Sighting a rifle scope: the theory

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  • ras_oscar

    Ultimate Member
    Apr 23, 2014
    1,666
    I have a AR10 with a Nikon M308 scope. I am pondering how to sight the scope. The only range available to me at present is max 200 yards, so that's the target distance (pun intended) I'd like to get accurate at.

    When looking at the ballistics chart here: http://www.gunnersden.com/index.htm.308winchester-ballistics.html

    I noticed that the drop at 100 yards for all ammo was zero. At 200 yards the drop varied from 2.3" to 10". I ultimately plan to load my own ammo and this table tells me to target a light projectile with a high muzzle velocity to get a flat trajectory ( Duh!!!)

    Should I zero the rifle at 100 yards and do a hold over at 200? I'd like the ability to engage targets at all distances, so I hesitate to fine tune for 200 yards, particularly since the drop numbers vary so much. I'd prefer to set the scope turrets once and leave them alone permanently, if that's feasible.
     

    ironpony

    Member
    MDS Supporter
    Jun 8, 2013
    7,187
    Davidsonville
    I have a AR10 with a Nikon M308 scope. I am pondering how to sight the scope. The only range available to me at present is max 200 yards, so that's the target distance (pun intended) I'd like to get accurate at.

    When looking at the ballistics chart here: http://www.gunnersden.com/index.htm.308winchester-ballistics.html

    I noticed that the drop at 100 yards for all ammo was zero. At 200 yards the drop varied from 2.3" to 10". I ultimately plan to load my own ammo and this table tells me to target a light projectile with a high muzzle velocity to get a flat trajectory ( Duh!!!)

    Should I zero the rifle at 100 yards and do a hold over at 200? I'd like the ability to engage targets at all distances, so I hesitate to fine tune for 200 yards, particularly since the drop numbers vary so much. I'd prefer to set the scope turrets once and leave them alone permanently, if that's feasible.

    Winner, winner sounds like you do know how to hold over from there. Others with experience may chime in soon.
     

    danb

    dont be a dumbass
    Feb 24, 2013
    22,704
    google is your friend, I am not.
    The one with a 10" drop is a low recoil load with muzzle velocity of only 2000 fps. Most 308 is in the 2600-2700 fps range (with 24" barrel). Exclude that one and its all about 3-5" range
    drop.

    The "the drop at 100 yards for all ammo was zero" because they assumed it was sighted at zero. This is a better calculator, it lets you change the zero range to 200 yds (or whatever) and see the trajectory. It also lets you adjust the speed etc.

    http://www.shooterscalculator.com/ballistic-trajectory-chart.php

    you can choose some of the 308 pre-set ones. So for example for a 168 gr bullet @2600 fps, zeroed at 200yds, the chart says it will be +2.24" at 100 yds. On my rifles which are accurate enough to zero at 200yds, I zero it at 200 (max for AGC), since +/-3" out to 250 yds is perfectly acceptable for hitting anything with almost no holdover.

    Keep in mind also, the posted muzzle velocities probably assume a 24" barrel. If you are shooting a 16" barrel, you will probably be 150 fps slower than the posted #s. https://rifleshooter.com/2014/12/30...ato-barrel-length-versus-velocity-28-to-16-5/
     

    lazarus

    Ultimate Member
    Jun 23, 2015
    13,673
    All loads I care about in .308 and .223 I zero at 200, which results in roughly 2” high at 100yds. Then 8” low at 300yds.

    Not going to take a shot at a game animal past that.

    Nothing I care about shooting with a .308 is going to miss the vitals if I am off by an inch or two on the point of impact vs point of aim.

    If it does miss the vitals, it has nothing to do with not knowing the dope.

    That said, this last year I was hunting the last day of deer season in Greenridge when a fox came out and sat at the bottom of the valley I was hunting for about an hour so I finally decided to take it out as I was pretty sure it and the dozen gibberijg squirrels were going to keep anything from coming down the deer trail. I gave it a hair cut because I DID forget it was zeroed for 200yds and the fox was only at 90-100. Aiming right between the eyes so I could keep the pelt. I should have aimed at its neck (it was sitting on its haunches).

    I litterally saw a small puff of fur in my scope and the leaves explode immediately behind its head as it ran off.
     

    Uncle Duke

    Ultimate Member
    MDS Supporter
    Feb 2, 2013
    11,662
    Not Far Enough from the City
    All loads I care about in .308 and .223 I zero at 200, which results in roughly 2” high at 100yds. Then 8” low at 300yds.

    Not going to take a shot at a game animal past that.

