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Old April 8, 2013, 11:09 PM   #1
golfcalc
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point of impact shift with .460

I have a question. I shoot two bullet weights from my .460 Weatherby Magnum. They are 500 g Hornady round nose at 2570 and 300 g Hornady hollow points at 3150. I don't use a muzzle break and both loads reproducibly shoot 1.5" 5 shot groups at 100 yards either leaning over the hood of the car with a 6" bipod or using a lead sled. Both impact at the same point horizontally, but the 500 g bullets hit 16" lower at 100 yards! Any explanations. I have shot them enough to know that these results are reproducible. Thanks for any ideas.
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Old April 9, 2013, 03:11 AM   #2
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Well there is a 200grain and over 500fps diffrence in those loads therefore the trajectory between them is going to be quite diffrent. Expect it to be even worse further than 100yds.
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Old April 9, 2013, 12:06 PM   #3
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Thanks for the reply. Unfortunately the drop in 100 yds at these velocities is only ~ 2" and hence way to small to account for the 16" difference.
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Old April 9, 2013, 07:33 PM   #4
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The drop between the 2 loads may be close but that doesn't mean the point of impact will be similar. I shoot a 308 with everything from a 110gr hollowpoint up to 180gr softpoint and while drop is somewhat similiar out to 300yds the point of impact is over a foot diffrent.
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Old April 9, 2013, 10:51 PM   #5
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You are seeing the affects of barrel harmonics. As the bullet travels down the barrel it causes the barrel to "whip" around. All guns do it, some more than others. Some change poi left or right, some are more akin to a shotgun with various weights of bullets. This is why many of us use only 1 bullet in a given rifle. I have 3 guns in 243 win. Each for different bullet weights and different uses.
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Old April 10, 2013, 08:12 PM   #6
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I'm not so sure about barrel harmonics causing a 16" drop, but anyway. Barrel harmonics will effect group sizes, not so much point of impact. YMMV.

In my experience trajectories on ballistics charts and point of impact are always two different things. Point of impact is "real life", ballistics are science. The science may or may not apply to real life because of things that the science didn't take into account. Bullet time of flight and amount of barrel rise before the bullet leaves the barrel are two factors I can think of right off the top of my head. How hard I'm hanging on for dear life (if I'm shooting that damned 460 Weatherby - what a tooth rattler!) may be another.

Quote:
300 g Hornady hollow points at 3150.
I'm more interested in what load you are using for the 300 gr to get that speed. I couldn't find anything close to that as a reloading recipe. Is that a factory load? I'd be interested in more info on that.

If one of the bench rest guys comes along, I'm sure that they have all this nailed down. Rifles are my toys anymore. I haven't had to shoot any appreciable distance in many years.
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Old April 10, 2013, 08:52 PM   #7
golfcalc
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load for 300 grain

Thanks for the reply. The load is 118 grains of RL12. They chronograph at 3162 +- 25. The basic problem with most of the explanations is making the group size consistent with the extreme drop. It can't be random else the group size would scale with the difference in drop. The fact that there is little (<1.5") spread horizontally tells me that it is not barrel vibration. That would have to be basically circular since the orientation of the bullet relative to the bore is random. It can't be difference in drop since the drop is 16 times time of flight squared. For 100 yds this is only ~16*(300/3000)*(300/3000) = .16' =2.4". It can't be barrel rise since this would be more for the heavier loads. That seems to leave just a barrel droop as with a fly rod. I haven't had time to finish that calculation yet.
Thanks again.
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Old April 10, 2013, 10:42 PM   #8
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Take any sporter weight barreled rifle and shoot 4 different weight bullets for groups. All on the same target. Use the same poa for all loads. Each bullet weight will have its own poi, in different locations, probably similar group sizes.

As a bullet travels down the barrel the muzzle moves in a pattern that is consistent. The speed of the bullet determines where in that movement the bullet exits the barrel. Therefore heavier, slower bullets leave the barrel at a different axis from the line of sight. This is part of what makes ammunition that is more consistent in velocity more accurate. Not the only thing but it certainly helps.

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Old April 11, 2013, 01:18 PM   #9
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Thanks for the reply. Certainly different bullets will have different POI. the question is one of scale. In my experience they never differ by 16" at 100yds. Have you found differently?
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Old April 11, 2013, 02:23 PM   #10
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Are you using a scope or iron sights?
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Old April 11, 2013, 02:36 PM   #11
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16" is a lot. One other factor to consider is that the rifles muzzle will rise during recoil while the bullet is still in the barrel. With different recoil the muzzle will rise at a different speeds and rise to different heights before the bullet leaves the muzzle. This can make a difference in impact.

