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Techies Only: Barrel Physics?

Discussion in 'Uncategorized Threads' started by EXFDX, Oct 13, 2007.

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  1. EXFDX

    EXFDX Member

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    I've asked this before on this forum, but sort of got irritated responses. Anyway, I'd still like to know so I'll ask one more time…

    I'm curious about what goes on with the shot string from the time the shell is touched off until it (hopefully) intercepts the target. I know the shot string is a fluid, expanding, decelerating mass, and I believe I have a fair notion about what it looks like in flight. But what I'm really curious about is how choke affects the overall shape of the string.

    Near as I can figure, this is the approximate math of the shot as it leaves the shell until it exits the barrel: Once the primer is detonated, the shot accelerates the 32 - 34" down the barrell from 0 to about 900 mph, in about 1/12,000th of a second. Where I have difficulty is in believing that the last inch-and-a-half has some measurable effect on the shot string in terms of its shape or relative size. I have done more research on this than I am willing to admit -- we are approaching obsession territory here. But one interesting factoid I have learned is that somewhere around .031 to .033 of constriction, the string stops getting narrow and begins to widenout again leaving the barrell. In other words, that may be the upper real limit of tightening the pattern leaving the barrell, and that makes a certain amount of implicit sense to me.

    What I can more comfortably theorize is this: Increased choke separates the wad more quickly from the ejecta as it leaves the barrell. In other words, the longer the wad remains a part of the shot string, the more likely it may be "pushing", i.e., widening, the string from behind.

    That's all I can come up with. I can deal with a certain amount of unexplainable "magic" in my life - in fact I welcome it. But on the other hand, the unexamined life is not worth living to some, and I guess that's where this question is coming from. So if you can provide some insight or point me to some really (really!) out of the way research, I'd really appreciate it.

    Thanks,

    Steve
     
  2. smokerz

    smokerz TS Member

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    What you have said is probably correct, and is based on empiral observation rather than a full understanding of all the forces at work. In fact AFAIK, there is not much theory which will predict shot string/pattern. Barrel and choke shapes, velocity, rates of acceleration, wad type and shape, roundness, hardness, size and uniformity of shot are all factors, not fully understood by anyone.

    OTOH there is lots of empirical data which demonstrates what works and what doesn't. If you're trying to accomplish something in particular as regards patterns and strings, there's info available.. If you're looking for a unified theory - good luck.
     
  3. Neil Winston

    Neil Winston Well-Known Member

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    The time is 3 or 4/1000th second.

    Why would the wad "push" the shot after it leaves the barrel?

    Both my tests and those of Dr. Jones have resulted in the tightest chokes being 0.040, not the popular 0.033, though I might have one of the more open ones which match the tighter ones.

    Open chokes have longer shot strings at trapshooting distances than full chokes. I'll post the documentation, which includes the reasons, if you wish.

    Neil
     
  4. HSLDS

    HSLDS Well-Known Member

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    two questions for you to think about...

    How do you explain the effect of chokes on 'old' style shells - no plastic wad, just a layer of paper and/or felt to separate the powder from the shot? Chokes still work here and the shot is basically rattling it's way down the barrel.


    To the other extreme, how do you explain the effect of Chamber Mates - sub-gauge shells in what is essentially an over-sized barrel (http://www.chambermates.com/background.htm)? There should be no friction of the choke/barrel on the wad as it exits the muzzle...
     
  5. Straight99

    Straight99 Member

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    I have been patterning shotgun chokes for a long time also. I have made full chokes shoot like a skeet choke and skeet chokes shoot like full. By changing speed, wad and shot size the pattern will change and not always the way you think it will. I have some old Herters unslit wads that realy hold a tight pattern. I have a Green Dot load with 7 1/2 shot that inkball 16 yard shots but is weak from handycap. By changing to 8's it becomes my handycap load without changing anything else but is way to tight at 16 yards. You would think it would be the other way around. I have some chokes that will not pattern at any yardage. I'm sorry I can not help with the why. If you can find any of the old Don Zutz magazine articles or books maybe that will help.
     
  6. EXFDX

    EXFDX Member

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    I didn't say it was a great theory LOL. But after some reflection, yes, it's not likely the wad could be moving faster than the shot. So that one doesn't do very well either on the Sniff Test nor the Plausibility Scale, does it? And Neil, I'm very grateful for your response. I'd love to see the documentation. Never have been very good at Math. I took a Statistics pre-test early in my doctoral program and one of the early "questions" on the test was "4!". I blinked and looked at that for a while, and finally answered "Oh crap! A4!"

