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Scope mounting ?

Discussion in 'Long Gun Accessories and Optics' started by joneb, Aug 15, 2018.

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  1. joneb

    joneb Member

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    Does anyone mount their scope to cancel out spin drift?
    At 1000 yds with a RH twist the bullet will hit to the right of the target around 9" with no wind effect. By clocking the scope to the left or counter clockwise will cancel out spin drift and coriolis acceleration.
    Here is an short article on the subject, http://www.appliedballisticsllc.com/Articles/ABDOC108_GyroscopicAndCoriolis.pdf
     
  2. bearcreek

    bearcreek Member

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    Seems like it'd be more hassle having to work backwards at shorter ranges than it'd be worth. Unless, I suppose, a person was only going to shoot at 800+, which seems unlikely.
     
  3. Varminterror

    Varminterror Member

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    @joneb - have you ever done any 1,000yrd shooting?
     
  4. joneb

    joneb Member

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    Not yet, but I plan to.
    With a Howa 1500 lighting .308 Win I can get a 5-6" at 500 yds with 3 shots then I need to let the barrel cool down.
    I'm thinking of a new rifle and optic better suited for longer ranges, and was curious about setting up the scope.
     
  5. Varminterror

    Varminterror Member

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    Most guys in precision matches I have spoken with don’t have spin drift turned on in their calculators, more common in ELR games. I don’t correct for spin drift, and have no problem making impacts at 1,000+.

    The difference for me, for example, shooting my 6 Dasher is supposed to be just under 7” at 1,000yrds (StrelokPro). That’s equivalent to estimating the net wind to within 0.8mph. Even if I do choose to dial for spin drift, it’s only 2 clicks on my scope. Say I’m shooting a full size IPSC at 1,000yrds, that’s an 18”x30” target - aka 5 clicks wide. I will have dialed 85 clicks of elevation (8.54mil), estimated my wind call to the nearest full mile per hour. The spin drift would be a mere 2 clicks on a calm shot at 1,000. But only a bit less than 1 click at 500yrds. Do you want to have it optimized for 1,000 and off at 500? What about shooting at 1400? 300? How about you just develop real world DOPE (Data On Previous Engagements) to know where your rifle prints?

    If I send a shot dead center, planning on zero wind at 1,000, I still hit the IPSC target even if I don’t account for spin drift. What I can’t know is whether I have zero wind or .75mph wind across the field - so if I hit right side on a zero wind called shot, I don’t call it spin drift, I call it a 1 Mile wind (mph) and hold left side for follow up shots - which at 1,000 is unreliable enough for me to simply ignore and keep holding Center, because shifting my POA is more apt to slip one off of the side of the plate than spin drift or a 1mile wind. If I’m holding 1moa at 1,000 with a .5moa at 100yrd rifle, I’m doing phenomenally - that’s a 10” group, however. So do your first shot hit 7” to the right - are you sure it was 7” of spin drift, or was it an inch of off-Center hold, plus 5” of group radius, and an inch of poor trigger finger placement? Maybe it was a 1 mph wind to the left, 3” of bad trigger technique to the right (.3moa), 4” of bad POA at the shot to the right (less than two widths of your crosshair), minus 7” of spin drift? Or maybe you have a very slight cant in your reticle such the further you dial, the more you drift? Or maybe your scope takes a little windage drift with elevation because of their errector design (see below for machining concerns). Or maybe you just need to shoot more and figure out where your rifle really hits before making a correction on ONE shot - the hit was a hit, and you don’t have any real information from that one shot to say it was wind or spin or any other one thing. You’re already shooting >1moa at 500yrds, you’ll likely be 1.5-2moa at 1,000. Spin drift is the least of your concerns for today.

    Now, let’s set that aside for a second... consider your optic mount and the precision in machining equipment... Let’s say your scope rings are 4” apart. So you want to accommodate those 2 clicks. 2 clicks at .1mil/click = .2mil correction built into your scope mount. Ok... so 1 mil is 3.6” at 100yrds, .1 mil then is .36” at 100yrds, and .2mil is .72” at 100yrds. .72” / (100yrds * 36”/yrds) = 0.0002” correction per inch of mount length. So if you have 4” between rings, that’s 0.0008” correction. First: do you believe you can mechanically correct your mount by 8 ten-thousandths? Secondly: do you think your rail, rings, receiver, and receiver holes are precisely machined to have started out within 0.0008” to begin with?

