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Posted: 5/8/2022 11:26:00 PM EDT
Is this guy correct? I guess he's basically arguing that he has a perfect parallel zero as opposed to converging. If you just zero at one known range without accounting for perfect mechanical offset, and confirming at multiple ranges, aren't you converging no matter what you do at some point? And if that's the case, you will be diverging beyond that and all of your scope tracking/stadia line holdovers will be off?

Dumb this down for my lizard brain

Link Posted: 5/9/2022 1:29:57 AM EDT
[Last Edit: AKSnowRider] [#1]
Basically he is correct..your level needs to be aligned with the reticle(more lined up with the adjuster mechanism that moves the reticle)... there is a bit more to it than that but most rifles will shoot accurate enough even if the rifle/action is not level to the reticle...The easy way to check and verify is to do a tall target test(I combine with a box test at the same time), basically you are going to have one aim point which you will do all your aiming at, precisely measured from that aim point you will place bullseyes at 5-10-15-20 moa or MIL depending on what your scope measures in..you will also do a target 5 to the right and 5 to the left of the bullseye with another 20 moa/mil directly above them..your test is at precisely 100 yards or meters depending on wheather you use yards or meters and the target must be perfectly level/horizontal for the test...

Here is a video that explains it better than I can...

Scope Tracking: Tall Target Test - Long-range Shooting - Applied Ballistics

This is my combined tall target/box test...

Attachment Attached File
Link Posted: 5/9/2022 1:56:09 AM EDT
[Last Edit: uglygun] [#2]
Converging on a vertical plane is the idea so trajectory is in line with bore center line axis and scope center line axis, ideally these line up with the pull of gravity ignoring spin drift or wind.   If that plane is off due to canting the rifle then things are wonky after POA through the reticle/zero point.

99% of the shooters out there you can likely put a level on the rifle base and then set your crosshairs off a plumline and call it good.   If mechanicals are off a degree from the reticle it is not gonna make much practical difference for most shooters(do the math for 1 deg of incline due even 1MOA of windage, if a shooter can split that hair of adjustment then all I can say is god damn).  Hopefully your scope base is drilled on center/vertical allowing for a square/level rifle mounting point to get a truly leveled rifle.   And hopefully scope rings are concentric and centerlines are precise, what if both front and rear rings are .005in off?

It is possible there could still be some cant, that is where testing on a target can start to ferret out the possible issue if the shooter/load/rifle are precise enough to determine scope/canting issues while testing the mechanicals.

Eliminating rifle cant, within reason, is a good goal.

I would love to see a practical test done with a decent shooter, above average but not some gifted type, with a rifle setup to be absolutely god awful damn near perfect.   And then the same rifle/load/scope with the scope reticle maybe 1deg off from perfect, still able to be shot level thanks to a bubble level on the rifle, not the scope.   Or maybe rings or base is just ever so slightly off.

See whether they can shoot the difference across a course of fire.   At what range does the error stack enough to make a meaningful difference?

Eliminating sources of error is all well and good.  But what is your budget and just how far are you willing to take it before calling it good?
Link Posted: 5/9/2022 10:49:18 AM EDT
Someone getting up their own ass about something that is not helpful to 99% of shooters. There's a reason everyone does it one way even though there are many ways. Because it works.
Link Posted: 6/11/2022 8:59:59 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By ballisticxlr:
Someone getting up their own ass about something that is not helpful to 99% of shooters. There's a reason everyone does it one way even though there are many ways. Because it works.
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If your natural point of aim or body form induces rifle cant it’s nice to know that eliminating scope cant is more key than a mechanical arrangement that is visually aesthetic. The bullet doesn't know that the scope tube and barrel/receiver are not mechanically aligned on the exterior. All that matters is that the reticle internally is aligned to the trajectory and that the reticle is externally aligned to a plumb line for shots at ranges beyond the zero distance.

Plenty of people mechanically align their rigs and then for a variety of reasons take a canted hold for a shot.  Unless there are visual or physical cues many people won’t even look at a spirit level or notice the cant and will take the shot.  Most people never have issues because for the ranges they shoot at a slight cant either doesn't matter, or at ranges where it could matter wind drift compensation errors dominate the horizontal dispersion.  

It’s more important to align spirit level to a plumb reticle than align scope to receiver if one is a very good long range marksman.  For most of us good enough is good enough.
Link Posted: 7/10/2022 5:33:03 PM EDT
[Last Edit: DakotaFAL] [#5]
Technically he is correct.  If the center of the reticle isn’t precisely over the center of the bore, then the bullet will have to move from left to right (in most  cases with right handed shooters) to reach the line of sight at the zero distance, and then will keep moving to the right past it.

Does it matter? That depends on a few things.

How far off bore centerline is the reticle and what is your zero distance?

