Scope Adjustment Limits from Its Mechanical & Optical Centers

[Bart B.]

A few days ago calculating two Weaver Model T target scope's objective lens focal length, I had zeroed those windage (W) and elevation (E) adjustments to mechanical centering of the inner tube in the main outside tube so it's on the optical as well as mechanical center of the scopes. Then I turned each knob in towards the scope center (down on the E knob pointing the inner tube lower to lower bullet impact from POI, left on the W knob to move bullet impact to the left pointing the inner tube to the left) recorded the MOA change, backed it out the same amount to zero, then up to its stops to record that number. Finally returned it back to mechanical/optical zero. I'd done this before spinning them in fixed rings (instead of a pair of V blocks) to get the adjustment limits but lost the paper they were recorded on. Here's their MOA adjustment limits from optical/mechanical zero with the erector tube centered in the main tube at both ends:

T16 scope, original;
E; 11 down, 60 up, 71 total spread.
W; 26 left, 46 right, 72 total spread.

T16 scope (T10 bumped up to 16X);
E; 27 down, 53 up. 7 more to stop but no reticle change, 87 total spread.
W; 30 left, 45 right, 75 total spread.

The second T16's inner erector tube stops against the outer main tube wall while the adjustment knob continues up for another 7 MOA or 28 clicks. I’m convinced the reason for more inner tube movement from zero towards the knobs is due to that stop’s against the adjustment screw flats that let the tube move further towards them while the opposite direction is filled with spring hardware to push the tube against the adjustment flats.

What does this say about the age old method of “zeroing scope adjustments” by centering each in the middle of the knob's mechanical limits?

 

[wally]

What does this say about the age old method of “zeroing scope adjustments” by centering each in the middle of the knob's mechanical limits?

That it'd be big wrong if the scope is not designed for "symmetrical" adjustments. IMHO its very reasonable to have asymmetrical elevation adjustment with more up adjustments unless you plan on using sloped bases. OTOH asymmetric windage adjustments would always seem to be bad and indicate poor quality unless the specified MOA windage adjustment range was less than the minimum measured adjustment range.

IMHO bore sighting is a better starting point than centering by counting clicks, so I've never considered using the "age old method"

 

[Bart B.]

Of course, bore sight your scope to the barrel axis to start with. No set of installed scope rings' centers align parallel in all axes to the bore center.

I've spun dozens of scopes, new and used, to zero their optical/mechanical to center their inner tubes to their outer tube over the years and none of them were equidistant from that zero to elevation nor windage clicks/units to their stops. All had a bit or a lot more from center to the inner tube's stop against the adjustment at its furthest point mechanically allowed. With an ey up close to the objective lens and a tiny flashlight pointed into the fronts of those scope, the inner tube was physically well centered in the outer tube. The inner tube's back end is always dead center.