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Thread: Queston about the rollover prone

  1. #1
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    Queston about the rollover prone

    First, an apology if this is posted in the wrong area or if someone has already answered this, I did do a quick search on "rollover" yet came up with nothing. I am kind of new to forums and am not all that up-to-speed on finding what I am looking for, so if I missed the answer, sorry, provide the link and I will follow...

    First the story/facts, then a question below:

    Many months ago, after zeroing for point of aim/point of impact at 100, I began a rollover prone 10-shot group experiment at 100 yards. The intent was to keep the same point of aim of the shot group that was center mass for both the left and right rollover prone in order to show the impact difference when rolled over to the right and left were applied.

    When shooting with the ejection port side down, I got the expected grouping (low and right).

    However when shooting rollover prone left - ejection port up - I was shooting high and left (almost as high left as I was low right).

    Being puzzled and wondering about counter-intuitive results, I then repeated experiment, except this time I supported the rifle by sandbagging to help take out any human error/input. Yet, I got the same results, albeit with tighter grouping.

    The left, ejection port up, results made no sense to me... I kind of mulled it over, and I guess I filed it in the "talk about it later" category of my mind.

    Then a couple months ago I was attending training in Louisiana at the state police facility, and while I was shooting I had an instructor join me on the line. I watched as he ran his own test on the roll over prone (strangely enough he was conducting the same experiment for the same reasons). I was very curious to see the results he would get, especially on the left side. However I kept my mouth shut and didn't tell him my results because I didn't want to "poison the well."

    I respect this guy, he is a good shooter, and he was shooting with with a free-floating barrel, something I did not have in the original experiment. When we went down range I could see that his grouping was almost identical to mine, I watched his face as he examined his target and realized the look he had on his face must have been the same as mine when I shot the experiment. Next, as I went back to 100 to shoot more, I saw him set up sandbags and shoot the same test, now supported (smart guy). Now it was no shocker to me that he came up with the same results but with tighter groups. At that juncture I explained to him my results of months earlier were identical to his. We were both stumped.

    I brought up the subject to some fellow instructors and teammates here in Iraq (all, snipers, competition shooters, IPSC, etc..., most - if not all all - with combat trigger time), yet no one can seem to figure out what was going on.

    I have asked a fellow instructor to shoot the experiment without sharing the results of my experiments with him, and hope to hear back from him soon.

    I am baffled, this makes no sense to me. I have to think that there is something we are missing in this equation.

    My initial assumption is that there is some shooter input (shooter error) that we are not accounting for. No one is perfect, I am not, but I am an accomplished shooter, as was the instructor in Louisiana... so I am still stumped, as are all the quality shooters I am around.

    Now the question:

    Has anyone else experienced this themselves, or seen it in students? If so, what's going on with the left side prone that I am not understanding or missing? What was the input (shooter error) that caused this, and did you do to correct it? Or is there some exterior ballistic magic that I have never heard of before going on here?

    Let me be perfectly clear, I am not asking for theory alone. I am asking for someone who has seen the same phenomena (either in himself or his student), figured it out, and has an real tested and proven solution that I can try out as soon as I get back to the range.

    Preferably someone who has photo or video documentation of the problem and solution set... and crayons, I need lost of crayons with a coloring book because I am a visual learner. (Okay, just joking about the crayons, but all the documentation you can give would be much appreciated.)

    Thanks in advance for your time,

    - SB

  2. #2
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    SB,

    Havent' done your test but have another question and possible answer. What type of sighting systems did you guys use? Reason I ask is that we see things differently at different angles with regard to lighting.

    I don't have 100m here in the backyard (only 25m) to conduct the test. But I do seem to remember something about it maybe in one of my old sniper manuals, I'll look.


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    Iraq: 91,03-06,08 & 09'
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  3. #3
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    SB,

    I have also NOT done your test, but if you will bear with me for a moment I will try to explain what you are seeing.

    When you sight a rifle in to point of aim, you are compensating for TWO things, (wind notwithstanding), the two things are gravity, and longitudinal bullet movement due to the spinning bullet grabbing air and steering it to one side or the other.

    Ok, lets say that you are shooting a right hand twist barrel (lets remove gravity from the equation for now) the spinning bullet grabs the air and steers it in one direction, you compensate for it with the sights. good, then you have compensated for gravity as well, so in reality when the bullet leaves your gun it is not traveling as if it were a laser from the barrel, got that?

    Note: the bullet will always leave the barrel in the same direction, no matter how you position the gun.

