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13 yard POI question

Discussion in 'Uncategorized Threads' started by handlepuller, Mar 1, 2008.

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  1. handlepuller

    handlepuller Well-Known Member

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    So I patterned my gun at 13 yards today and it looks like it hit about six inches high.

    Unfortunately I forgot my full choke tube so I was just looking for approximately the center of the pattern with my 16 yard choke.

    Anyway, I guess this would be about 18 inches high at 40 yards so would I say my gun shoots 18/30 = 60% high?

    It was nice to confirm that the windage seemed pretty much right on. I expected it to hit lower than that honestly because I really shoot the bottom off a lot of targets and tend to have problems with really high ones.

    Neil?
     
  2. Jim101

    Jim101 Active Member

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    It won't be 18 inches high at 40 Yds. You have to account for about 3-4 inches of shot drop.





    Jim
     
  3. Neil Winston

    Neil Winston Well-Known Member

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    I depends on whether it's a over single or an unsingle (or high-rib equivalent), John, but Jim's about right. Those who talk in percent (I don't) would call it 100% high.

    Neil
     
  4. handlepuller

    handlepuller Well-Known Member

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    It's a top single Neil.

    Would that mean it will be relatively higher than an unsingle? Would you consider this a pretty high-shooting gun?

    I shot from the kneeling position, without a rest.
     
  5. phirel

    phirel TS Member

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    Handlepuller- Shoot several tests and use a rest. It is too easy to pull a shot off the target without a rest. Eighteen inches high (+-100%) is high but many shooters shoot guns that have a POI similar to this.

    Multiplying the number of inches high at 13 yards by 3 is not a bad estimate of the POI. A bit more precise estimate would by the inches high at 13 yards times three, minus 3 inches (fall due to gravity) plus three times the distance from the center of the bore to the front sight.

    Pat Ireland
     
  6. grnberetcj

    grnberetcj Active Member

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    I have to ask:

    As most of us break 16 yd. targets at approx. 32 - 35 yds. from where we stand, wouldn't it be more adviseable to pattern the gun at that distance instead of 32 yds.??

    I have a 682 Gold E Top Single and with the comb all the way down, it shoots a whole pattern above the point of aim mark. In my thinking, that would be 100% high.

    Curt
     
  7. Neil Winston

    Neil Winston Well-Known Member

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    Sure, Curt, about 30 yards works fine, though at 40 finding the pattern center (for sure) is a little iffy. Shooting closer encourages more shots; you can easily shoot a dozen or two at paper up close and thus be pretty sure, while a couple at 30 (using the same amount of paper) is less certain, if only because there's less data.

    A second advantage of shooting close is that you have to abandon the 60/40 or 80/20 kind of thinking and revert to "an inch" or "three inches." Look for example at your "it shoots a whole pattern above the point of aim mark. In my thinking, that would be 100% high." If all the pallets are there, it may shoot 15 inches high, it may shoot 30 inches high, all we know is that all are high. If you said "the center of the pattern appears to be 18 inches above the point of aim" then we'd know a lot more, specifically where the gun (probably, since it depends on your care in doing this) shoots.

    Neil
     
  8. zzt

    zzt Well-Known Member

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    Gentlemen, shot drop is automatically factored in by the process, so you do not have to add it in again.

    When you shoot for POI at distances between 13 and 16 yards you are referencing your POI against a zero at that yardage. For instance, with an unsingle barrel 1.5" below LOS and a 1200fps load, zeroing the gun at 15 yards (actually 6" less if you want to be precise) gives you the flattest trajectory. The shot will cross the LOS at 15 yards and recross at 45 yards, never having risen more than .66" between those points. That's essentially dead flat at any distance a trap shooter cares about. A top single varies so little it doesn't matter.

    Since the trajectory already includes shot drop, adding it in again produces incorrect results. Since the trajectory is flat at ranges we care about, you need only measure. No compensation of any kind is required, unless you are worried about the .66" above LOS at singles distances.
     
  9. mad frank

    mad frank TS Member

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    Thanks ZZT, you just answered a question That I have been mulling over for quite some time now.
    Frank M
     
  10. Neil Winston

    Neil Winston Well-Known Member

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    Could you go through the math on that for us, zzt?

    Neil
     
  11. zzt

    zzt Well-Known Member

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    See the thread above (recrossing distances were rounded).

    Since I happen to have the info for my 2000RS barrel handy, here is the info for that.

    For 7 1/2 shot @ 1200fps, the flattest trajectory requires a 20 yard zero.

    The center of the barrel is 1.875" (1 7/8") below the line of sight, so the shot starts out that low, strikes the POA at 20 yards, rises to .36" at 35 yards, recrosses LOS at 40 yds. 2'6" and drops to .3" low at 45 yards.

    To achieve the same trajectory shooting at 13yards I would have to print .5" low. At 15 yards POI would be .32" low.

    BTW, to expand on the folly of "deducting" for shot drop, bear in mind that you are approximating POI when shooting at short yardage. At anything above a 50/50 shooter, your approximation will be low.

