Confused about where to zero

[michaelmew]

[ricsmall]

you will first need the real world velocities of all the loads you plan to shoot. Then load all your info into a ballistics calculator, several free online, and this should get you close to make some dope cards. That being said, there IS NO substitute for actual shooting all loads at distance to see the difference.

[stokesrj]

I'll start with answering question #2. There is no way to calculate where different loads will impact. This is because the barrel is vibrating as the bullet travels down the barrel and the point of impact, especially at short range is more dependent on the barrel position as the bullet exits than the external ballistics of the bullet.
The m855 load you mentioned is notoriously inaccurate but other loads using quality bullets at moderate velocities will be very close to the Mk262 load which is a very accurate load. For instance I shoot 52grain Sierra Match Kings at 3,200 fps, 77 grain match kings at 2,750 fps, and 80 grain match kings at 2,800 fps and they are all within 1/4 MOA at 100 yards. But that M855 is over 3 MOA different in point of impact. At 600 yards the 77 and 80 grain loads are within 1.5 MOA of one another and the M855 is about three feet lower and the 52 SMK is about 5 feet lower if it even makes it on the target it has to be relatively calm.

My advise would be to choose a load that your rifle shoots well using a bullet that has a reasonable BC and stick with it. Zero it at 100 yards since that is the range you have and then calculate point of impacts at the yard lines you plan to use. JBM is one of the most respected ballistic calculators out there. Just inter the required information starting with the bullet you are using and the muzzle velocity and hit calculate, that will put you within 1 MOA or less out to 1000 yards for normal conditions. If you want to be more precise then you will need to measure your sight height above the center line of your bore, enter your temperature, humidity, and altitude to get you very close.
But nothing beats actually shooting at the actual yard lines. It is all pretty meaningless anyway until you do because you will not understand how to read the conditions to properly dope them.

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

Bob

[LR1955]

Let me preface by saying that I am an amateur shooter with no formal training.

I want to re-establish all the zero distances on my rifles for various reasons and work up some type of dope chart for various distance and bullet combinations. I do not have access to a range past 100 yards at this time. Ideally, I would be able to shoot to each distance and see where I am on paper and then make a note in a dope chart, right? That is not a possibility, so I need to mathematically estimate to the best of my abilities what those results will be.

I have a couple of issues that are concerning me, so I'd like to float these questions out to you guys:

#1. Min & Max PBR is the distance that I can take a shot without the need to adjust POA (assuming a center mass or kill zone hold). Correct? I would think that +/- 5" would yield good results. Would you agree with that?

#2. If I zero my AR 5.56 at 50yds with 55gr M193 (3100fps), then POA=POI again around 245yds. Correct? If I do nothing other than load a 62gr M855 (3000fps) into the rifle and fire again, the 50yd zero is not the same, and the other POA=POI is different. Is there a way to calculate this difference? How much of a difference is that? What about if I am zero'd for the 77gr Mk 262, but decide to shoot the 55gr or 62gr… how much of a difference then?

This example is for a 5.56, but it could easily translate to the Grendel & other calibers, especially for someone who hand loads.


#3. Is it uncommon to use a different zero for your scope and your BUIS? I guess the assumption is that with BUIS you are engaging targets at a closer range, therefore your Min & Max PBR may shift?



I realize there are many unique factors that may come in to play, but try and give advice if you can. And, I would love to go out there and shoot and see what happens, but that just isn’t feasible right now. I need to have a calculated approximation of expected results before I get to the range. I appreciate your patience with an uninformed shooter trying to get started in the right direction for better marksmanship.


-michaelmew

MM:

1. You are correct.

2. Zero your blaster at 100 yards. I say 100 yards, not because it is the only way, but rather 100 yards is a distance where you can be sure your bullet has crossed your line of sight. Then you can look up the elevation from 100 that you need to get to your zero range. Given a 100 yard zero, if you want to get to 250 yards point of aim / impact, you will end up coming up about three minutes. And it probably won't matter in practical terms what load you are using. Note, however that your initial zero at 100 yards may well be different between different loads. Their come up to about 500 yards will be very close but initially they may zero differently. So, do not assume a hit if you change ammo and don't adjust your zero for the different ammo.

3. Depends on your conditions. One thing about a BUIS is that they are very crude devices and given the target somewhat blends into the background, your error becomes immense. Zero the BUIS for 200 yards and it will be fine on big targets with big kill zones.

LR1955

[michaelmew]

Both you guys suggested zeroing at 100 yards. What is the logic for that? I'm not sure how to phrase this and I probably do not fully understand everything that is happening, but doesn't that put the bullet flight path only crossing POA at 1 point (100 yards); as opposed to a 25 or 50 yard zero, in which case the bullet flight path would cross in 2 places. In my mind 2 places is better than one? Am I wrong?

