Different Bullet weights - different POI

Discussion in 'Reloading' started by NewportNewsMike, May 2, 2012.

  1. NewportNewsMike

    NewportNewsMike

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    The following applies to handgun bullets of varying weights and their differing point-of-impacts (POI).

    Like many folks on this Forum, I have observed that, from the same gun, using the same point-of-aim (POA), lighter weight bullets will have a lower POI than their heavier weight cousins. Sometimes, depending on weight difference, a large variation of POI – on the order of multiple inches at a range of 15 to 25 yards. This effect is very observable, and very repeatable (does it every time).

    Because I had no contrary evidence and it sounds very plausible, I have always accepted the explanation from many different sources that this phenomenon is the result of the presumably faster light weight projectile spending less time in the barrel which is rising from recoil when fired. Less time means earlier exit from the muzzle (which would be at a lower rotational position), and thus the bullet is given less “arc” on its way to the target, resulting in a lower POI.

    Until now –

    We are into the fourth season of “Top Shot” which is a television show aired on the History Channel. One of “Top Shots” treats is their outstanding high-speed photography. They have amazing footage of bullets in flight, bullets hitting targets, targets exploding / shattering, and bullets leaving the muzzle – all in glorious slow motion splendor.

    This high-speed photography showing bullets leaving the muzzle is what this post is all about. In all the various guns shown – semi-automatic pistols, single-action revolvers, double-action revolvers – the bullet is well away (an inch or more) from the muzzle before the gun starts its recoil rotation / movement. This shows that barrel rotation / elevation caused by gun recoil cannot be having any effect on the flight path of the bullet. The bullet is well out of the barrel before there is any barrel or shooter’s arm movement.

    This seems to be in direct opposition to the well-publicized and accepted theory above on why lighter bullets have different POI than their heavier cousins.

    Has anyone else observed this on the high-speed photography?

    Any other thoughts or theories about what is happening here to cause differing POI’s?
     
  2. F106 Fan

    F106 Fan

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    If we assume a bullet leaves the barrel at 1000 fps, the average speed through the barrel is 500 fps. If the gun has a 6" barrel, it takes 1/1000 of a second to clear the end. I wouldn't quibble if someone came up with a slightly different answer but the point is the same: The gun CAN'T move very far in 1/1000 of a second.

    I'm not sure I agree that the POI should be much different. I use the Sierra V6 software and set a zero at 1 yard and then run through various bullets and velocities. I don't get much more than an inch of difference for a given bullet over a radical change in velocity (500 fps versus 1000 fps). For a modest change in velocity (say 100 fps), the difference is about 1/4". I worked on this example a couple of weeks ago for a 15 yard range. The math doesn't support much of a change in POI versus velocity. Similarly, it won't support much of a change in POI versus bullet weight (which, ultimately, gets back to velocity).

    I guess the only way I could be convinced that POI varies radically with bullet weight over such a short distance is if someone shot the gun in a Ransom Rest and the velocities were captured with a chronograph showing equal velocity.

    What you suggest may actually occur but I haven't seen it in the magnitude you describe. Whenever my shots go low, I just blame it on the shooter (me) and lousy trigger control (mine).

    Richard
     
    Last edited: May 2, 2012

  3. fredj338

    fredj338

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    You are getting sucked into the slow mo pics thing. Recoil starts @ ignition & plays out thru the entire firing sequence. If you think it's bbl time, here is the easiest proof. Take a revolver, the tend to show more recoil, fire say a 200gr bullet @ 1000fps, then a 300gr bullet @ 1000fps. Same vel, same bbl time. The heavier bullet shoots higher. Slow the heavier bullet down 50fps, more bbl time right? The bullet will still impacts higher @ a mere 25yds. That is telling me recoil is playing a significant role, regardless of the great photo work on TS, about the only imnpressive thing I have seen watching the show. The shooting is pretty average.
     
    Last edited: May 3, 2012
  4. sellersm

    sellersm disciplinare

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    I'm with Fred on this one... I don't watch the show, can't watch it unless it's on the internet somewhere!
     
  5. F106 Fan

    F106 Fan

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    More barrel time?

    I have heard this discussion for many years. I don't doubt that it occurs but I would rather see the test instrumented and shot from a Ransom Rest. But then, the recoil might be more constrained. So, maybe it has something to do with grip. Perhaps the effect is more apparent when shooting one handed - like the bullseye shooters. Maybe Jack Weaver was on to something...

    I just don't know. I still wouldn't expect a multi-inch difference at 15 yards. Maybe out at 25 yards but 15 yards is awfully close.

    I liked the golf ball at 100 yards with a Valquartsen 10-22. I'm too old and beat up for the more physical events.

    Richard
     
  6. fredj338

    fredj338

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    My little SP101 is a good example. With 158gr bullets @ magnum speeds, it shoots POA/POI @ 15yds. Move to a slower 158gr 38sp or 125gr magnum, it shoots lower, like 4-5" lower. That is not bbl time, but recoil.
     
  7. F106 Fan

    F106 Fan

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    Recoil is the only explanation because interior and exterior ballistics just won't account for the variation.

    So, is it a function of grip? If you really mangle the grip, can you get less of a difference? I'm not proposing that strangling the thing is a proper grip, just wondering.

    I was told, years ago, that if my 1911 had wood grip panels, I better see sawdust on my shoes. Procedures change over time but I still find that a very firm grip gives much better performance in action pistol. I think it counteracts my poor trigger squeeze (more like a slap).

    Richard
     
  8. fredj338

    fredj338

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    Yeah, typing faster than I think.:wavey: