 

[jelenko]

Ah. Yes, if offset.
I did assume that the discussion was about a scope mounted on the receiver but pointing not exactly parallel with the bore.

 

[4xforfun]

No , not exactly. Say the scope is offset to the left 1” ( exaggerated) and you sight in at 100 , poa = poi then at 200 you will be to the right of poa . And the poi will continue to be to the right even more the farther you go .
Now if your poi ( still 1” offset scope ) is 1” to the left of poa itll stay that way ( assuming no wind or spin drift )

"Offset" is TOTALY different than " Aimed slightly left of BBL". Two different subjects with totally different answers.