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Practical Consequences of Ignoring Rib Height

Discussion in 'Shooting Related Threads' started by zzt, Dec 20, 2009.

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  1. zzt

    zzt Well-Known Member

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    In the thread entitled ”Is the effect cant greater w/ high ribs?” I stated I thought the conclusions drawn misleading, and gave my reasons why. The statement you only had to worry about POI over POA is very misleading. POI over POA where? At any distance? At one distance? It appears the answer was one distance, since 17' and 40 yards were given as examples.

    When I think about comparing different rib heights, I visualize the problem as one rib with two separate barrels hung below it at different heights.. Why? The LOS is common to both guns and is the rib. The barrels are separate because they may be articulated differently. That is, they must be angled differently if they are ever to strike the same POI. It is the angles of the bores (actually the axes of the centerlines of the bores) that are important. Where they coincide is critical. I used the terms zero and zeroed in my replies in the referenced thread. I believe that term can be misleading, so I’ll not use it here.

    Here are two examples. One, IMO, you can safely ignore rib height as long as you are satisfied with accuracy to within 2”. The second, you should not ignore rib height.

    I think it safe to say most ATA trap shooters shoot flat or higher shooting guns. Practically speaking, that means the shot trajectories from guns with different rib heights will converge or cross somewhere within normal trap shooting ranges. The point at which the trajectories cross is important when deciding when rib height matters.

    40 yards is a “standard” distance for measuring or calculating many things trap. So let’s use it for example one, even though it minimizes the effect rib height has on POI. For any two guns with different rib heights set to shoot to the same POI @ 40 yards, the shot trajectories converge at 40 yards, regardless of the rib height or POI chosen. To remain consistent with the examples on previous threads, let’s use 10” high @ 40 for the POI, and 1” and 3” rib heights. Just remember, we could have used 5” high, or 12” high, or 24” high. The important point is both barrels are angled to hit the exact same spot at 40 yards. The POI over POA distance is irrelevant.

    At 40 yards, the shots converge. Each shot traveled a different path to get there, because the high ribbed barrel started out 2” lower than the other barrel. However, at this one point at 40 yards, and only that one point, the centers of the pattern are superimposed. So anything you do to both guns will have the same effect on POI. This is self-evident. Move your POA 2” to the left; both POIs move 2” to the left. Cant each gun 10 degrees counterclockwise (keeping the same POA) and both POIs move a little bit less than 2.5” left and about .2” lower. You can decide whether that is enough to make you fix your canting problem.

    When the shot trajectories from each barrel coincide, or cross, (shoot to the same POI) you don’t have to worry about rib height and any effect it has on POI and/or canting.

    Now each shot traveled a different path to get to this coincident point. Different because one barrel was lower than the other when the shots were fired. It is self-evident the POIs cannot be coincident at any distance other than 40 yards in this example. The shot from the high rib gun started out 2” lower and it took 40 yards for it to converge with the shot from the low ribbed gun. So for any distance farther than 40 yards, or closer than 40 yards, the POI of the high rib gun will be different than that of the low rib gun. Do we care? In this instance, coincident POIs at 40 yards, probably not. The high rib gun will print 1” lower than the low rib gun at 20 yards or ½” lower at 30 yards. If you set your two guns up this way, to the same POI at 40 yards, I don’t think you have to worry about rib heights. For the distances you will be breaking targets, it is close enough.

    Now let’s look at example 2. Two shooters decide to retire their 870s for new guns. They want to do things “right” so they read the accumulated wisdom on these threads. They discover they should find out where there old guns shot and make the new ones shoot there. Makes sense. They discover they should do this at 13 yards. They also read they really don’t have to worry about rib height, especially because it has no effect when canting. They are not sure what that means, but they are told they don’t have to worry about it, so they don’t. They think wow. I just saved $165 because I don’t have to get an adjustable butt plate to fix my cant. This is great.

    So they set up 13 yards away from a big piece of paper and fire away at various aiming points. They have the foresight to use their 870s first. Low and behold, they find each of their 870s shoot 3” above POA at 13 yards. So let’s make the new guns shoot there as well. Bang, bang, bang, bang, bang. Two boxes of shells and an hour later and both have their new guns set up so they shoot straight right and left, and 3” high at 13 yards. They head to the trap line. Both feel 25 straights coming on. They’re accomplished 27 yard handicap shooters and set up there to have at it.

    Shooter 1 is doing pretty well, but something is not quite right. He is hitting targets well, but gets one or two more chippers than he is used to. Shooter 2 is having problems.

