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.