LR, if I understand you correctly, you are saying to see where I am on paper at 100 yards and calculate to have a zero of 250 yards. So for example, I should shoot +8.16" high at 100 in order to shoot POA=POI at 250 (123gr A Max, 2540fps). Is that correct?

I'm not embarassed to admit that I do not completely understand MOA and Mils and clicks and what not, but I do understand inches very well. Probably has something to do with my occupation.

So, now I am going to crunch some numbers and see where that gets me virtually first. Then, I can shoot and see where that actually gets me. Does anyone have a good chart format they would like to share? I'd like to take from all your experiences as I put mine together.


Thanks,

-mm

[stokesrj]

If you zero at 100 yards with one of the loads you mentioned the MK262 as an example here is the trajectory you would have

Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(yd) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
0 -2.5 *** 0.0 *** 2759.2 2.471 1166.3 0.000 0.0 ***
10 -2.0 -19.4 0.0 0.1 2731.6 2.447 1143.0 0.011 1.9 18.4
20 -1.6 -7.7 0.0 0.2 2704.1 2.422 1120.1 0.022 3.9 18.5
30 -1.2 -3.9 0.1 0.3 2676.7 2.398 1097.5 0.033 5.8 18.6
40 -0.9 -2.1 0.2 0.4 2649.5 2.373 1075.3 0.044 7.8 18.6
50 -0.6 -1.2 0.2 0.5 2622.5 2.349 1053.5 0.056 9.8 18.7
60 -0.4 -0.6 0.4 0.6 2595.5 2.325 1032.0 0.067 11.8 18.8
70 -0.2 -0.3 0.5 0.7 2568.8 2.301 1010.8 0.079 13.9 18.9
80 -0.1 -0.1 0.6 0.8 2542.2 2.277 990.0 0.091 15.9 19.0
90 -0.0 -0.0 0.8 0.9 2515.7 2.253 969.5 0.102 18.0 19.1
100 -0.0 -0.0 1.0 1.0 2489.4 2.230 949.3 0.114 20.1 19.2
110 -0.0 -0.0 1.2 1.1 2463.2 2.206 929.5 0.127 22.3 19.3
120 -0.1 -0.1 1.5 1.2 2437.2 2.183 909.9 0.139 24.4 19.4
130 -0.3 -0.2 1.7 1.3 2411.4 2.160 890.7 0.151 26.6 19.5
140 -0.5 -0.3 2.0 1.4 2385.7 2.137 871.9 0.164 28.8 19.7
150 -0.8 -0.5 2.3 1.5 2360.2 2.114 853.3 0.176 31.0 19.8
160 -1.1 -0.7 2.7 1.6 2334.8 2.091 835.0 0.189 33.3 19.9
170 -1.5 -0.9 3.0 1.7 2309.6 2.069 817.1 0.202 35.6 20.0
180 -2.0 -1.1 3.4 1.8 2284.5 2.046 799.5 0.215 37.9 20.1
190 -2.5 -1.3 3.8 1.9 2259.6 2.024 782.1 0.228 40.2 20.2
200 -3.1 -1.5 4.3 2.0 2234.9 2.002 765.1 0.242 42.5 20.3

If you zero for 200 yards this is what your trajectory would look like

Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(yd) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
0 -2.5 *** 0.0 *** 2759.2 2.471 1166.3 0.000 0.0 ***
10 -1.9 -17.9 0.0 0.1 2731.6 2.447 1143.0 0.011 1.9 18.4
20 -1.3 -6.2 0.0 0.2 2704.1 2.422 1120.1 0.022 3.9 18.5
30 -0.8 -2.4 0.1 0.3 2676.7 2.398 1097.5 0.033 5.8 18.6
40 -0.3 -0.6 0.2 0.4 2649.5 2.373 1075.3 0.044 7.8 18.6
50 0.2 0.3 0.2 0.5 2622.5 2.349 1053.5 0.056 9.8 18.7
60 0.6 0.9 0.4 0.6 2595.5 2.325 1032.0 0.067 11.8 18.8
70 0.9 1.2 0.5 0.7 2568.8 2.301 1010.8 0.079 13.9 18.9
80 1.2 1.4 0.6 0.8 2542.2 2.277 990.0 0.091 15.9 19.0
90 1.4 1.5 0.8 0.9 2515.7 2.253 969.5 0.102 18.0 19.1
100 1.6 1.5 1.0 1.0 2489.4 2.230 949.3 0.114 20.1 19.2
110 1.7 1.5 1.2 1.1 2463.2 2.206 929.5 0.127 22.3 19.3
120 1.7 1.4 1.5 1.2 2437.2 2.183 909.9 0.139 24.4 19.4
130 1.7 1.3 1.7 1.3 2411.4 2.160 890.7 0.151 26.6 19.5
140 1.7 1.1 2.0 1.4 2385.7 2.137 871.9 0.164 28.8 19.7
150 1.6 1.0 2.3 1.5 2360.2 2.114 853.3 0.176 31.0 19.8
160 1.4 0.8 2.7 1.6 2334.8 2.091 835.0 0.189 33.3 19.9
170 1.1 0.6 3.0 1.7 2309.6 2.069 817.1 0.202 35.6 20.0
180 0.8 0.4 3.4 1.8 2284.5 2.046 799.5 0.215 37.9 20.1
190 0.4 0.2 3.8 1.9 2259.6 2.024 782.1 0.228 40.2 20.2
200 -0.0 -0.0 4.3 2.0 2234.9 2.002 765.1 0.242 42.5 20.3