Way back in the day before laser range finders and reliable finger adjustable turrets the basic zero distance for most snipers was fairly far out.  For example an M40 might be zeroed at 500 yards (as it wasn’t career enhancing to engage an enemy at any thing much less than 500 yards) and the max effective range was about 800 yards.   In that context, even if the rifle was canted a lot and the bullet had to move an inch and a quarter right to cross the line of sight at 500 yards, it was only going to be about  3/4” right at 800 yards.  Even 1.25” at 500 yards will be lost in the noise of normal dispersion.  You would see it however at 100 yards with the bullets hitting about 1” left.

Today, with finger adjustable turrets that are both accurate and reliable, it’s far more common for long range shooters to zero at 100 yards or 100 meters.  In that situation with a rifle canted so that the bore was 1” left of the center of the reticle in the scope, the bullet is moving 1” to the right in just 100 yards/meters  At 200 yards/meters it’s now going to be 1” to the right, 2” at 300, 3” at 400, 5” at 600, 6” at 700, etc.  at 1000 yards/meters  that 1” error due to canting the rifle will place the bullet 10” to the right.

Are we canting the rifle, or are we canting the reticle?

Back in the day with a fixed zero and estimating hold over, it didn’t make much difference.  

However today, with adjustable turrets and putting the hold over on the scope it can make a huge difference if the shooter isn’t shooting with the vertical cross hair perpendicular to the earth (and gravity).   Let’s say you want to make an 1100 yard shot with M118LR and you crank 46 MOA of elevation in the scope.  If your reticle is canted 10 degrees to the right, you’ve really added 45 MOA of elevation and 8 MOA of right windage.  That’s only about an 11.5” error in elevation, but it’s a 92” error in windage. Good luck with that.   Even at just a 5 degree cant angle for the reticle it’s a 46” windage error.

Now…that’s where the debate begins.  Some shooters will advocate leveling the scope on the rifle so that the center of the reticle is directly over the center.one of the bore when the rifle itself is level.  Other shooters will advise leveling the scope on the rifle so the reticle is level when the shooter is holding the rifle naturally.  Both approaches have pros and cons.

Ideally, the rifle stock will fit the shooter comfortably when the rifle and cross hairs are level.

However, if that’s not the case, then there is an argument that the shooter will be more consistent at keeping the reticle level when the rifle is comfortable in his grip, even if the rifle is canted relative to the scope. What matters most is that the cross hairs are vertical.   That approach is common with small bore target shooters, but canting errors don’t matter given that they shoot at known distances with known sight settings and adjustments for different distances and positions.

Either way, for a long distance shooter shooting at varied distances,  a level can be mounted on the scope, rail or rings to let the shooter verify the reticle is indeed level.   That’s just a modern offshoot of the old practice of putting a spirit level on the front sight of the old Creedmoor rifles where the rainbow trajectories of the black powder cartridges used required tall vernier rear sights, that resulted in high sight heights that magnified canting errors.

Personally, I normally mount a level on either the left side of the scope or the rail so that I can see the level with my left eye while looking through the scope with my right eye to verify the reticle is level.

How accurately aligned are the adjustments with the reticle?

Someone above mentioned a tall target tracking test.  Since having the reticle vertical is a given and should be a basic shooting skill, it has nothing to do with how the rifle is canted. A tall target test is really testing the scope to see if the internal adjustments are in fact precisely aligned with the reticle.  A degree or so  is pretty normal, even in a high end scope.  

In the 1100 yard shot above, a 1 degree misalignment of the reticle versus the adjustments is a .8 MOA canting error and at 1100 yards that is about a 9 1/4” horizontal error.  Obviously much more than a degree or so is going to quickly become a serious problem.

That’s why everyone should take any new scope out to the range and perform both a tall target test and a box test.

The tall target test verifies the reticle is precisely aligned with the adjustments, and that the amount of elevation adjustment on the scope actually produces the correct angular change.

The box test confirms that when you put, for example, 30 MOA of adjustment on the scope you get 30 MOA of change on the target, with no lag or hysteresis in the adjustments.  If you start with the target dot in the left lower corner of your sheet of paper, you’d shoot a group of five, then add 30 MOA of elevation, shoot another 5, add 30 MOA right windage and shoot another 5, take off the 30 MOA of elevation and shoot another 5, then take off the 30 MOA of windage and shoot the last 5 shots.

The groups should all be tight with no tendency for the first shot to lag or over shoot the rest of the group, and the last group should fall directly on top of the first group. The four groups should also form a square.  If you don’t have right angles in the resulting “box” formed by the groups, your adjustments are not perpendicular to the reticle and or your reticle is canted (not level).

The sharp crayons in the class will have just figured out that if the box is big enough, it’ll substitute for the tall target test as well.  With a box rest bigger is better and I would not consider anything less than 20 MOA or 6 mil to be sufficient to detect error in the scope adjustments. 30 MOA or 9 mil is better.

If you are a MOA shooter, setting the target at 95.5 yards will give 1” of change on the target for each MOA, which makes the measuring and math easier.
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