Scope zeroing

[PANZER22]

If the range I am using is only rated at 25 yards can I zero at 25 then move the height of the POI by raising the reticle up X number of clicks to move the reticle where the POI will be considered zeroed at 100 yards. (cross hairs on point of POA of 100 yard target)

Any comment will be appreciated.

Dan

 

[Dayman]

3 in down would be closer to 100 yards center as the bullett is still rising at 25 yards. Look for ballistics charts per your caliber/grain. Some one may know better though.

 

[Tigerstripe]

i use 1 inch low at 25 yds with a 308 to be crosshairs on center at 100 yds.

 

[armaborealis]

If the range I am using is only rated at 25 yards can I zero at 25 then move the height of the POI by raising the reticle up X number of clicks to move the reticle where the POI will be considered zeroed at 100 yards. (cross hairs on point of POA of 100 yard target)

Any comment will be appreciated.

Dan

Yes.

However, you will have to know the difference between your POI at 25 yards and your POI at 100 yards with YOUR rifle and YOUR scope and YOUR ammo, be able to convert that to minutes of angle, then dial in the correct number of clicks (and hope your scope has an accurate measurement of clicks per MOA!).

As one example, I typically keep my service rifle zeroed at 200 yards, as that is where the high power matches start. If I want to shoot at 100 yards, I know I can dial in two clicks and be close to dead on. Likewise, when I go from 200 to 300 yards, I need to dial in two clicks the other way.

I keep my M4gery zeroed at 50 yards. It is about 1.5" high at 100 yards. So, if I want to go from the 50 yard zero (POA=POI) to a 100 yard zero (POA=POI), I need to move 1.5 MOA, which is three clicks on the aimpoint (1/2 MOA per click).

As another example, based on the ballistics calculator, an AR shooting M855 ammo zeroed to be dead on at 25 yards should also be on around 350 yards. It should be about 5" high at 100 yards. That suggests that the 100 yard zero is 5 MOA different from the 25 yard zero. If I zero at 25, move my front sight about five clicks (on a 20" A2 style rifle) then I should theoretically be dead on at 100 yards. If I had a scope with 1/2 MOA clicks like my Aimpoint Pro, then I'd have to move 10 clicks (5 MOA x 2 clicks/MOA).

Of course, it is indispensable to verify the zero at actual range and takes notes as to how your actual rifle performs. A lot of rifle/ammo/sight calculations differ somewhat from the ballistics charts, and not all sights have an "as advertised" clicks per MOA. A lot of cheaper scopes which advertise 1/4 MOA clicks actually have 1/3 MOA clicks in practice.

If this sounds like gobbley-gook, come to an Appleseed. We teach the concept of inches-minutes-clicks and how to efficiently make sight adjustments as well as the adjustments to be made at range.

 

[bigfutz]

3 in down would be closer to 100 yards center as the bullett is still rising at 25 yards. Look for ballistics charts per your caliber/grain. Some one may know better though.

Technically speaking, the bullet never rises. It only appears to rise as your scope is x" above the barrel and essentially pointing down. A lot depends on the height of your scope above the bore. So if you zero at 25 yd with a really high scope, you will hit a lot lower at 100 yd than if you do the same with a lower scope, as the angle between line of sight and bore is more extreme in the first case.

For #1 5.56/55/fmj and #2 .308WIN/147/fmj you see an apparent rise at 100 yd with a 2" scope height sighted at 25 yd.


But the apparent rise is much less with a 1" scope height.

 

[PANZER22]

I use the same ballistics calculator that you do......I can't see any reason why it would not work by moving the reticle up click by click.......I have four scopes with 1/4 inch at 100 yards with each click.

 

[Tigerstripe]

Bigfutz, do that with the zero at 25 yds an inch low.

 

[Dayman]

Downloaded that ballistics chart after seeing this thread. Good info here. Any one got a good formula to determine exact scope height? Center bore to center of the scope? Buis center bore to top of the post?

 

[PANZER22]

Most of the BDCs I've seen have a notation some where on the page denoting 2" to 2.50" from center of bore to center of scope......I have no idea how you figure that in...I have always tried to mount my scopes close to the bore as possible figuring the less angle the better.

 

[armaborealis]

3 in down would be closer to 100 yards center as the bullett is still rising at 25 yards. Look for ballistics charts per your caliber/grain. Some one may know better though.

Technically speaking, the bullet never rises. It only appears to rise as your scope is x" above the barrel and essentially pointing down. A lot depends on the height of your scope above the bore. So if you zero at 25 yd with a really high scope, you will hit a lot lower at 100 yd than if you do the same with a lower scope, as the angle between line of sight and bore is more extreme in the first case.

Eh, I would not necessarily fully agree with the bolded text. Check out the illustrations at this link: http://en.wikipedia.org/wiki/Rifleman's_rule

The barrel on most rifles is actually angled up slightly with reference to the sights/optic and the plane of the earth. The bullet rises, passes the "near zero," reaches its apogee, then begins falling and crosses the line of sight again at the "far zero." This is basically the whole concept behind "battlesight zero" which is an extremely useful concept for hunters and soldiers alike. It is also easily verifiable... sight in at 25 yards, then shoot the same rifle at 100 yards at a similar sized target... you will be striking higher at 100 than you did on your 25 yard target.

Using the first chart big futz posted, for that postulated rifle/ammo/zero combination, there is about a 4 MOA difference between the 25 yard zero and the 100 yard zero. If the shooter zeroed at 25, then dialed in 4 MOA of elevation into their scope/sights (16 clicks on a 1/4 MOA scope), they should theoretically then be dead on at 100.

Of course, I say "theoretically..." I think it is very hard to beat going out and collecting data with your gear. For example, a lot of scopes (especially less expensive ones) which advertise 1/4 MOA clicks are really 1/3 MOA clicks. YMMV with different ammo. Your scope may not be 2" above the bore. Etc.

We could also work this backwards using some rules of thumb. The BSZ is that a 30 cal battle rifle zeroed at 25 should also be "on" at 200 (the near zero is 25, the far zero is 200). Some typical come ups* tell us that to go from a 200 yard zero to a 100 yard zero, we need to "come down" 3 MOA. So that would argue for a 3 MOA adjustment, which is pretty close to the 4 MOA derived from Big Futz's chart.

*30 cal come ups ROT: 100-->200 (3 MOA), 200-->300 (3 MOA), 300-->400 (3 MOA), 400-500 (4 MOA), 500-600 (5 MOA) -- all MOA cumulative, so to go from a 200 yard zero to a 600 yard zero equsl 3+3+4+5=15 MOA

 

[Dayman]

Sighted in a new scope this weekend at 100 yards (give or take) under variable conditions with an unconventional rest. Then went to an indoor range with more controlled factors to confirm the results. Used the ballistics calculator to confirm. That thing is pretty dead on. Looking forward to trying other calibers. If any one has a smart phone I highly reccomend that app good find. Thanks guys

 

[Dayman]

Sighted in a new scope this weekend at 100 yards (give or take) under variable conditions with an unconventional rest. Then went to an indoor range with more controlled factors to confirm the results. Used the ballistics calculator to confirm. That thing is pretty dead on. Looking forward to trying other calibers. If any one has a smart phone I highly reccomend that app good find. Thanks guys

Btw I was about an inch low at 50 yards with a 100 yard zero