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Notice any POI change with velocity???

Discussion in 'Uncategorized Threads' started by jappo, May 16, 2007.

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  1. jappo

    jappo TS Member

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    On more than one occasion I have noticed considerable POI difference with the same gun on same day between shells going around 1090 versus 1230fps... The lower velocity always being the higher POI. Anyone else ever notice this??? If so, what are the reasons? The only one I've heard so far is that the faster shells get out of the barrel quicker before the barrel has a chance to rise due to recoil. The difference that I noticed this last time was around 5 to 6 inches....
     
  2. Clay McCracken

    Clay McCracken TS Member

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    I noticed the same thing but was beaten within an inch of my life when I posted a similar observation.
     
  3. dedpair

    dedpair Active Member

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    That would be the simple answer and probably the correct one. The only way to know for sure would be to lock the gun in a rest. Jeff
     
  4. hmb

    hmb Well-Known Member

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    The slower the velocity the higher the launch angle do to muzzle rise is the correct answer. It's a common occurrence with pistol shooters, when switching to 800fps wadcutters from faster service ammo. You would always havve to lower the elevation for the slower ammoo, just the opposite of what most shooters thought. HMB
     
  5. Neil Winston

    Neil Winston Well-Known Member

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    duper, I tested Federal Extra-lights against Federal Super Handicaps off a rest at about 25 yards, at a greater distance than I usually use, but I wanted to give it the best chance of showing a result. Half a dozen of each at least.

    There was no systematic difference in the height of the patterns shot by the two shells, one very fast, the other very slow.

    Neil
     
  6. BDodd

    BDodd TS Member

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    HMB wins the cigar. This phenomenon is common in the handgun world and those that encounter it think it's counterintuitive; thinking that more velocity = more recoil and muzzle jump and therefore, the barrel should rise more and have more affect on the POI but the faster bullet will print lower every time. It is thought it's a relationship of how long the bullet is in the barrel and affected by any barrel rise. If that's true and I'm not saying it is, the slower bullet will be affected by any muzzle rise over a longer period of time (nanoseconds) and, therefore, print higher than the faster bullet. I've never pondered the same effect on a shot charge but with a standard configuration shotgun, barrel higher than where the shoulder pad meets pocket, perhaps so....breakemall....Bob Dodd
     
  7. jappo

    jappo TS Member

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    Seems like Clay has seen similar results while Neil didn't. I would've thought there would be consistent results. I wonder if if has anything to do with Neil being rested on a rest and the others being standing free hand??? I was doing it mainly to see if there was any difference when using the ultra light shells for 16's and the higher velocity for handicap. From what I seen anyway it has the opposite desired results, being that you normally want the higher impact on handicap. If nothing else it could be another excuse for a bad day at handicap. Ha..Thanks for all of the feedback......
     
  8. hmb

    hmb Well-Known Member

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    It would seem that Neil's results show you can get different results when you test from a rest and test free hand. Maybe it is best to test it the way you are going to use it. HMB
     
  9. AveragEd

    AveragEd Well-Known Member

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    I, too, have found handgun loads to exhibit this phenomenon the most. I once had an S&W Model 14 with an 8-3/8" barrel on which I had to replace the rear sight leaf with one that had a lower notch so the 110-grain hollow points I wanted to use for varmint hunting when the hay was high would shoot to center at 25 yards. Target loads with 148-grain wadcutters were fine with the standard sight.

    One local trapshooter who is a state HoF member and an occasional ATA All-American swears that one-ounce trap loads shoot higher than 1-1/8 ounce fodder. I've never experienced that while shooting via evaluation of my breaks but then I've never bench-tested the theory, either.

    Ed
     
  10. zzt

    zzt Well-Known Member

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    duper, your observations are correct and hmb explains why. Shoot bunker loads sometime and see how much lower they shoot. Do not pay any attention to those suggesting they cannot reproduce the effect while shooting from a bench. That is very much like comparing apples and oranges and concluding they are the same.
     
  11. jappo

    jappo TS Member

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    AverageEd, I mentioned the speed of 1090 vs 1230 but I didn't mention the slower one was 1 ounce and the faster was 1 1/8 oz. That goes along with what the state HOF'r says.
    I was doing it originally to see any difference with my normal loads for 16 vs handicap. I shoot 1 oz at 16 and 1/1/8 at handicap.
    So it appears others have noticed this at times also. Thanks.
     
