Reticle to Bore Perpendicularity

[bb223]

I've read as much as I could find on the subject, including really detailed guides with math and all kinds of graphics but I'm sort of dead set on how I want to set up my rifle and I'm not finding the answer.

Problem:

I drew a nearly 4ft tall line with a Sharpie on a target at 100 yards using a 4ft carpenter level and confirmed the straightness of the line by hanging a plumb line in front of it to match up and gave myself an aiming point at the bottom of said line.

Fired 3 shots at the bottom aiming point and the drilled dead center and all touching, I dialed up 5 mils and fired 3 shots aiming at the bottom aiming point and aligning the vertical stadia in my 10x SWFA with the vertical line on the target and the group equally as tight but a hair to the right of the vertical target line.

Repeated it again this time with 10 total mils dialed and the 3 shot group was again tight but this time offset to the right twice the distance as the previous 3 (5mil dialed) shots.

Darrell Holland shows this exercise and indicates shots falling right with elevation dialed means the scope needs to be rotated a hair clockwise (much like drifting a front sight towards the error for zeroing windage on an AK).

Looking at my scope it does appear that I am already slightly rotated clockwise in the rings as opposed to having the elevation turret top dead center so that's making me wonder if I should actually rotate the scope counter-clockwise in the rings to bring everything back to center.


What I am going to do is just rotate it both ways until it's tracking straight on the plumb line of the target but I was just trying to picture in my head how the process would work and what is the right way to do it.

Rotate the scope in the rings towards the error or opposite of the error?

Gonna be a few weeks before I'm on the range again so any input would be greatly appreciated

[mark5pt56]

Rotate to the left, iron are a different story. On iron, you move the front opposite of the direction you need to go, on the rear, it's the same direction. Also, many ways of setting up a scope. Remember-you may be canting the rifle as you shoulder it, does it have an adjustable butt plate?

[markm]

If you can secure the rifle solidly, you could track your scope come up with no shots fired and a plum line at home. Do you also have a level on the gun?

[bb223]

Rotate to the left, iron are a different story. On iron, you move the front opposite of the direction you need to go, on the rear, it's the same direction. Also, many ways of setting up a scope. Remember-you may be canting the rifle as you shoulder it, does it have an adjustable butt plate?

Unsure if I am canting the rifle. I figured that if I am holding the vertical stadia in my scope against the vertical target line that my reticle is a least "plumb to the world" and that would help me determine how much I would need rotate the scope (or maybe "rotate the rifle under the scope") in order to hopefully square everything up.

I can shoulder the rifle and cant it a bit in either direction and it is physically comfortable so I was just going to attempt to square everything up with my, as described, shoot the vertical line test.

Very little on the rifle is adjustable, it's basically a beginner setup, it's a Ruger Predator .308 in a Magpul Hunter stock.

[bb223]

If you can secure the rifle solidly, you could track your scope come up with no shots fired and a plum line at home. Do you also have a level on the gun?

No level on the gun and unfortunately I don't really have anywhere here to lock the rifle up and try and track the scope against a line. I tried pulling the action in some barrel jaws and leveling it with a Precision Hardcore "Tank" level on the bare rail and then putting on the scope and rings and trying to rotate square on a plumb line but I didn't really have enough distance to hang the plumb line to get any meaningful results besides blurriness as it was barely 10 yards out.

[mark5pt56]

One method is to get on the gun, have the plumb line and have a buddy move the scope as you are on the gun, meaning shouldered and held as if you would shoot it. Remember and this is a rabbit hole, the gun under recoil will find the weak spot, if you are muscling the gun onto the target it will under recoil want to return to the natural point of aim if it's not there at the start. Know how to adjust the NPA and obtain bone support and make it repeatable.

Not to debate any scope mounting method. Mechanically leveled, depending on the shooter they will get on the gun and say "my scope is crooked" It's because it's going into the shoulder differently, hence the adjustable butt plates.