    HSLDS, you are exactly correct and that was always the biggest hole in my "theory" -- what about pre-wad days? But do we know what effect the introduction of wad technology had on shot string or pattern?

    And Smokerz if you can point me to any empirical stuff, I'd appreciate that. Do you need an email addy?

    Steve
     
  7. Neil Winston

    Neil Winston Well-Known Member

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    This article is about 18 years old, appearing in December issue of American Rifleman, 1989. Ed Lowry is _the_ expert in all the areas the article discusses. As far as I know, nothing has ever been published to match this in terms of clarity, usefulness, and if I dare say so, accuracy.

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    [​IMG]

    Neil
     
  8. BBowen

    BBowen Member

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    steve,

    i have a couple of outstanding pictures i can send to you if you are interested.

    i was present when they were taken and plan to get the proper equipment and gather much more infomation myself.

    the pictures were taken with a canon digital 35 mm camera from approximately 50 yards behind the shooter who was shooting 16 yard trap targets. these photos clearly show the shot cloud, the wad, and in one case the target just as it is getting hit. the camera is capable of 10 frames per second and i think about 100 pics were taken each time because the photographer had to anticipate when the shooter would call for the target. we then simply looked to find the one we wanted and printed it out. you can see individual pellets and the shooter!!

    the lense was a cheap celestial telescope adapted to the camera. i don't know the specs on the lense but it was about 30 inches long and not real expensive.

    as i said, these photos were all taken from about 50 yards behind the shooter. we are going to the range next week, if the weather is good, and try some more from the side. we will lock the trap for straight aways and see what happens.

    i would post the pics here but the scan doesn't do them justice and i don't have the digital files. if we get some good stuff ourselves, i will try to get in on ts.com

    we shoot lots of patterns here in our underground range and have pattern analysis software. however, there is nothing like seeing the actual shot cloud in a photograph. also, i think this would be a valuable training aid to difinitively show where the person is shooting in regard to the target. (might give the experts a little trouble though--you know, the ones that always stand behind you and claim to know where you missed the target!!)

    good shooting,

    bruce bowen www.bbguns.net
     
  9. pulllit

    pulllit Member

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    Bruce,

    I would like to see a shot string photo taken behind a shooter swinging on a hard right from post five. On a foggy/overcast day I stood behind a shooter on 16 yard post five with the trap locked on hard right and I could see the estimated 8-10 foot shot string. The shot string had a definite curvature to it similar to the tracer rounds fired from a C130 gun ship at night. I have read on here where the TS.com experts have stated that there is no way that a moving barrel can impart any side stringing effect into the shot cloud. What I observed that day proved to me that there is a slight curve similar to a water hose stream. I can not provide the calculations to substantiate my theory that pellets at the very front of the shot column have a different line of flight than the pellets at the bottom of the shot column with a fast moving barrel.

    Jim W
     
  10. Neil Winston

    Neil Winston Well-Known Member

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    Jim W, you are right when you cite:

    " a slight curve similar to a water hose stream."

    ...since the apparent curve in a water hose stream is an illusion.

    Neil
     
  11. JerryP

    JerryP Active Member

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    Steve, I've asked the same question here about choke restriction reaching the point of diminishing return. I got varied opinions but if that's true why do I read about people using .100 chokes at meat shoots. I remember an article in Shotgun Sports Magazine about smiths who specialize in making barrels for those shoots. The barrels looked like a heavy barrelled varmit rifle. Very thick barrel walls at the choke end and .100 choke.


    tomk2, When the picture was posted I was wondering if the choke could cause those pellets at the outer edge of the rear to break away from the shot string. I don't think anyone actually KNOWS.
     
  12. JerryP

    JerryP Active Member

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    Jim W, To achieve what you describe the gun would have to be spun on an axis at tremendous rpm so that the shot at the front would leave the gun at a different time in space than the shot at the rear. It's not humanly possible.

    The tracer rounds are like the water hose, as Neil stated, an illusion.
     
  13. pulllit

    pulllit Member

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    Maybe Bruce can get a picture that would show what the actual shot string would look like from behind the shooter on post 5 hard right angles. If it shows a slight curve in the picture then it would not be an illusion, right?

    Neil without getting too complicated in your answer, could you explain why I observe a slight curve illusion to the shot string on the hard right? On a straight away under bright lights I can see a elongated streak especially with homemade shot that has not been graphited.