    Now to make my point, If you did the exact same experiment with a PERFECTLY identical barrel, and bullet but with LEFT hand twist, your results would be the same BUT opposite.

    I hope that makes sense.
    "Why is it, when your kid has an imaginary friend people think they are cute, but if they have an imaginary enemy, they are paraniod?"

  4. #4
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    None of this can be true. I learned my physics from watching Arnold Schwarzenegger and Sylvester Stallone. None of this makes sense.

    Sarcasm aside, +1 to you Claymore. I was a poor math student. I can understand practical principles when explained where my jarhead brain can understand it. I didn't even think about gravity or rifling when it came to bullets leaving guns. I knew that bullets went up a little bit (which is why you can BZO at a shorter range) but I didn't think about the rifling twist.
    Yes, I'm a grown man that loves My Little Pony: Friendship is Magic. Got a problem?

  5. #5
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    @ Combat Diver,

    Thanks for the help, I do appreciate it.

    The first time I ran the experiment (without the sandbags) I was using an EOTech mounted on the upper receiver. The second time I ran the experiment (with the sandbags) I flipped up my irons (I co-witness my sights) in order to verify that my EOTech was not moving due to mechanical issues (I have had sights move on detachable rail systems [on the handguard rails] where the steel sleeve/spring has stripped allowing the sight to shift left and right).

    During the second set of experiments on the sandbag, I left the iron sights up throughout the experiment as a cross-reference. The optical appeared solid (no noticeable deviation from the iron sights).

    The instructor that I watched run his own experiment was using EOTech's throughout.

    Anything you can find out of your old manuals which could shed some light on this would be much appreciated.

    @ Claymore,

    Thank you for taking your time to address this issue. Your comments are appreciated and you make some good observations. You obviously put some though into your reply, so I ask your indulgence to allow me to ramble and toss some ideas around with you (and anyone else who may have experience in this area).

    I understand the truth of your statement that "lets say that you are shooting a right hand twist barrel (lets remove gravity from the equation for now) the spinning bullet grabs the air and steers it in one direction, you compensate for it with the sights. good, then you have compensated for gravity as well, so in reality when the bullet leaves your gun it is not traveling as if it were a laser from the barrel."

    The phenomena you refer to with the right/left hand spin is what A.A. Yur' Yev (of Russian Olympic shooting team fame) called "drift" in his book Competitive Shooting. (Used to be available from the NRA... Now apparently the used copies are selling for almost $900 at Amazon. Perhaps it's time to photocopy my copy and put the original up for sale.)

    While it has been a number of years since I have read the book - if I am remembering correctly, in the section where he addresses external ballistics, Yur' Yev claims that drift, while a factor, is a relatively minor one; not greater than 1 or 2 MOA. My personal experience with shooting at 1,000 yard seems to back up (anecdotally at least) Mr. Yur' Yev's observations and your statement.

    Both I and the shooters/instructors I work with over here have had this same discussion, so bear with me here, and please double check my reasoning as I ramble on, to make sure I have this correct.

    Please forgive me in advance for breaking this down the way I am about to, I know it's long, but it's long because doing so sometimes helps (guys like me) to walk through a process mentally, step by step, to its conclusion. I don't do this because I am talking down to anyone in this forum, I do it because if I don't, I could miss a detail; and by being overly meticulous, smarter men than me (and there are plenty of them out there,) can catch my missed step, or failed reasoning, and then easily tell me where I am wrong.

    Having stated that, I would like to walk us through a hypothetical experiment. For those of us who are visual learners, I will use simple and somewhat unrealistic numbers only to make it easier to follow. Furthermore, I will make this experiment less realistic by pretending we are taking out all variables that could lead to shot dispersion, assuming everything at all times is perfect; the rifle is shooting to it's fullest potential with the correctly matched ammunition, on a windless and comfortably warm day, we are on the top of our game (no hangover, no caffeine high) a bench rest shooters dream day:

    Not knowing the exact numbers for drift, and in order to keep things simple in this experiment, let's say drift = 1 MOA, obviously in the direction of spin.

    Just for giggles, we have viced the rifle in the worlds most accurate and advanced Ransom Rest vice at 100 yards. Then we take the worlds most accurate bore sight laser, insert it into the chamber, and let the bolt go home. We then move the bench rest - with it's built in mechanical adjustments - in order to make the laser bore tool point at the center of our desired area of impact at 100 yards.

    We now notice that - to our amazement - with a mechanical zero on our rifles sighs, our sights just happen to be dead on, directly centered on our intended point of impact (the little red dot of our bore laser tool and our sights are in the center of our desired point of impact).