Again, many methods and styles.

[JiminAZ]

Another possibility: what is going on is internal to your scope - your reticle is not perfectly aligned to your internal elevation adjustment mechanism.

If you roll the optic CW (or any direction for that matter), your problem will persist if my supposition is true.

I can see how a Clockwise roll would potentially correct your situation if indeed your scope was not level to the bore (i.e., your vertical stadia does not project through the true center of your bore).

So try rolling it a touch. It will either get better or worse or stay the same. If better yay, if worse yay also, just roll the other way. If it stays the same your internal alignment of reticle to elevation mechanism is the problem.

[T2C]

When setting up the scope in the shop, the scope is first set up using playing cards placed between the scope base and the flat on the bottom of the scope. I attach a piece of cardboard with a 3' vertical line drawn on it to the wall. I plumb the line on the cardboard with a level that is at least 3' long. If I am too close to the plumb line and cannot adjust the scope to view the plumb line clearly, I move the setup outdoors. I like to view the plumb line through the bore and make certain the plumb line is centered left to right in the bore while being covered by the vertical scope crosshair.

One way I determine if the rifle is level at the range while making minute adjustments to the scope is to take a 3' piece of drill rod and lay it in one of the grooves on the scope base. If the rifle has two stand-alone bases, the drill rod is attached to another flat surface on the rifle. I set the rifle up on a pair of homemade V blocks, place the vertical crosshair over my 100-yard target plumb line, then step back several feet from the rifle and observe the drill rod to determine if it is level to the horizon.

When physically moving the scope to correct tracking error, I imagine the rifle moving under the scope like a pendulum. If I rotate the scope counterclockwise, I think of the rifle bore moving to the left at the lower part of the pendulum when the crosshairs cover the target plumb line. When tightening the scope ring screws, I use feeler gauges to make certain I see the same gap between the lower and upper scope rings and everything is tightened evenly in several steps.

I perform a tracking test at 200 yards to determine if I need to make a minute adjustment.

I've had a few rifles with scope ring screws that appeared worn, because I was not happy with the results the first two or three tries. Some people are reluctant to loosen the scope ring screws multiple times to get the ideal setup, don't be.

If you have to move the scope to change eye relief in the future, witness marks placed on the scope and rings with a thin lead mechanical pencil will save you a lot of time.

[mark5pt56]

If you have to move the scope to change eye relief in the future, witness marks placed on the scope and rings with a thin lead mechanical pencil will save you a lot of time.


Learned this long ago, really helps to tweak the eye relief, especially at the range.

[bb223]

One method is to get on the gun, have the plumb line and have a buddy move the scope as you are on the gun, meaning shouldered and held as if you would shoot it. Remember and this is a rabbit hole, the gun under recoil will find the weak spot, if you are muscling the gun onto the target it will under recoil want to return to the natural point of aim if it's not there at the start. Know how to adjust the NPA and obtain bone support and make it repeatable.

Not to debate any scope mounting method. Mechanically leveled, depending on the shooter they will get on the gun and say "my scope is crooked" It's because it's going into the shoulder differently, hence the adjustable butt plates.

Again, many methods and styles.

This is actually a really good idea, never thought of having a helper when scope mounting.

Anyway to clarify I went ahead and put a Sharpie mark on the scope and ring and rotated the scope counter-clockwise just a hair, maybe less than 3/4 the width of a Sharpie line and ran the test again and it was a dead nuts straight with the POI as I could possibly hope.

In my head rotating away from the error always made sense but I got confused by Darrel Holland's article recommending turning the scope towards the error as he's Darrel Holland and I'm not.

But yeah, getting on the rifle is perfectly comfortable as is and it seems my reticle bisects my bore and tracks straight with gravity, so I'm happy.

And I have frequently used feeler gauges in the past for mounting flat bottom scopes square to the rail but my SWFA SS is the classic model and has no flat.

Thanks everyone!