    Well, their 870s were low ribbed, only ½” between bead and centerline of the bore. Shooter 1 bought a “high” ribbed top single where the bore was 1” below the bead. Shooter 2 bought a higher ribbed top single where the bore was 3” below the bead. When at the patterning board, both shot trajectories converge until at 13 yards from the muzzle they intersect/cross/coincide. Thereafter, they diverge. This is exactly what happened in example one, except here they intersect at 13 yards instead of 40.

    Gravity affects all these shots equally and in the same direction so we’ll discount it here so we can work with straight lines. The magnitude of the changes will be accurate, but the POIs in real life will be lower because the shot will fall more at 40 yards than it did at 13.

    The shot from the 870s started out ½” below the bead (rib if you prefer). It printed 3” high at 13 yards. The shot rose 3.5” in those 13 yards, so it will rise to 10.5” above POA at 40 yards (6.6” if you insist on including gravity). In Shooter 1’s new gun, the shot starts out 1” below the bead, so it must rise 4” in 13 yards to print 3” high. At 40 yards it to rises 12” above POA (8.1” with gravity). His POI is only 1.5” higher than where his 870 shot. That’s not enough to notice most of the time.

    Shooter two is beside himself. He did everything “right”. Plus, he got the same results at the pattern board as Shooter 1 did. Could it be his occasional 10 degree cant. No, he read here it doesn’t generally matter and rib height doesn’t count. Besides, his buddy is from Missouri and had to see for himself. They both canted their guns the same amount and low and behold, the POIs shifted the same amount at 13 yards. TS.com was right. Rib height didn’t matter. Besides, the offset wasn’t much, so he forgot about it. He did everything the same as Shooter 1. He is just as good a shot, so it has to be the gun. It’s the choke! It has to be bad! That explains his mystery misses. What is going on?

    The shot from Shooter 2’s unsingle started out 3” below the bead, and had to rise 6” in 13 yards to print 3” high. It continues to rise and reaches 18” above POA at 40 yards (14.1” with gravity). That’s a whopping 7.5” different than he was used to with his 870, more than enough to account for a lot of his chips and mystery misses. But wait. He occasionally forgets to correct his 10 degree cant. That messes him up even more because when he does cant, his shot is a little over 3” left (and 1/3” lower) than when he doesn’t. It’s no wonder he is frustrated.

    Shooter 2’s shot is about is 14” high (with gravity) and 3” left of center out where he should be breaking targets from the 27, when he cants his gun. It is 14” high and dead center when he doesn’t. He borrows Shooter 1’s gun and fires away. Now his shot is 8” high (with gravity) and a little more than 2” left when he cants. That is a 6” difference in vertical offset and a 1” difference in horizontal offset, all because of rib height.

    Now, to be fair, the majority of each offset is due to the shot not hitting the same spot out where they are breaking their birds, but some is not. As I’ve said in previous threads, the effect rib height has on canting at any distance other than where the two shot trajectories meet is small and can usually be ignored.

    Rib height in general cannot be ignored. As we see with these two shooters, doing so can cause problems. Both followed instruction found in these threads. Both thought they understood the implied meaning. Both set their guns up at 3” high at 13 yards, an identical POI over POA. But when they went to the line, the practical consequences of not considering rib height became evident. That is one of the reasons I suggest you measure your POI at half the distance you are interested in. It minimizes errors when you extrapolate to full target distance, and eliminates rib height induced surprises.

    Rib height affects canting offsets in a relatively small way. Disregarding other aspects of rib height induced changes can be consequential.
     
  2. crusha

    crusha TS Member

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    Just shoot the damn target.
     
  3. 870aa

    870aa TS Member

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    Man I have been underthinking trapshooting the whole time.
     
  4. hmb

    hmb Well-Known Member

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    In your first example, cant each gun 10 degrees counterclockwise, keeping the POA the same, the POI will move to the right and up, IMHO. HMB
     
  5. zzt

    zzt Well-Known Member

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    HMB, I think of the rib (LOS) as the axis of rotation and the barrel moves. So rotating counterclockwise moves the barrel to the right of the rib and up a little. That puts the shot left and a little low vis a vis the original shot. The same should hold if you think about moving the rib around the barrel. The rib moves left, and the barrel still ends up on the right side of the rib and a touch higher.
     
  6. Tony Fortino

    Tony Fortino Member

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    awhata wahta?
     
  7. jimrich60

    jimrich60 Member

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    Setting aside the purely mathmatical effects of gun canting due to rib height (if any, as shown by Neil) as well as the possibly small POI effect that canting shows in a machine rest, perhaps in actual target shooting, canting may have a greater effect on POI than Neils testing may indicate. I posit this as an effect of the head/face placement on the stock, which will vary with both the direction of, and amount of, cant. Considering that it only takes a relatively small amount of cast change (say by movement of an adjustable comb) for instance (even 1/8 inch has an effect on POI) then canting the gun, it seems to me, might well alter the head position, in relation to the rib alignment, enough to cause a change in POI. If that shift results in the pattern moving left or right by 4-5 inches or more, it could well cause a miss, especially on a left or right angle target.