    Nothing I care about shooting with a .308 is going to miss the vitals if I am off by an inch or two on the point of impact vs point of aim.

    If it does miss the vitals, it has nothing to do with not knowing the dope.

    That said, this last year I was hunting the last day of deer season in Greenridge when a fox came out and sat at the bottom of the valley I was hunting for about an hour so I finally decided to take it out as I was pretty sure it and the dozen gibberijg squirrels were going to keep anything from coming down the deer trail. I gave it a hair cut because I DID forget it was zeroed for 200yds and the fox was only at 90-100. Aiming right between the eyes so I could keep the pelt. I should have aimed at its neck (it was sitting on its haunches).

    I litterally saw a small puff of fur in my scope and the leaves explode immediately behind its head as it ran off.

    Which leads to 2 important sight in considerations for OP....target size and range estimation.

    Need fox between the eyes sort of precision? A deer size kill zone? Man size?

    One thing to shoot at a consistently known distance, where zero can remain consistently zero. Beyond that, it's a lot about how close is close enough for the task at hand.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,810
    All loads I care about in .308 and .223 I zero at 200, which results in roughly 2” high at 100yds. Then 8” low at 300yds.

    Not going to take a shot at a game animal past that.

    Nothing I care about shooting with a .308 is going to miss the vitals if I am off by an inch or two on the point of impact vs point of aim.

    If it does miss the vitals, it has nothing to do with not knowing the dope.

    That said, this last year I was hunting the last day of deer season in Greenridge when a fox came out and sat at the bottom of the valley I was hunting for about an hour so I finally decided to take it out as I was pretty sure it and the dozen gibberijg squirrels were going to keep anything from coming down the deer trail. I gave it a hair cut because I DID forget it was zeroed for 200yds and the fox was only at 90-100. Aiming right between the eyes so I could keep the pelt. I should have aimed at its neck (it was sitting on its haunches).

    I litterally saw a small puff of fur in my scope and the leaves explode immediately behind its head as it ran off.

    I keep 99.9% of my hunting rifles 'zeroed' 1 inch high at 100yds. That's been my magic setting almost forever, and that includes my black powder rifle.
     

    davsco

    Ultimate Member
    Oct 21, 2010
    8,607
    Loudoun, VA
    probably should tell us what distances you want to shoot at, and what size targets you're looking to hit. is it deer at 300 yards, 12" steel at 1000 yards, what?

    also, what reticle do you have? is it a bdc (bullet drop compensator), or does it have moa or mil lines, or just a set of crosshairs?

    depending on how precise you want to be, actual bullet speed comes into play. i think you have a 20" barrel, right, most ammo makers when they put velocities on their box, use 24" if not 26" barrels, so right off the bat you cannot use the fps per the box and be super precise on where to dial or hold. if you're just shooting at deer sized animals within 300 yds, not a big deal, but smaller or farther away, your actual velocity starts to matter. ditto for a bdc reticle. it is set up for a specific bullet weight and speed, so if your actual bullet weight and speed doesn't match what they used, obviously you most likely won't be spot on using the various elevation lines.

    even if you have gone thru all the math and calculations above, you really should actually shoot your rifle at the various distances and see where you are hitting. especially if you will be hunting with it, so you don't wound a game animal because your theoretical drops were off a little.

    as an example, my 3 gun rifle has a bdc scope on it. i zeroed at 200 yds per the manufacturer's spec. so it's obviously bang on at 200. it's also bang on at 300. to hit a 400yd target i have to hold roughly halfway between the 400 and 500 yard hash marks on the scope, and same for a 500yd target, have to hold halfway between 500 and 600yd hashmarks. i have also shot it at 25, 50 and 100 yards just to have a better idea where it hits at those ranges. all that data is called dope. write it down like a cheat sheet and you will be GTG.
     

    Major03

    Ultimate Member
    How I would approach it...

    1) Figure out what the purpose of that weapons system is for...is it for long range or precision shooting or for putting meat on the table / hunting?

    2) If for long range / precision, the zero is just a point of reference from where you dial in your elevation corrections or establish your holdovers. Many shooters will chose 100 yds as their zero if that's the case, although really it could be anything you want it to be.

    2 a) Zero at the distance you want, and then reference the ballistic calculator that DanB references, record the changes by distances (yards or meters). Some like to convert that to clicks for quicker adjustment in the field.

    2 b) Confirm your adjustments in the field and apply good DOPE (data on previous engagements) to your recorded changes.