This is more common with handguns and even at that 16" seems like too much.

Have you checked your velocities with a chronograph, or just guessing based on factroy specs or what a loading manual says you should be getting.
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Old April 11, 2013, 08:03 PM   #12
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Most of my rifles shoot different weight bullets to various poi. My rem 700 in 243 win puts 55 gr bullets 9 inches lower than 100 gr. Both are around 3/4 inch groups at 100, shot the same day, same target. I only have one rifle that puts everything within 2 inches for poi. That is a Ruger 77 in 338 wm. Every rifle is different.
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Last edited by big al hunter; April 11, 2013 at 08:41 PM.
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Old April 11, 2013, 10:56 PM   #13
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Thanks for the replies people. I'm using a 1-4x nikon african scope. The velocities are chonographed. I have considered the barrel rise assuming it acts like a ridgid rod. In that event the effect is too small and also in the wrong direction - the heavier load would make the barrel rise more and hence hit higher. In practice it is the light load which is high. I suspect the problem is that the barrel bends like a fly rod - which would make the heavy bullets lower. This is a harder calculation and I haven't finished it yet. My intuition is that for a barrel as heavy as this one the bend would not be enough, but I'm not sure. This is a very interesting physics problem. If we ever figure it out I may have to give it to some of my students!
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Old April 12, 2013, 12:01 AM   #14
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I would propose a theory that as the bullet travels down the barrel it applies one of Newton's theories, every force has an opposite and equal reaction. As rifling imparts spin on the bullet, the bullet causes oscillation in the barrel. Using this idea on your 460, how much does the barrel have to oscillate to cause 16 inches of difference in poi ?

To start out lets assume that your highest poi is caused by the bullet exiting the barrel at the top of the oscillation. And assuming the low poi is caused by the bullet exiting at the bottom of the oscillation. Therefore the barrel is moving a little less than 8 moa each direction beyond the straight centerline of the bore. 1 moa is just over 1 inch at 100 yards. 1 inch divided by 100 tells us that 1 moa at 3 feet is .01 inches. For a 24 inch barrel 1 moa is 2/3 of what it is at 3 feet=.0066 inches. 8 moa multiplied by .0066 is the amount of oscillation needed to cause 16 inches at 100 yards. 8x .0066= .0528 inches Do you suppose your barrel could move 5 hundredths of an inch? These numbers are not exact but very close.

Pretty long story problem. What kind of class do you teach?
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Old April 12, 2013, 12:51 PM   #15
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16" is a lot more than is common. I have no good explanation for the amount of difference, other than it may be that your rifle and load combinations are just a "perfect storm" of factors combining.

500fps difference in velocity and 40% differnce in bullet weight means that odds are they are going to strike in quite different places.

Drop may be similar, but drop is due to gravity, a constant, varied results coming from velocity over the time of flight. And remember drop is referenced to point of aim and point of impact being the same at a given distance. At another distance, the difference between impact and original point of aim is the "drop".

Vibration (harmonics) is, I think the culprit here. The fact that your groups are small means that it is consistant, not that it isn't the cause of the radical impact shift. It could very well be that as that big heavy bullet rumbles down the bore your rifle is actually pointed in a much different place when it leaves the muzzle than when using the lighter, faster bullet.

Any difference (change) in the way the rifle is held can make a difference as well. Shooting offhand with a tight sling can mean a different point of impact than shooting from a bipod, which could also be different than shooting from a bench. Each rifle & load combination is an individual in this regard, some being very different with each set of conditions, some not so much.

16" difference in the point of impact at 100 yards is way more than normal, but if consistant, can be lived with, although I wouldn't be all that happy with it.

Do you have enough adjustment in your scope to sight in (bullet hits point of aim at specific distance) for both loads? If so, memorize the clicks needed to change and go back and forth as you change ammo.

If not, decide on which load best suits your shooting, and stick with it. While most rifles are pretty flexible about changing loads, some are not, and it seems yours might be one of those rare ones which isn't.
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Old April 12, 2013, 11:20 PM   #16
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Try fixing a heavy weight on the end of your barrel. I recommend something in the two kilogram range or heavier.

If it is harmonics the extra mass will dampen them and shift POI between the two loads closer together.

Simple experiment for a physicist.

If the POI difference remains the same, clearly it is a mechanism other than harmonics at work.

