25 yard zero...

[Patrick Chadwick]

I apologize for this not being a straight answer to your question, but I do not think that trying to zero a rifle with open sights at 25 yards is a very good idea, UNLESS you use a target that is reduced in size AND know the trajectory of your ammo AND the height of the foresight blade above the barrel centre-line.

If, for instance, you take the standard ISSF target with an 8" black and set it up at 100 yds, then a 6 o'clock POA is in fact a sight line that is below dead centre of the target by an angle given by InversTangent 1/900 (4"= 1/3ft viewed at 300ft). If you move the same target back to 25 yds, then the angle is InversTangent 1/225 (4"= 1/3ft viewed at 75ft) i.e 4 times as much. And at 25 yds you need to take into account that the barrel centre line is about 1" below the sight line, so the effect is even stronger. In effect, the barrel is aiming much lower than you think.

If you want to fire a G11 at 100 you are going to need a higher foresight blade to get on target anyway. So to make it even roughly accurate, you need to know the exact height of the foresight above the barrel centre line AND the ballistics of your ammo. In addition, the sight picture of,say, an 8" black at 25 yds with a 6 o'clock hold, is different to that at 100 yds, and very different again to the sight picture at 300 meters (G11). This effect alone will cause a shift if you go straight from 25 to 300. So using a reduced target goes some way to ameliorating this effect.

And forgive me for asking, but why do you want to zero at 25yds at all? I have been there and I have done that - and with a G11 to boot. And my experience was that at 25 yds/meters you can't do much more than make a horizontal correction and check that the shots are in the black. About 1.5" above the POA is my estimate - but you will not be able to set it up to be dead on at 100, much less 300, unless you really, really know the trajectory of the ammo you are using. Which means knowing the actual muzzle velocity as well.