    3) If it's for hunting / defense or another similar "close enough is good enough, but it needs to be fast" purpose, then we would want to spend a little more time understanding the ballistics of the round you're shooting and what you're goals are for precision (or what the size of the vitals). Based on what you wrote, this is what you're going for. Generally, most like to have point of aim equaling point of impact. It doesn't have to be...but most will chose POI/POA to coincide. The goal is to find the distance where you can aim at your target and give or take the bullet will impact where you aim.

    3 a) Figure out how far out you want to be able to engage targets. Here in the Eastern U.S., if you're hunting, then you're probably not shooting anything beyond a couple of hundred yards. 200yds seems right on the money for your purposes. Certainly wouldn't go beyond 300.

    3 b) Look at the ballistics calculator that danb provided a link for, and plug in the numbers. It's worth doing some homework to get accurate data....a 147 gr NATO .308 surplus will have a different trajectory than a 180 gr Accubond bullet. Most manufacturers will have BC, velocity and bullet weight readily available.

    Obviously the further out your zero, the more the differences in bullet weight, ballistic coefficient, and velocity will matter. Once you start reloading, invest in a chronograph. Being able to measure the velocity of your round gives you good feedback that goes beyond data you can use to calculate an accurate trajectory.

    3 c) Plug all that good data into the calculator and make a choice as to where you'd like your zero to be...based on the drop/rise from zero and the size of the target you want to be able to hit and the yardage you want to be able to engage it at. Generally I like to have +/- 3", which gives you a 6" radius from point of aim. Adjust your zero yardage based on what gives you the most amount of flexibility...it's subjective. Some like it closer and will sacrifice having to hold over at longer ranges for being able to "hit what you're aiming at" at closer ranges, and vice versa.

    BTW, you don't have to have a 300 yd range if you want a 300 yd zero. Just figure out where the bullet should impact relative to the zero you want at the range you can shoot at...and have your point of impact hit there relative to your point of aim. So, you could have any zero you wanted and still "dial it in" at your 200 yd range.

    Basically, this is all just explaining what's in the link that mini14tac sent on Maximum Point Blank Range....or what in the military refers to as a BZO, or battle site zero / battle zero.

    Clear as mud right?
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,810
    How I would approach it...

    1) Figure out what the purpose of that weapons system is for...is it for long range or precision shooting or for putting meat on the table / hunting?...

    True. I first thought this was to be a hunting rifle, but OP hasn't really said other than wanting to "get accurate" with it.
     

    dreadpirate

    Ultimate Member
    Mar 7, 2010
    5,521
    Cuba on the Chesapeake
    I would say zero it at 25 yards and you will have a flat trajectory from 0 to 200-ish yards depending on particular ammo.

    [EDIT] FWIW; For faster bullets like the 5.56 at approx 3,000 fps, I would say zero at 50 yards. Same rationale flat shooting out to 200+ yards.
     

    Biggfoot44

    Ultimate Member
    Aug 2, 2009
    32,877
    ( Presumptions- deer sized targets ( deer chest and human chest will be comparable) , and "normal-ish distances . Head shots on groundhogs, or really long distances will be different discussions.)

    I don't totally buy into the PBR theories ( even though they provide interesting comparisons of loads and cals ) . I have my divergence because I don't place equal priority on Over and under . I prefer closer congruence of POI/POA at closer distances .

    My thinking is close-ish shots will have probability of being quicker ( to aquire, aim, fire ) . I prefer to aim and squeeze, and not have to further make allowence for hold under . Conversely for ( shots beyond the easy zone ) , will be taking comparitive more time to estimate range, think about holdover , using steady position practical for conditions and location .

    Outrider58 has the right concept . Except my default is 1.5in high at 100yds ( for rifles of this broad catagory) . People using this way of thinking will pick a default between 1 inch and 2 inches . Somewhat a matter of personal preference , but for me 1.5in is sweet spot between close precision , and useful distance capability .

    In the example of 150gr .308 , 1.5in high will cross back down to POA just short of 200yds , and allow for aiming without holdover to 225yds, or a bit more .
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    I use JMB Ballistics for my calculations. Or Ballistic AE app on my phone. They both use the same engine.

    For precision shooting, you zero at some distance, typically 100 yards. Then dial or hold over for other ranges. You use the ballistics calculator to determine the hold over at each range. You can either make a table, or just dial the range into the phone app.

    You can also use the ballistics calculator to find your actual MV. Zero at 100, shoot at 200, measure drop, and with a known bullet, they can figure the MV.

    For hunting, IMO, the best way is maximum point blank range. This is zero that gives you a point of impact a certain +/- from point of aim. So for deer, with about a 10" vital area, if the shot impact +/- 5" from the point of aim, you will still have a good shot. The ballistics calculator will give you the Zero range, and the Maximum range for that +/- 5" (or whatever range you choose).