  12. zzt

    zzt Well-Known Member

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    hairy, no it isn't. Not for me, anyway. Some, most notably Hap Mec Tweaks swear there is no difference. For him, there may well be none, but I think there are too many variables involved for a one-size-fits-all answer.

    When I shoot from a bench I'm sitting, hunkered down and leaning well forward. I hold the gun differently. My face is in a slightly different position on the stock and my cheek puts more weight on the stock because I'm leaning forward. More of my mass is in line with the barrel. There is simply no way the gun can rotate upward under recoil in the same manner as when I stand on the line. My POI vs. POA is always lower from a bench than standing. YMMV.
     
  13. Hauxfan

    Hauxfan Well-Known Member

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    Dr.Jones, I disagree a tad.

    A laser device is only meant to get you in the vicinity, it was never intended as a sole means of sighting in any gun, be it a shotgun or a rifle.

    It is just another tool to use before you fine tune the gun to shoot where you want it to shoot.

    Hauxfan!
     
  14. phirel

    phirel TS Member

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    I have shot the three loads I use for singles, handicap and the first shot of doubles from a rest at 13 yards. I recognize that the faster loads would fall very slightly less due to gravity and the slower loads might shoot a slight bit high due to time in the barrel. However, measuring the POI to an accuracy level of 1/2 inch is the very best I can do and measuring to the level of 1 inch is probably best. At these levels of measurement accuracy, my slow and fast shells all have the same POI.

    Dr.longshot- You mentioned that when shooting faster shells, the shot will get to the target faster. I have shot several "fast" handicap loads and the shot never did get to the target. Have I missed something in your explanation?

    Pat Ireland
     
  15. BDodd

    BDodd TS Member

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    Dr. Jones, the laser aiming device on handguns will have instructions that it works well with the same ammo when accurizing in the first place. Sure, if you use different ammo every time you load up, the device will not be as "perfect." Then, if we're talking about 1/4 or 1/2 inch at say 15 yards, a suitable distance for self defense, it's plenty close enough, eh?....breakemall....Bob Dodd
     
  16. phirel

    phirel TS Member

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    MIA- You need to study gravity a little more. Both of the bullets described by Ed will fall at 32 feet/sec/sec.

    Pat Ireland
     
  17. Trucido

    Trucido TS Member

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    Assuming that the barrel is clamped down and cannot jump, the faster shot WILL have a high POI. Period end of story.

    The reason is that, as Pat said, EVERYTHING accelerates towards the center of the earth at 32 ft/s (assuming you are not at EXTREME elevation...like in a spacecraft). This does not matter if the object is moving horizontally (towards a clay) or not.

    We can therefore setup a thought-experiment:
    Laser-sight a barrel to the center of a target. Clamp down the barrel to stop barrel jump. Say that target is 40 yards from the front of the barrel (objects are unaffected by gravity in a barrel, because the barrel counters the force of gravity). A bullet moving at 1200 ft/second will reach the target in 0.1 seconds (approximately... will be slightly more due to its following a parabolic path rather than a straight-line path, but less than 1% difference). Using classical physics equations, we can see that the bullet will drop (1/2)*32*(0.1)^2 = .16 ft or approximately 1.92". A bullet moving at 1100 ft/second will reach the target in approximately .11 seconds and will therefore drop (1/2)*32*(.11)^2 = .1936 ft or 2.32". Since all masses drop at the same rate, we can extrapolate the drop of a bullet to the drop of each individual pellet of shot.

    Therefore, using a clamped barrel at a distance of 40yards, the 1200 f/s shot will drop ~.4" less than the 1100 f/s shot...which is probably not noticeable in a pattern of shot due to externals (uncontrolled variables in shot size, powder, wind patterns, air density, moisture, etc).

    With regard to barrel rise, in order for POI to change 6" at 40 yards between a 1200 f/s and 1100 f/s shot, assuming a 30" barrel, 18" LOP (makes numbers easy) and a fixed rear pivot (gun does not move on your shoulder), we can calculate the amount that the barrel must rise of the sighting plane:

    We can set up 2 similar triangles such that we have a 120' long baseline, with a 4' baseline behind to the left. Then on the right side, we draw a line 6" tall. We connect the left end of the 4' baseline to the top of the 6" vertical line. This means that we have a right triangle than is 124' long and 6" tall. We then draw a vertical line down from the diagonal to the intersection of the 4' line and 120' line. This height would be 0.2". This is the difference in barrel jump between a 1200 f/s load and a 1100 f/s load, assuming a POI jump of 6" for the slower load. This seems like a reasonable number to me, but I will not further delve into the equations regulations what sort of velocity and acceleration the barrel would have to have that change in height in a 0.00019 seconds time-span (the difference between the time the 1200 f/s and 1100 f/s loads are in the 30" barrel), as there are multiple factors to consider, including the fact that the barrel would move at differing velocity depending on the 'kick' we receive, which causes us to swing the gun up and which is variable based on the velocity of the loads (and the weight).