    Example: same barrel and loads as above: if your POI is 1" high at 15 yards and you wanted to extrapolate to 40 yards, you would multiply by 2.6667 (40/15=2.6667) and say your POI was 2.6667" high at 40 yards. In actuality, your POI is 3.52" high at 40 yards. As you shoot higher and higher POIs, the difference decreases slightly, but the "error" is still there. For a POI of 7" high at 15 yards, the actual POI at 40 is 19.42" high, not the 18.67 you would expect from doing the math.

    So if you "do the math" and then subtract 3-4" for "shot drop" you have an even larger error. 4-5" difference in POI at 40 yards is a lot.
     
  12. Hap MecTweaks

    Hap MecTweaks Well-Known Member

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    ZZT, subtracting shot fall again is something I've said for years was wrong to do also.

    I'm sure a lot of you guys recall some telling Joe Kuhn he'd never be able to break a 25 with his extremely high sighted gun? I think 99 was his best score with the dot being 5 or 6 inches above the centerline of the shotguns bore! The important part was the dot regulated to the bore centerline to hit where he broke targets, not at close range.

    Shot fall is measured from a perfectly level barrel and from the centerline of the bore. At 40 yards it falls 3-4 inches, same as a rifled slug will with same weight and velocity. Regulated sights ribs/beads or scopes make subtracting that drop measurement again, null and void, at 40 yards as ZZT said above. Hap
     
  13. grnberetcj

    grnberetcj Active Member

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    Neil & zzt....

    Thanks for the info. Now as I'm somewhat of a slow learner, if I pattern a gun at the recommended 13 yd. distance and the center of the pattern is 4 inches higher than the point of aim, am I to multiply the 4 inches by 3 to ascertain the most correct POI?

    Thanks,

    Curt
     
  14. phirel

    phirel TS Member

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    Curt- Until you get a possibly better answer from someone much smarter than I am, I will give you your answer using my simplified system.

    4 inches high at 13 yards = 12 inches high at 40 yards less three inches (gravity) plus three times the distance from the center of your bore to the front sight used to aim. If that distance is one inch, then I would conclude your gun shoots 12 inches high at 40 yards. Some would call this a 90/10 pattern and that is not at all bad.

    Hap- It is true that gravity results in a shot fall at 13 yards, but this rate of fall is not linear, it is exponential.

    Pat Ireland
     
  15. grnberetcj

    grnberetcj Active Member

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    Pat,

    Thanks! I like simple and that equation appears simple enough for me.

    I'll have to try the 13 yd. system and check the difference in my 32 yd. trials. 'sides, Greg Hissem did some stock work for me and I'll play around with that also.

    Shoot well in Florida, I can't make it this year, too many irons in the fire.

    Regards,

    Curt
     
  16. zzt

    zzt Well-Known Member

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    Curt, the complexities of Pat's "simple" formula are not necessary. As I've already explained, shot drop and the distance between bore centerline and Line Of Sight are automatically factored in when you "zero" your shotgun at a specific range. The trajectory which results is governed by the laws of physics and requires no compensation.

    Curt, the trajectory you get with your Gold E top single with a 13 yard zero is different than what I would get with my unsingle. Yours will be lower. As it happens, with your barrel, and the load I mentioned above, your very best approximation can be obtained by simply measuring your POI @ 13 yards and multiplying by 3 for POI @ 40 yards. That's 12" @ 40 yards and compares very well with the 11.67" high your pattern will actually be (because of your flatter trajectory).

    So just multiply by 3 and don't worry about the fractions. Same for all other low rib top singles. If the centerline of your barrel is 1" or less below the rib, shoot at 13 yards, multiply by 3 and be done with it. If you shoot an unsingle with a rib higher than an MX-15 you are better off zeroing at 15 to 20 yards. That is also true of O/U barrels. If you shoot for POI at 13 yards and both barrels shoot to the same height, the top barrel will print lower than the bottom at 40 yards.

    I find 20 yards to be the sweet spot for zeroing, checking barrel convergence and the like, from a bench. It cures a lot of ills. However, I still do quick checks at while standing at 13 or 15, just because it is easier.
     
  17. Neil Winston

    Neil Winston Well-Known Member

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    zzt, I'm without my PC and so my Lowry program so I can't get into a detailed analysis. However, if any others are as doubtful of your numbers as I am, they can mouse over to the link above which you provided where they will, I think, quickly see a big problem.

    I should be home tomorrow if the wind here in Missouri drops enough to drive my Tioga in a single lane.

    Neil
     
  18. Hap MecTweaks

    Hap MecTweaks Well-Known Member

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    A fired object propelled out of a barrel begins to drop as soon as it leaves the confines of the barrel. At 13 yards, that drop figure is so minuscule it's hardly worth mentioning. If you apply the same thinking to a slug at the same weight and velocity, should it also drop another 3 to 4 inches at 40 yards? One ounce is one ounce if speed is the same? Consider two rifle bullets of the same weight. One fired at 1000 FPS and the other fired at 4000 FPS, which hits the ground first? Both will hit the earth at the same time, the fastest one, much farther out if elevation degree is the same. Both rifles and shotgun loads work from bore center lines, the elevation degree of that line determines POI for both. Hap
     
  19. zzt

    zzt Well-Known Member

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    Care to say what the big problem is Neil?
     
  20. Jim101

    Jim101 Active Member

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    Hap your right as far as you go, But the determining factor is the projectile time of flight to the target. Ballistic coefficient is a critical factor in time of flight to the target.




    Jim
     
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