You can zero at any range you want using a ballistic calculator found at

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

[LR1955]

Both you guys suggested zeroing at 100 yards. What is the logic for that? I'm not sure how to phrase this and I probably do not fully understand everything that is happening, but doesn't that put the bullet flight path only crossing POA at 1 point (100 yards); as opposed to a 25 or 50 yard zero, in which case the bullet flight path would cross in 2 places. In my mind 2 places is better than one? Am I wrong?

LR, if I understand you correctly, you are saying to see where I am on paper at 100 yards and calculate to have a zero of 250 yards. So for example, I should shoot +8.16" high at 100 in order to shoot POA=POI at 250 (123gr A Max, 2540fps). Is that correct?

I'm not embarassed to admit that I do not completely understand MOA and Mils and clicks and what not, but I do understand inches very well. Probably has something to do with my occupation.

So, now I am going to crunch some numbers and see where that gets me virtually first. Then, I can shoot and see where that actually gets me. Does anyone have a good chart format they would like to share? I'd like to take from all your experiences as I put mine together.


Thanks,

-mm

MM

In most cases with AR blasters, the bullet has crossed the line of sight on the rising branch of the trajectory somewhere between 30 and 40 yards for a zero somewhere between 250 and 300 yards. So yes, you can use fifty yards but it won't be as accurate as 100 yards, which isn't as accurate as 250 yards. The closer you zero to what ever range you want a zero, the more accurate the zero. If you decide to try and get a long range zero at such a short distance that the bullet hasn't yet crossed your line of sight, you will need an offset on the target or will have to adjust elevation after you have zeroed. They are not accurate zero's. Zero it at 100 and look up the come up between 100 and 250, then put that on your sights. I will bet you that given a starting zero of 100 yards, your AMAX load will need about 3 minutes of elevation to hit (theoretically) point of aim / impact at 250 yards. This means shots that will be about 3 inches high at 100 yards.

If you shoot groups eight inches high at 100 yards (8 minutes), you have somewhere around a 450 yard zero.

LR1955

[michaelmew]

MM

...I will bet you that given a starting zero of 100 yards, your AMAX load will need about 3 minutes of elevation to hit (theoretically) point of aim / impact at 250 yards. This means shots that will be about 3 inches high at 100 yards.

If you shoot groups eight inches high at 100 yards (8 minutes), you have somewhere around a 450 yard zero.

LR1955

I guess I need to go back and double check my values in the ballistic calculator I used. I think it was the simple trajectory from JBM, but I will re-compute. Thanks Bob & LR

-mm

[JASmith]

We are all making assumptions about the kind of shooting you plan to do. For example,

• if you plan to do only casual plinking and varmint shooting, you will be fine out to about 150 yards. Beyond that range you will need to adjust your aim point to break a clay pigeon.
• If you want to hunt deer and plan to keep your shots under about 275 yards, then you might want to "zero" with the POI 2.0-2.25 inches high at 100 yards. That would let you aim for the top of the heart and not have to adjust the aim point until the range is past 250-275 yards.
• If you plan to shoot beyond 300 up to 400-500 yards, you will need to decide how to accurately estimate range and use hold over or adjust your tactical scope settings. There is a way to visualize the holdover at http://shootersnotes.com/calculator/...ight-pictures/
• If you are considering taking more than casual plinking shots beyond about 450 yards with the Grendel, the 100 yard zero works fine because you will likely be using a turreted scope and making adjustments in the field. You will also want to seek some formal training. LR1955, Bwaites, LRRPF52 and a host of others on this forum have had that training and routinely conduct classes. They should be able to recommend someone near you or if you are lucky you are close enough to work with one of them directly.