    Jim W
     
  14. phirel

    phirel TS Member

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    I have also seen a gray streak that is at least part the shot string. It disappears so fast that I could not accurately describe its shape and could never detect a slight curve. Many others have better eyesight than I do and you could be one of them. My logic, and that is sometimes faulty, suggests that to get a curve in the shot string due to barrel movement, the end of the barrel would have to be traveling 1200 Ft/sec or faster. I do shoot fast, but I do not move my barrel that fast.

    Remember, a water hose ejects a continuous stream of water, not a single pulse of water moving at 1200 Ft/sec.

    Pat Ireland
     
  15. Neil Winston

    Neil Winston Well-Known Member

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    Even if it shows up in a photo, Jim, it still might well be an illusion. As is, in my opinion, almost every photo I've seen posted here of a "shot string." That's why I want to see Bruce's - it might be "real" being taken from so far back.

    What people don't remember is that shutter speeds are slow compared to shot, and that means that an individual pellet will show as a streak, not a point. So you see what appears to be an elongated cloud, when in fact what you are seeing a lot of pellets shooting up in a lot of places while the shutter is open.

    There are three way to approach your "curving shot" speculation.

    1. The shot starts out going straight, then starts to curve. This is impossible (Newton's First Law). The shot, no matter what (air drag effects on non-spherical pellets aside,) goes straight.

    2. The swing of the gun lets the first shot out and then the gun movement causes the "back" shot out later, "stringing" the shot cloud. Let's say the shot charge in the choke is an inch long. Let's say the barrel, at shot exit is going 3 feet per second, probably an overestimation. The first part exits, the the later part, about 1/12,000 second later. You see the problem: the muzzle has only moved 0.003 inches. Not much displacement here, I think you will agree.

    3. The muzzle movement applies a horizontal speed to the shot. This is true. The shot is still going straight out, but (let's say) to the right, at a slight angle, as well. Using the same 3 ft/sec muzzle speed, the shot also now has that 3 ft/sec muzzle speed. In going 40 yards the shot flies for about 0.15 seconds. That figure, times 36 inches, predicts a 5-in displacement due to muzzle movement at 40 yards.

    But here's the problem. Imagine you are in a parking lot and a car, starting right ahead of you, begins driving away from you, pointing not straight out, but rather heading to a point five inches to the right of straight out. As you imagine this, you will see that the car does not appear to be following a curved line, it just looks like it is going straight. Because it is.

    Neil
     
  16. EXFDX

    EXFDX Member

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    Neil - WOW! I can't believe I missed that piece, but it looks pretty definitive. Neil, do you have a scanned or pdf copy of that piece? If so would you mind sending me an email copy? Bruce - please, send me any photos, any time. My addy is: sjoehmen@comcast.net

    I stood behind a guy at our club and just kept taking digital photos one afternoon. When I checked them later, one of the images appeared to be a breaking target. When I blew it up, the wad and the last of the shot string tearing into a breaking target could be clearly seen. I'll be happy to share with anyone who might like to see it, but I don't know how to post it on this site. I'm using a Mac -- does that matter?

    My real interest in the shot string is what's going on at thirty to forty yards out when the shot (hopefuly) meets up with the target. Debriefing with Phil Kiner on the some of the video he took of me while taking his clinic, I did learn that it appears my fixed choke, .034 constriction Ljutic has a string about 1-3/4 target diameters wide. But beyond that little nugget, I'm clueless.

    Again, I can't thank you all enough for your responses, and I hope to hear from you via email.

    Thanks,

    Steve
     
  17. EXFDX

    EXFDX Member

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    I didn't think this one would draw you out, Tron. That's a pretty sharp hook you got there, though ;-)

    Steve
     
  18. Harold

    Harold TS Member

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    A full choke chronographs faster than a cylinder bore with an optical type chronograph. What that should tell you is that the leading pellets are getting squirted out ahead. There cannot be a net increase in momentum so it must be that the trailing pellets are transferring some of their momentum. This causes the trailing pellets to lag and the shot to string out, just ahead of the muzzle, which is more aerodynamic. This leads to less dispersal due to air resistance.
     
  19. smsnyder

    smsnyder Well-Known Member

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    neil is it possible to spray the shot like water comming out of a garden hose if moved quickly to the left or right? i once had a conversation about this and i said it has to be impossible since the shot is moving facter than we can ever move?
     
  20. Neil Winston

    Neil Winston Well-Known Member

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    Could you reproduce the data here, D's A? How much did the barrel "wander" before treatment - ten or twenty (better) POI's off a rest will be an example, then the same after treatment.

    Neil
     
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