Changing front sight for 100 yard zero

[RBruce]

Any idea how tall the front sight on a Model 1891 should be to get a 100 yard zero? My rifle shoots about four inches high at 25 yards with the front sight just barely showing in the rear sight V.

Information

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[jmoore]

Inches high on the target/inches to the target = change in sight heigth in inches/sight radius in inches

For a 100yd target:

change in sight heigth = (4" x Sight radius") / 3600"

You will have to add the depth of the notch (minus a little) to the answer to compensate for the odd sight picture.

To get the sight radius, measure from the back of the front sight to the back of the rear sight.

That's it. Works for any distance and any iron sights. Can be done in metric units if you prefer, just don't mix the units of measure! (i.e mixing inches with yards) Keep everything in inches or meters.

[RBruce]

OK, that seems straight forward. However isn't that calculation for 4" low at 100 yards? I'm 4" low a 25 yards.
If there was no gravity or wind resistance, that would translate into 32" low at 100 yards.
I fitted a Brownells Mauser high front sight. It is about 1/4" higher than the original and twice as wide (extra width should be good for bad eyesight).
I'll report on the results.

[Patrick Chadwick]

My rifle shoots about four inches high at 25 yards with the front sight just barely showing in the rear sight V.

That appears to be seriously out of the ordiinary. Before worrying about the sights, I would check the muzzle crown and touch it up if necessary,

[RBruce]

Perhaps my measurements or reasoning is off, but here is the actual target I shot at 25 yards.Attachment 34184

[jmoore]

Just plug in "900" where it says "3600" (900 inches to the 25yard target.)

And the answer will be four times more... A fairly sizable change! Your front sight will be quite tall if you need to come down that much. But if you can live with a six o'clock hold whilst making "Xs" the change will be several MOA less.

I'd put up a big target at 100 before making changes if possible.

[Patrick Chadwick]

If you use a six o'clock hold on a rifle that is correct at 100 yards and then move an 8" target (the one normally used for 100 yards/meters) back to 25 yards, then the POI will indeed be in the 7 ring at six o'clock. Because that is indeed where the barrel is pointing - slightly above the POA!

So, reversing the process, if you use the six o'clock hold to center the POI at 25 yards on an 8" target, the POI will be very high at 100 - because you are in effect raising the aiming line as the target shrinks in the distance.


Patrick