    What we have at this time is: A rifle which now has the center line of it's bore and the line of sight intersecting perfectly at 100 yards.

    Without moving the rifle at all, we unload the laser bore sight tool, and then insert our first round (for the sake of argument let's say M855), confirm that nothing has moved via our sights, we then gently press the trigger. We repeat the above steps two more times (laser bore sight and all), giving us a beautiful clover-leafed three shot group.

    When we observe the group down range, we see to our amazement that the group is both a little low (+/- 3") and a little right (+/- 1") for some reason...

    "Ahhh... I know what it is! It's gravity and drift that we didn't take into account." we reason to ourselves.

    Now we go back and adjust our sights up 3 MOA, and left 1 MOA, then we recenter our sights (bench rest vice and all) on the center of our target.

    If, at this time we put the laser bore sighting tool back in the chamber, closed the bolt and observed the red dot, we would expect to find that the red laser dot, instead of being in the center of our point of aim and desired point of impact, is now pointing about three inches high and one inch left of our desired point of impact. Would we not? Or have I already biffed this?

    If the above is correct, then what we are saying here is - in order to compensate for gravity and drift, we have moved the sights so that relative to the center line of the bore the sights are pointing down and right. Therefore when we compensate in order to place our sights on our desired point of impact (cross-hairs/iron sights/red dot, whatever) we will have to move the rifle's center line of bore proportionately high and left.

    (Do I have this right so far? If I do, you can keep reading, if I am off, stop here and correct me, because everything else that will follow will quickly become irrelevant if I am wrong.)

    Okay, now we are going to hypothetically shoot the rollover prone experiment.

    First, keeping the rifle benched (avoiding touching the barrel in any way or otherwise interfering with the harmonics) , I am going to roll the rifle 90 degrees clockwise from vertical (ejection port down), and ensuring that my sights are still in the center point of aim, I am going to reinsert the laser bore sighting tool. At this time I notice that my laser sight (representing the center line of bore), is pointing about three inches to the right of my intended point of impact, and about one inch high.

    Because the center line of the boar is roughly parallel with the deck now (what was my + elevation when vertical, has now become my right windage.). And I now only have about one inch of + elevation (that was my windage before laying the rifle on it's side,) So what was my three minutes of elevation when the rifle was vertical, has now become three minutes of right windage, and what was one minute left has now become one minute of + elevation.

    When I shoot I should end up with a group that is both slightly low (gravity again) and right (now due more to the three minutes of windage that was my + elevation and drift) which combine to give me a total of about four inches of right windage.

    Reverse the experiment: Roll the rifle from vertical to 90 degrees counterclockwise (ejection port down).

    With the sights still pointing center of my desired point of impact, I should see the laser bore tool now giving me a red dot about three inches to the left (what was the + elevation when vertical has now become left windage) and about an inch low (what was my left windage when vertical, has now become my - elevation).

    Now, because my center line of bore is lower than both of the two previous tests (it is now, for the first time, pointing below my intended point of aim/impact), when I shoot I should see my rounds impacting lower than ever, and with drift as a constant like gravity, my rounds should impact about two inches out to the left side (the three minutes of elevation that became my left windage, minus the one minute of drift).

    Assuming that the drift factor is still in effect (and I believe it is), I contend that the drift will always be to the right when you have a right hand twist. I see no reason that the projectile should drift in any other direction other than to the right, regardless of the angle of the roll of the barrel. (Obviously vice versa for a left twist.)

    Assuming that I am wrong about the above and drift is reliant on the roll of the barrel, I still would be amazed to see the rounds end up more than 1 MOA above the center line of the bore when the rifle is canted on its left side, as that would mean that the drift is sufficient enough to overcome gravity and throw the round above the center line of the bore. I personally can not imagine that, but I am sure that there are plenty of things that I can't imagine that probably happen.

    Having written all of that (and if you have read this far, thanks for hanging in) I once again come to the conclusion that there must have been shooter error on both my part and the instructors part (don't get me wrong, I am torn between the pride of thinking I shoot well and the fear of looking like a dumba55 for not understanding simple external ballistics.)

    If the reasoning above is solid - Then the only logical explanation in my mind is shooter input/error (most likely), or both myself and the instructor happened to have the same type of mechanical malfunction in our firearms causing the same grouping (highly unlikely.)

    So once again, I find myself asking: "What am I missing?"