    Basically, it is sufficient to say that the difference in POI not due to barrel jump between a 1200 f/s load and a 1100 f/s load is 0.4". In order for the 1100 f/s shot to actually pattern 6" above the 1200 f/s shot at 40 yards, it would require a 0.2" change in barrel jump between the two loads, which seems reasonable...though this might not necessarily be the case. Someone should ask someone with a physics degree...I'm just a math major.

    -Kyle
     
  18. phirel

    phirel TS Member

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    Kyle- Nice job. I did not check your numbers but I will when I have a little more time. We could really confuse some readers if we factored in the variation in gravity at different sites on the earth. Shot will fall a little faster in regions where the magma core is closer to the surface and will fall a bit faster at Spanish Forks than near the coast. We could make this a very complex question.

    Pat Ireland
     
  19. Trucido

    Trucido TS Member

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    Pat,

    Basically, I decided that the graphs to describe the barrel position/velocity/acceleration would be too hard to describe...since I can't upload at work. If I was bored enough, I could actually workout an approximate acceleration function to integrate and describe the position of the barrel at any time. Using that, we could then figure out where the barrel was positioned at the time that the 1200 f/s shot left the barrel, as opposed to a few ten-thousandths of a second later when the 1100 f/s shot would leave the barrel. I think it could be done reasonably easily...but being involved in numerical analysis myself, I think that the potential 'front-end' or input error would significantly overwhelm any roundoff, calculatory (I'm pretty sure I made up that word...but it works) or machine errors involved...effectively the old applied mathematician being at fault rather than the the numerical analyst. I'm not a wonderful applied mathematician...but I do pretty well with numerical analysis (which is the FUN part!)

    Anyway - looking back at the original statement of the slower shot being 6" higher than faster shot is a little extreme. It seems to me that this is more likely due to the differences in how hard the shooter tries to control the recoil/barrel jump of the gun, i.e. how much the person tries to pull the gun down when they pull the trigger. I would bet that the slower load may pattern a bit above the upper load (discounting the effects of gravity for a moment) if the guns were both shot from a rest with the butt of the gun stabilized and the barrel free to jump. However, given the effects of gravity which I calc'd above, I would say that the difference in POI is overall negligible, especially in instance of shotgun patterns with hundred of pellets. I would almost guarantee that the difference in the actual POI of a 1100 f/s load and 1200 f/s nearly completely overlap in normal shooting conditions...if you are going to hit the bird with an 1200 f/s load...you are most likely going to hit it with an 1100 f/s load. That being said...the amount of force that the pellets hit the bird with will be decidedly different and can certainly account for 'dusting' some targets, where a faster load would break the clay.

    IMO, I think people should just pattern with a few different loads and take the one that fits their shooting style the best. Personally, I shoot loads at ~1185 f/s and 1oz. I think it shoots flat enough for me and certainly smokes the clays when I do my part. As with most things in trap, I think people over-think and forget that thinking is what ruins scores...its all about subconsciously allowing the gun to follow your eyes and pulling the trigger when the bird is in the center of your vision - nothing much else to it...just practice.

    -Kyle
     
  20. Trucido

    Trucido TS Member

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    I would also point out (just quickly for everyone...so I don't bore you all too much) that two loads of pellets moving at the same velocity, but different weights of lead (1oz vs 9/8 oz) should pattern at nearly the exact same spot, since gravity will have no effect, nor will the time the shot is in the barrel. The third factor (not discussed above) is that actual difference in the amount the barrel jumps due to the difference in the recoil experienced at the shoulder point and therefor the difference in the force applied in the upward direction of the barrel...this force obviously changes between load weights and velocities (you feel more recoil for faster loads and heavier loads, which means the barrel jumps more).

    Regardless...as I stated before, I think that this is all over the top and really people shouldn't need to try to make everything about their shooting mathematically precise and instead just make the gun comfortable to shoot correctly and spent the rest of your time practicing rather than analyzing why you hit or missed a target...unless the winds kicks the target...which always pisses me off.

    Thanks for this delightful conversation everyone,
    Kyle
     
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