    Use a quality rifle, use quality rings, load quality ammo, and go shoot. Connecting on steel at 1,000yrds really isn’t that hard - hell, if I can do it, anybody can.
     
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  6. cdb1

    cdb1 Member

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    No.
     
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  7. joneb

    joneb Member

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    I live east of the cascade range in Oregon, shots on dear and antelope are often out of range in excess of 600 yds. I'm looking for a rifle, cartridge and optic that would allow for a clean kill at this range. I feel comfortable with the Howa .308 Win. at 400 yds, But I would like to stretch this out farther with a 6.5 CM and a reticle I can range with, the scope on my Howa is a duplex reticle.
    So I'm looking for the best first shot, I thought clocking the reticle ever so slightly might cancel out a few variables.
    I forgot to mention we like to dispatch coyotes out here, and a these can be had at long range.
     
    Last edited: Aug 16, 2018
  8. Varminterror

    Varminterror Member

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    There’s no variable - as long as your rifle still shoots the same velocity, your DOPE will remain true. Nobody should be shooting targets at any range they haven’t actually fired at the range, so you’ll have DOPE. Download StrelokPro or AB Mobile and learn to shoot long range, get your group size down, and learn how to adjust for density altitude and how to read the wind. All of that will be necessary for throwing bullets at live game past 600.

    Personally, for killing deer past 600, I don’t care for the 308win or the 6.5 creed. They’ll do it, but if you’re buying a rifle just for this intention, I’d look for something more, preferably something which starts with 7 and ends with Rem Mag.
     
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  9. climbnjump

    climbnjump Member

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    That is your answer.

    What you are calculating in advance on a firearm you haven't yet fired may not be what you end up with anyway. There are theoretical calculations and then there is the real world. Mount the scope normally and shoot to see what you get.
     
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  10. joneb

    joneb Member

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    I realize this, but couldn't spin drift be compensated for with a slight clocking to the scope? and wouldn't this correct for spin drift at any distance?
     
  11. climbnjump

    climbnjump Member

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    At any distance? No. It isn't linear, it's a curve. The actual calculations are pretty complex. Bryan Litz (whose article you referenced) has come up with a simplified formula for it that can be found in his book, "Applied Ballistics for Long-Range Shooting". But the amount of drift will depend on the bullet you use, barrel twist rate and the velocity - or more accurately the time of flight. So if you went to all of the trouble to calculate how to "clock" your scope and then switched loads, you'd have to go through it all over again.

    But the real problem is, how are you ever going to verify that you have done it correctly? You will need to have loads with very low standard deviation, a perfectly level and stable - probably machine type rest - for the rifle, a rifle that can shoot less than 1/4 MOA groups, a scope that tracks perfectly and then perfect no wind conditions at your chosen 1000 yard test range. It's a pretty tough nut to crack.
     
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  12. Varminterror

    Varminterror Member

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    No - because the spin drift is not constant. If you build in 900yrds of drift, you’ll be over-compensated at 500, and under at 1200. So your DOPE will be what matters, still.

    If you really want to waste your time, buy Burris signature XTR rings and tilt your inserts just a tad. You’ll be giving up your option to add angle to the rings to help you shoot long range, but you’ll get what you wanted. Absolute waste of time, energy, and good gear, because, as I’ve stated no less than twice above, the offset will only apply at ONE range, and be over or under compensated everywhere else.
     
    Last edited: Aug 17, 2018
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  13. LoonWulf
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    LoonWulf Contributing Member

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    Yes I absolutely mount my scopes with the reticles canted....
    It gives me something to blame when I miss.

    As others have said
    Dope
    DOPE
    !!!DOPE, And learn to read the wind well. Wind will be far more of an issue, than anything else.
     
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  14. taliv

    taliv Moderator

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    seems like you could put it in a fixture, aimed at a plumb line, and just look at it on paper. dial your 1000 yard elevation and see how far the crosshair sits relative to the plumb line. adjust til it matches what the calculator says.
    to your earlier point it might not match at every distance. however, it doesn't really need to match at closer ranges. so if you got it to match in the 700-900 range, it might be worth doing. a fast twist 308 will have more drift than the cheater calibers we shoot at matches. some people do need to account for it
     
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  15. Space Ghost

    Space Ghost Member

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    I've never shot at those kinds of distances, but I know that's not a cheap game. Shouldn't a scope in that price range have enough adjustability in its turrets to cover that?