    As most do not want to try to measure 247 years (for example) for zero, you look at the table and see it says that at 100 yards, the round will impact say 1" high, so you zero your rifle to be 1" high at 100 yards and you are good to go.
     

    Mini14tac

    Ultimate Member
    MDS Supporter
    May 14, 2013
    2,146
    North County
    I usually hunt with a 7mm Rem Mag. Being it's a pretty flat shooter for the round that I hunt with using the MPBR method puts me about 350 yards before the round will hit to low with no holdover. I have shot quite a few deer with this setup out to over 200 yards and the 7 always puts them down with just putting the cross hairs dead center in the kill zone.

    Now for my long range 6.5 CM that I have been messing with, it is zeroed @ 200 so that I can use the ballistics tables to know what the MOA dial up is at each 100 yard increment out to about 1300 where my scope runs into that last 10% of adjustment that is unreliable. I do have a 20 MOA base that allows me to use the majority of the MOA built into the scope adjustment. Now just need to find a range with enough distance that I can attempt to test this theory.
     

    deerassassin22

    Active Member
    Apr 12, 2016
    700
    Littlestown, PA
    I have a DPMS GII Recon II with a 16inch Barrel and Nikon M308 BDC and I have no issues with the scope by zeroing at 100yards. Do you have the BDC scope of just crosshairs? I have shot my M-308 out to 600 yards and the BDC is accurate with the Nikon App shooting the below load. I have also attached my drops so I can just turn the turrets if I just want to use the crosshairs without the BDC. I would zero at 100yards and adjust your drop as necessary for further engagements. I practice this way and then I return my scope to zero and utilize my BDC or Dial my dope at matches if necessary. The Nikon App will help you if you have the BDC version to figure out which magnification will be what yardages for the BDC.

    RANGE ZERO ADJ MOA
    Yards CLICKS

    100 0 0
    150 4 1
    200 10 2.5
    300 24 6
    400 46 10.2
    500 58 14.5
    600 78 19.5

    Load Data:

    168grn Hornady A-Max
    Hornaday Brass (Chamfered, Deburred, Flashole Resized, Primer Pocket Uniformed, Annealed, Neck Sized/Bump)
    Varget 42.0grns
    CCI LR Primer
    Velocity 2,448fps
     

    Biggfoot44

    Ultimate Member
    Aug 2, 2009
    32,877
    5 in is a whole lot of hold under for deer sized, considering the preponderence of shots at 100-ish or less vs 300 + .

    It's not just the theoretical trajectory , also consider allowence for aiming error, and the inherent accuracy. Unless you train yourself to default aim at the bottom edge of the chest, you will have lots of misses over the top and marginal hits unless you magic BB the spine .

    Hence my preference for closer POI at typical distances . Rather than equal above and below , more useful would be thinking in terms of +2/-6 . Aim essentially where you want to hit up to 200 or so , and at extreme range hold cross hairs level with top of back . ( Exact yardage subject to calculation and experementation with your particular load .)

    Of course, if a situation like ridge to ridge , where you would only get at shot at a specific long yardage , but that is specialized, not an all around " set & leave alone " .
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    5 in is a whole lot of hold under for deer sized, considering the preponderence of shots at 100-ish or less vs 300 + .

    It's not just the theoretical trajectory , also consider allowence for aiming error, and the inherent accuracy. Unless you train yourself to default aim at the bottom edge of the chest, you will have lots of misses over the top and marginal hits unless you magic BB the spine .

    Hence my preference for closer POI at typical distances . Rather than equal above and below , more useful would be thinking in terms of +2/-6 . Aim essentially where you want to hit up to 200 or so , and at extreme range hold cross hairs level with top of back . ( Exact yardage subject to calculation and experementation with your particular load .)

    Of course, if a situation like ridge to ridge , where you would only get at shot at a specific long yardage , but that is specialized, not an all around " set & leave alone " .

    You +2/-6 would work also.

    The idea with MPBR range, with +5/-5", is no hold over or under within the MPBR. Just aim center of vital area, assuming vital area is 10" you will always (minus shooter error) be in the vital area. You could also do a MBPR calc for an 8" vital area for a max +4/-4 of the point of aim.
     

    Biggfoot44

    Ultimate Member
    Aug 2, 2009
    32,877
    I'm not that optimistic about " just aim at center, and ignore all other variables" . 10in diameter for trajectory, plus shooter variables, and rifle variables would ( to me ) be reasonable ... For Moose. ( The large 4 legged creature, not human members of the fraternal organization.)
     

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