Can someone explain horizontal POI shift when changing bullet weights

Vicko

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I'm trying to wrap my head around POI shift and need some references to read up.

Shooting a new to me 30-06 featherweight model 70 yesterday which seems to group fairly nicely.
Started with some 125gn loads that were proving to be particularly accurate, then jumped all the way to 180gn. Noticed that the 180gn were still grouping nicely, but the POI had shifted to the right by 4-4.5 inches at 100m... that seems absurd. Shooting both off bipods. No rear bag.
 

Shootnwrench

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Barrels bend and twist. Those two particular loads are releasing the bullet with the barrel pointed in different spots. Only way to know where POI will be with a load is to shoot it. Same weight doesn’t mean anything either.
 

Vicko

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So can anyone give me an explanation as to how and why that happens (in terms of left to right shifts of that magnitude)?
I'm keen to understand what's physically happening.
 

steelsnob47

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i dont understand the physics entirely, but ive been recently testing different 556 loads out of a 10.5 pistol and have noted large horizontal shifts with different ammo types despite remaining strangely consistent in vertical plane. im talking 4 inches at 50 yards sometimes. wouldnt have likely noticed or thought it was me if i hadnt decided to "bench" test these for accuracy. prior to these recent ammo tests i hadnt fired from a rest in years. mostly offhand standing usually.
 
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Jordanwickham

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I believe my math is right here but that much dispersion at 100m =1/20" at the muzzle.
I'm tired and that could be wrong.
If it is right that is pretty conceivable considering a 30-06 with an ultralight barrel is really going to shitwhip itself.
You can see POI shifts in the range of 3/4 easy in little calibers with heavy barrels just by changing powder charges .6 grains. That's the whole premiss of OCW, basically finding powder charges that shitwhip the least.
 
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Jordanwickham

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I believe my math is right here but that much dispersion at 100m =1/20" at the muzzle.
I'm tired and that could be wrong.
If it is right that is pretty conceivable considering a 30-06 with an ultralight barrel is really going to shitwhip itself.
You can see POI shifts in the range of 3/4 easy in little calibers with heavy barrels just by changing powder charges .6 grains. That's the whole premiss of OCW, basically finding powder charges that shitwhip the least.
I don't think that math is right lol. That would be for anything at 100 of anything; inches, meters, miles, ECT.
I think .001" is correct.
4/(39x100) inches into meters x 100
 
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paulwardd

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I've noticed that too, shooting .223 with 55gr vs 75gr, the 75gr shifted right and down about 2 inches at 100. Best guess is heavier bullet = more recoil, which moves the rifle to a bigger extent and therefore shifts. Just thinking out loud here, but maybe better recoil management?
 

Vicko

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But recoil in itself (without additional input that I now understand can be explained by harmonics) should really only effect things in a vertical plane shouldn't it? This is why I asked the original question.
fwiw - I did notice that even though I preloaded the bipod pretty well, the rifle jumped left on the heavier loads that went right. Hope that makes sense. I had my left hand under the butt stock. Wonder what would happen if I rested my hand on the scope. Is that why I see some shooters doing that?
 

Shootnwrench

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Dthomas3523

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I've noticed that too, shooting .223 with 55gr vs 75gr, the 75gr shifted right and down about 2 inches at 100. Best guess is heavier bullet = more recoil, which moves the rifle to a bigger extent and therefore shifts. Just thinking out loud here, but maybe better recoil management?
It’s not recoil.

Answer has already been given. The barrel moves. When you change anything at all, you may change the timing of when the bullet leaves the barrel. If the timing changes, the bullet exits the barrel as it’s “pointed” a different way than before.

This produces a POI shift.
 

Dthomas3523

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But recoil in itself (without additional input that I now understand can be explained by harmonics) should really only effect things in a vertical plane shouldn't it? This is why I asked the original question.
fwiw - I did notice that even though I preloaded the bipod pretty well, the rifle jumped left on the heavier loads that went right. Hope that makes sense. I had my left hand under the butt stock. Wonder what would happen if I rested my hand on the scope. Is that why I see some shooters doing that?
Hand on scope is used primarily when shooting on props. The balance point of the rifle is usually in the same area as the optic.

It’s not done to help with recoil management.

And no, recoil does not only affect vertical if you are not controlling recoil properly.
 
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eastexsteve

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I'm trying to wrap my head around POI shift and need some references to read up.

Shooting a new to me 30-06 featherweight model 70 yesterday which seems to group fairly nicely.
Started with some 125gn loads that were proving to be particularly accurate, then jumped all the way to 180gn. Noticed that the 180gn were still grouping nicely, but the POI had shifted to the right by 4-4.5 inches at 100m... that seems absurd. Shooting both off bipods. No rear bag.
I usually get 1 to 2 moa shifts right or left. I can only guess that slimmer profile barrels whip more.
 

Vicko

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Thanks folks. Thinking I will get a muzzle brake on it to help settle the recoil down a bit, I read that it might dampen the harmonics a little too.
Will keep putting rounds through it and trying different loads.
 

Dthomas3523

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Thanks folks. Thinking I will get a muzzle brake on it to help settle the recoil down a bit, I read that it might dampen the harmonics a little too.
Will keep putting rounds through it and trying different loads.
Harmonics are harmonics. Even if a brake “dampens” it, it’s still going to change poi from load to load.
 

hlee

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My bull barreled 308 has a 3” horizontal shift when switching from 168 Amax to 168 tsx loads. Note it and drive forward. Don’t make this harder than it is.
 

Jordanwickham

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A muzzle brake dampens your recoil. It in itself will change your POI even will same load. I would just dial out the difference there's no magical solution to POI change.