    Jim R
     
  8. zzt

    zzt Well-Known Member

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    flincher100, sorry I missed your post earlier. Long ago i read that you needed .5 ft/lb of retained energy to reliably break a clay target. I know for a fact they break more easily than that, but have no way of knowing if less energy would give 100% results.

    Personally, I use .6 ft/lb as the minimum. Even #8 shot launched at 1145fps carries .6 out to 50 yards, so I just don't worry about it.
     
  9. zzt

    zzt Well-Known Member

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    Jim R, that is what I call "bad" canting. That's what I do on occasion, and every time I do, I miss the bird. Canting as HMB suggests keeps the beads lined up. When I catch myself canting, the beads aren't. That is definitely not good.
     
  10. Neil Winston

    Neil Winston Well-Known Member

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    zzt, the first half of your post is fine, because it's what I wrote on the earlier thread. I doubt that an inch in POI change when you shoot a target at half the usual distance would pass most shooter's standard for the "practical consequences" you promised in the thread's title but since you determine you own points of impact to 1/100 inch a change of 100 times that is probably pretty consequential in the world you inhabit.

    The second half is far more problematic, not because it's numerically wrong, but what it says about you and your failure to cite sources, essentially appropriating the ideas of others and presenting them later as your own. You do this all the time. Since I've not directly involved in this particular example of theft of intellectual property, I think I can call myself a non-involved party.

    First, I think it's fair to say you present the above as a product of your own work, since you don't credit anyone else at all.

    Let's turn the clock back to March 1, 2008 when my friend handlepuller reported some results of 13-yard POI testing asked me what I thought they meant.

    Pat Ireland and I have long shared a basic approach to POI testing and getting it right, and after a bit, answering a request for a more accurate way than the multiplication by three, he offered this:

    "Subject: 13 yard POI question?From: Pat Ireland?Email: ?Date: Sun, Mar 02, 2008 - 05:46 AM ET?Website Address:

    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"

    Now Pat and my approaches are not exactly the same, nor do we use exactly the same formula, but it's clear that two years ago he was warning people of the same problem you now alert us to, that rib height makes a difference when you check POI at 13 yards.

    Amazingly, at that time you were moved to respond with a contrary view, after a lot of intermediate stuff,

    "Subject: 13 yard POI question?From: zzt?Email: ?Date: Mon, Mar 03, 2008 - 09:12 AM ET?Website Address:

    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."

    Do you remember writing that and see why you make me so mad? You take Pat's perfectly useful formula which includes rib-height compensation, substitute your own of multiplying by three, and then today chide us for not warning people that rib height made a difference. You took Pat's idea, said it was wrong, then come back here two years later and parade it as your own. That's what I call intellectual dishonesty, a charge I've leveled at you for years now and you've never seen fit to fix.

    The thread goes on and on - so long in fact that even when I change the font to 11 it dragged on for 52 printed pages. But the heart of it is right here, in the sections I quoted.

    If you were moved to point out in this thread that rib height is a consideration at close distances, you owed the source of the idea credit. It doesn't have to be elaborate: "As I learned from Pat Ireland a couple of years ago . . . " in the introduction to that section would have done nicely and paid the debt you owe to your instructor in this. Heck, if you had done that I might have even overlooked the fact that you used to disagree.

    Neil
     
  11. zzt

    zzt Well-Known Member

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    You amaze me Neil. There is nothing inconsistent in what I wrote 2 years ago, as you claim, and what I am saying now. If you read the passage to Curt you cite, it shows that. For a low rib height of less than an inch, multiply by 3 and don't worry about it. For an unsingle with a rib taller than an MX-15, you are better shooting for POI at 15 or 20 yards. I'm not going to check that thread, because I've said similar things for years. Anyone who reads these threads knows you are the proponent of 13 yards and I like greater distances, for good reason.
     
  12. Neil Winston

    Neil Winston Well-Known Member

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    Oh hogwash, Steve. Pat gave a perfectly general and easy solution to the 13-yard problem which you did not understand because you didn't see how his gravity factor worked. So you told Curt to use yours, even though it's far more restricted. And yours will get you into trouble if you follow it and Pat's won't. Now you post a warning of an effect which you pretend is yours, but is Pat's discovery. You owed it to him, and indeed, all the people you steal ideas from, to give credit where it's due.

    Neil
     
  13. Post  2

    Post 2 TS Member

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    Call for the bird, identify it, break the bird. Post-2
     
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