    I mean 9 inches of drift isn't like 9 feet of drop. You shouldn't need the 20 mil base equivalent for azimuth. Should you?
     
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  16. climbnjump

    climbnjump Member

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    I guess I'm too dense to understand your method based on what's there. I'd like to know more if you'd care to expound.

    True, it wouldn't need to match at the closer distances. I knew of one custom builder of long range hunting rifles who claimed he cants his scopes to "zero out" the spin drift at about 800 yards. (And it's good for only the one load that he develops for that particular rifle.) But he had years of experience in the business. The OP doesn't have the equipment, time or expertise to pull this off or he wouldn't have asked the question in the first place. (Which isn't to say that he COULDN'T have those things if he wanted to put in the time, money and effort.)

    And as you say, most people shooting long distance probably want to account for spin drift. I know I try to anyway... But I don't put the adjustment in the mounting of my scope, I keep the approximate numbers in my head or on a dope card and add or subtract to the wind call. For me, I start to compensate for it at 600 yards. Shorter than that, it's just lost in the wind. (And quite frankly, that's also mostly the case beyond that as well as the compensation amounts to about 1 mph on the wind call - for a full value wind - and personally, I'm just not good enough at calling the wind that close over those ranges when it comes to first round hits.)
     
  17. Varminterror

    Varminterror Member

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    I bolded the last bit there - that’s what should be done.

    Canting your scope will give you a linear offset, just like offsetting your scope centerline, but the spin drift isn’t linear. Whatever cant puts you on at 900 will be off at 600 and off at 1200. Unfortunately, with an offset system, you’ll have more geometry and algebra to do to make your corrections, because every click up won’t actually raise you a tenth of a mil any more, and every hash of wind won’t actually move you a half mil for wind any more like it should. You’ll be stuck dialing both knobs to make elevation corrections, because every click up will move you right or left too. You’re making your world MUCH more complicated than it needs to be, before you’ve even been born.

    If you want to account for spin drift like the rest of us who are shooting long range, turn on spin drift in your calculator and it’ll give you the wind corrections for each respective range.
     
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  18. taliv

    taliv Moderator

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    i 100% agree with varminterror.

    but as a thought exercise, hang a tall piece of paper 100 yards away, with a vertical line at least 40" tall and make sure it's plumb.
    put your rifle in a vice so it can't move and make sure the scope is level. aim at the bottom of the line.
    now calculate your 800 yard come up and drift. lets say for an example, you need to come up 25 MOA and have 1/2 MOA of right drift.

    so roughly, to compensate for the right drift you need to hold left. at 800 yards the 1/2moa is 4" but at 100 it's only 1/2" so draw another line on your paper 1/2" left of the center line.

    now, dial up 25 MOA on your elevation knob. if your scope is tracking properly, your crosshair should still be covering the center line, just 25" higher up the paper.

    at this point, you could rotate the scope left til your crosshair is on the 1/2" left line.

    you could do this at any other distance of course and just change the math.
     
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  19. Varminterror

    Varminterror Member

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    Shot a match this wknd out to 955yrds. Spin drift compensation turned off in StrelokPro, impact on every distance of target. 6 creed running super slow, 105 Hybrid at 2853fps. DA varied 2500-3500 throughout the two days. Spin drift does happen, but it’s HIGHLY over-rated by new shooters. Leveling your rifle, having your crosshair vertical pass through your bore centerline, ammo velocity spread, reading the net wind, building solid positions, trigger discipline (breaking on target ONLY), etc all have far more impact on dispersion and require far more of the shooter to correct than spin drift ever will.

    In short - your priorities are way out of whack at your stage in the game.
     
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  20. Varminterror

    Varminterror Member

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    I revisited this thread today and the above jumped off of the page at me.

    First, and briefly - the 6.5 Creed wouldn’t be my first choice for 600+ deer killing. If you’re talking about normally 0-600yrds, with one in 5-10yrs at 800, sure. If you expect to shoot 600-800yrds every season, get something with a larger case.

    Secondly, ranging with the reticle, while an avid pastime of mine, is NOT suitable for hunting game at 600yrds. It’s simply too imprecise and inaccurate. If you’re trying to hit a deer, sure. If you’re trying to hit the HEART of a deer, no way. That’s not to say we don’t need graduated reticle for long range shooting, but we shouldn’t be using them in place of a laser rangefinder for long range hunting. You’ll only use the reticle stadia for holding over range and wind, preferably just wind, and dialing the range/elevation correction.
     
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