    I would truly like to know if anyone has experienced the same grouping that I have observed, or perhaps has had a student shoot with the same results as myself and the instructor. If you have seen this before, and you have found a solution to the puzzle, please do share. I would be very grateful.

    - SB

  6. #6
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    SB,

    We are on the same page, And to clarify, I am neither a physics major nor a ballistics expert.

    What I posted was simply observations from 40 odd years of recreational shooting, that said, in the event I am proven wrong, I chalk it up to "lesson learned" and I take no offense.

    I also know that my observations will vary from gun to gun, and using different ammo, for instance a slow moving, fast spinning bullet will "drift" farther than a fast moving slow spinning bullet.

    While I may not be able to add anything constructive to my previous post with further information, you didn't say what you were shooting OR how far off the p.o.i. was from the p.o.a. with the rifle on it's side.

    that would mean that the drift is sufficient enough to overcome gravity and throw the round above the center line of the bore.
    Keep in mind that it's not the drift that's doing it, the sight adjustment to compensate for the drift will come into play here.

    In fact, the drift will now (on it's side) still be moving in the same direction as it was with the rifle in the upright position, which should cause the bullet to move farther right or left, depending on what you are shooting.
    "Why is it, when your kid has an imaginary friend people think they are cute, but if they have an imaginary enemy, they are paraniod?"

  7. #7
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    I should note,

    I am not arguing my position as an answer, I am simply adding relevant points to the thought process.

    A much wiser man than me once said "A conclusion is not an answer, it is simply a convenient place to stop thinking".
    "Why is it, when your kid has an imaginary friend people think they are cute, but if they have an imaginary enemy, they are paraniod?"

  8. #8
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    Claymore,

    It sounds like my initial reply may have read as if it were a rough reply, or rebuke... it most certainly was not mean to be. I was simply trying to walk the reader through my thought process in regards to the theories of external ballistics as they apply to the OP (as I understand them as a layman).

    If I gave any offense, I sincerely apologize, and beg your pardon.

    You are indeed correct, I didn't mention the p.o.i.. I failed to mention them because I didn't I didn't take measurements at the time, as I was more interested in shooting the second string on the bags to see if, as I initially assumed, there was shooter error involved, in an effort to try and understand what was going on. After the second group, both instructors and students were arriving and it was more important to clean up and prepare for work.

    What stood out in my mind was the fact that the shots were roughly where I mentioned them in the OP. Now going off of memory months old, I would be guessing that for the rollover prone right the p.o.i was about 5" right and about the same low. And at least equally as high and left for the roll over prone left. Again, what stuck out in my mind was the stark contrast to what I had expected.

    I think that we could plug in any numbers to my first reply, and if the results were different than the expected results, I would still be on this forum asking and probing.

    Thanks again for your time,

    - SB
    Last edited by Silent_Bob; January 28th, 2012 at 10:35 AM.

  9. #9
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    I know you didn't want theory, but I've never observed this, and these two resources explain it pretty well (the PDF article is by Kyle Lamb, so you can assume the author has actually performed the experiment :). I can't explain why you might be experiencing what you're describing. I'm stumped. Anyway, if I'm following your explanation correctly, you've got the right idea.

    http://www.m4carbine.net/showthread.php?t=33747
    http://www.vikingtactics.com/oldsite...-Roll-Over.pdf

    My first suggestion would be to try it again at 200m+ and see if the greater effects of bullet drop overcomes any possible shooter error, putting you back into a place where theory matches experiment.

    With regards to what might cause that "shooter error", I think Combat Diver raised a good point; varying lighting situations can cause POI shift of as much as 1 MOA in my experience (using magnified optics). I don't know if my experience translates into something meaningful in this particular situation, though.
    Speed is fine, Accuracy is final.

  10. #10
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    SB,

    Fear not, I took no offense, as I said, I am no expert by any means, I was just stating observations that I have seen in my shooting.

    I cannot and will not claim that they are the answer, I am only putting fourth some points to ponder, I know that gravity and bullet drift play a part in sighting, and they need to be compensated for, so in my opinion, when you lay the rifle on it's side the sight picture will not put your shots to p.o.a.

    Kind of like if you sight a scope so p.o.i. is exactly on the center of the cross hairs, then rotate the scope 90 degrees, you will be way off.

    I don't believe that there is a way to correct the situation, as it seems to follow the laws of physics, you just need to know how far off you are going to be, and compensate with good ol' Kentucky windage. (if you will)
    "Why is it, when your kid has an imaginary friend people think they are cute, but if they have an imaginary enemy, they are paraniod?"

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