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Old April 13, 2013, 10:18 PM   #17
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replies

Thanks for the replies. I'll take each in turn.
Al
I agree with your estimate and I'll calculate the force needed to produce such a bend. The bend seems pretty much to me, but it is not trivial to estimate exactly what force is available.
In answer to what classes I teach, the answer is that I've been a university Physics professor for 50years (and still am) and as a result I've taught pretty much everything from beginning undergrad to advanced grad courses.

Amp
I agree that it pretty much has to be vibration. The odd thing is that the rifle is a Weatherby MarkV and as such has a free floated barrel. It seems to me that it is difficult to get that to vibrate only vertically. It certainly won't happen as a result of bullet alignment with the bore. I think the upward recoil the Weatherby stock produces must be the ultimate cause. My scope will adjust enough to handle the difference easily and that is what I will do. I'm using the 300 grain for practice because they are cheap. Obviously I won't use them for hunting - probably won't hunt much with this gun anyway.
Another interesting fact. Today I ran the same kind of experiment on my .416 Weatherby MarkV. It was slightly different in that I used the same bullet, but very different powder charges. 350 grain Speer with charges of 85grain RL22 for 2190ft/sec, 110g RL22 for 2660, and 120.5g for 2840. In this case the POI are in the expected order (85 lowest, 110 next, and 120.5 at the top) but again the spread vertically is too large (about 14"). The group sizes were larger, but that may be due to a very strong wind shaking me around a bit. The results for the .460 were the same as before or a bit more (18"), but again the wind may have been responsible.

Jimro
A great suggestion and one I am going to try. I considered doing it today, but the wind was so strong I decided it would introduce too much error.
Thanks to all of you.
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Old April 14, 2013, 01:09 AM   #18
big al hunter
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Let us know what results you and your students come up with.

If it was recoil induced the heavier projectile would hit higher on the target, because it is in the barrel longer. Just as in handguns.

Hmmm....maybe we need a video in super slow motion, so we can see what it does.
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Old April 14, 2013, 09:41 AM   #19
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Quote:
I agree that it pretty much has to be vibration. The odd thing is that the rifle is a Weatherby MarkV and as such has a free floated barrel. It seems to me that it is difficult to get that to vibrate only vertically
Something is happening, but specifically what? When we talk about harmonics, we usually focus on the barrel, because that is where changes usually have the most obvious effect, but vibration includes everything about the rifle as well.

It could be that the entire barreled action is "flexing" in the stock enough to affect the radical point of impact shift. Since you mention another Mark V having a similar wide shift with different speeds of the same bullet, it might be something resulting from the way Weatherby's are built.

Several inches of impact shift are within the normal range at 100yds, with widely different loads, but shifts of over a foot? Now I am wondering if this is something Weatherby's do, or do in heavy calibers?

Here's something else to try (and keeping with only changing one thing at a time), try the same bullets with a different powder. Right now, all I see in common is Weatherby Mk Vs, RL-22 and a huge point of impact shift. Change the powder, and shoot. Similar results means we can eliminate that one factor, at least. Plus, its the easiest thing to change.

Anyone out there with a different rifle in these calibers? (yeah, I know, not likely)
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Old April 14, 2013, 05:37 PM   #20
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A 500 grain bullet at 2570 fps!?

You sure that 16" consistent lower impact is not due to a consistent flinch?

That's what I would be doing

Barrel harmonics are weird, with the amount of horsepower the action is absorbing, I would check to ensure that the barrel is not making anysort of contact anywhere, even if it is supposed to be "free floating", and I would bed the action in a 1/4" thick layer of stainless steel filled Devcon epoxy.
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Old April 14, 2013, 10:02 PM   #21
big al hunter
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Quote:
You sure that 16" consistent lower impact is not due to a consistent flinch?
The OP said he got the same results using a lead sled. Doubt he flinched with that.
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Old April 15, 2013, 01:31 AM   #22
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Can I just say you have one of the most awesome rifles known to man.
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Old April 15, 2013, 03:07 PM   #23
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If your rifle is bedded well and free floated, you are going to have a pretty significant barrel harmonic caused by the recoil impulse in the Y-axis. I'm imagining a springboard here. Depending on where your bullet comes out of the muzzle, you are going to see vertical change, with the barrel harmonic being in the vertical axis.

I wouldn't want to try this, but shoot the rifle sideways and see what happens. If you survive, tell us what happens.

This is a high-speed video of a 50 BMG. It has a totally different action (plus a lower center of recoil with less rotational effect) but seems to illustrate what I am thinking. Watch the scope flex (Y-axis) and then at the very end, watch the barrel do the same.
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Old April 15, 2013, 03:09 PM   #24
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Forgot the link.

www.youtube.com/watch?v=s5pVya7eask
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