Zeroing for 200 yards with only 100 yards range to use.

[Robert303]

Come on chaps let's have some replies. I'm in the same position and so could do with as much help as possible. I'm an ex pistol shooter who'se never really been taught to shoot rifle. I really must learn how to use Parker Hale 5s on my SMLE and No 4.

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[Patrick Chadwick]

Come on chaps let's have some replies.

Don't be so impatient everybody! I've only just seen these posts.

First point: If you are using the 6 'o'clock hold on a 6" target at 100 and then on a 12" target at 200, then the angle between the line of sight - POA - and the line to the target center - the hoped-for POI - is the same in both cases, so no correction is required for target size.

Secondly, unless you are using some very odd ammo, then the muzzle velocity will be in the range 2300-2600 fps for a 174gn .311 bullet fired by a reasonable charge. At this point, we who have no ballistics software turn to the Sierra exterior ballistics tables. The drop at 200 yards for a 174gn MatchKing is given as 11.23" (2600 fps) to 14.44" (2300 fps). Relative to the bore centreline, of course. If you were to draw a straight line from muzzle to POI, this looks like half as much at 100, i.e. 5.6" to 7.2" (there really is no point in calculating to .01").

But the drop at 100 is in the range 2.66" to 3.41". So you need to elevate the muzzle to put the aiming line about 3" to 3.6" higher at 100 yards. Since the sight radius on a No. 4 is about 28", the peep hole need to be raised in proportion. I.e. 3 to 3.6 divided by 100 times 28/36.
In other words, about 0.023" to 0.028". On a standard No. 4 backsight that means 3 to 4 clicks.

Ideally, one should correct for the fact that the line of sight is actually about 1" above the barrel. For the greater precision expected with a scope, and the fact that a scope is mounted much higher, it is indeed necessary to take this into account. But for iron sights - forget it.

There is no point in making involved calculations to the micron - no software can take into account the fact that your sight picture for the 6 o'clock hold will not be quite the same at 200 yards as for 100 yards, because the blade/target contrast is different, so your eyesight and the light conditions also come into the equation. Just raise the backsight 4 clicks and get going!

I know this is a rough method, and make no claim that it would work going from, say 100 to 1000 yards, but for 100 to 200 it will provide a good first approximation. I do this sort of thing frequently, in BP competitions at 100, 200 and 300 meters, with "best ten out of 13" scoring, so there isn't much to play with. Your first shot may not be a 10X, but it should be well enough in the black.