What Does This Change In Impact Point Mean?

Theory#1

Your barrel resonates as the bullet travels down the muzzle. Literally flips up and down. Precision rifles are designed so that with the same load the harmonics are the same--it flips roughly the same way every time, and you zero based on the degree the rifle barrel is pointing when the bullet leaves the muzzle. As long as it's repeatable you will get the same POI.

Basically this different round is causing different harmonics and the muzzle is at a different point in the arc when it leaves the barrel. It has a slightly different weight, slightly different velocity and slightly different pressure so it's going to cause the barrel to have different harmonics and the muzzle will be pointing a different direction when the round leaves the barrel.

You can also see POI shift left and right.

Harmonics is 1 answer.

Recoil movement is another. In most firearms the slower heavier bullets tend to hit higher then their lighter faster counterparts. It's known as dwell time of the bullet in the bbl. The longer the dwell time the better the chance of the bullet being affected by muzzle flip/movement from recoil.

With that 16" bbl you're looking at +/- .6/1000"s of an inch of movement in the bbl to get a difference of that 1.5" point of impact.

Different bullet,different powder,different POI. It's that simple .

IF one finds no other indications of pressure, abuse to rifle, and so forth,
IF one finds the accuracy - group size - is suitable, and
IF one plans to use this load as the primary load in that rifle;

Readjust the sights on the rifle to shoot the new ammunition as desired and continue to march.
One might make a note of how to move the sights back to the prior ammunition zero. The sights ARE adjustable and one may have to adapt over time.

Sometimes you get lucky but more often than not 2 different bullets will not shoot to the same point of impact. Sometimes when you are handloading 24 grains of powder will shoot an inch different point of aim than 23 did. Its not unusual for a heavier bullet to hit higher than a lighter bullet either because your on the upward swing of the barrel harmonics instead of the the downward swing or because the rifle is recoiling while the bullet is still in the barrel and a heavier bullet spends more time in the barrel and exerts more recoil. Your see this really dramatically in magnum revolvers where a 158 grain 357 magnum will sometimes shoot like 6 inches higher at 20 yards than a 125 grain 38 special.

The worst rifle I have ever seen for this is my 1898 krag. I have loaded a bunch of different bullet weights and powders in that rifle and there will be a foot difference at 100 yards between a 150 grain and a certain powder and 180 grain with a different powder. Even with one bullet and different charge weights you can easily push the point of impact 6 inches just with different charge weights. It took a lot of experimentation and trying different stuff with that rifle to find an accuracy node that shot well over a decent variation of charge weight and temperature. Otherwise I was always chasing the point of impact around with the scope when the temp would change.

Bullets are NOT impacted by recoil. The recoil process doesn't happen until the bullet has departed the muzzle(Newtons third law of motion). Dwell time impacts harmonics which in turn can affect accuracy, but recoil does NOT impact physical accuracy in the projectile. Obviously it CAN impact the shooter in the way of a flinch which can and will affect accuracy, but the bullet is already well out of the barrel before recoil happens.

CryptKeeper5 said:

Bullets are NOT impacted by recoil. The recoil process doesn't happen until the bullet has departed the muzzle(Newtons third law of motion). Dwell time impacts harmonics which in turn can affect accuracy, but recoil does NOT impact physical accuracy in the projectile. Obviously it CAN impact the shooter in the way of a flinch which can and will affect accuracy, but the bullet is already well out of the barrel before recoil happens.

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That is incorrect, there is no delay in physics. You cannot accelerate the projectile without an instantaneous reaction in the opposite direction.

I made this video a few years ago to demonstrate this. You can see the muzzle has moved up about a mm before the bullet exits the barrel.

I'd rather see the same demonstration from a hard rest...

CryptKeeper5 said:

I'd rather see the same demonstration from a hard rest...

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That's not going to change anything. If you think its the shooter moving the gun watch the gun moving and the hand remaining stationary.

If this were true, then stretch that barrel out to 20" as in a .44rifle and you'd have 5mm of upward movement roughly assuming the same load beginning velocity. EVERYTHING I've ever read or observed has told me it's had zero affect on accuracy. But everyone's entitled to their own opinions...No matter how steady you think you are, I can't accept a free hand experiment as gospel.

I have observed this same effect with a rimfire rifle on a mechanical rest.. Thanks for explaining it. I was puzzled as to why it happens. I have also noticed that 5,56 have a very different POI than .223 does. Yes dwell time is a real thing as are the effects of recoil.

CryptKeeper5 said:

I'd rather see the same demonstration from a hard rest...

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Ditto that. Takes out most of the variables.
With most of the rifles I shoot, 1.5 inches at 100 yds isn't something I even consider worth doing something about, hah hah. But seriously, bench-rest the rifle and repeat. Then you can address other variables. As an aside, I talked with a guy at the range today who was using a Caldwell "Lead-Sled" and I was pretty impressed with the setup, and he was getting great groups.

I'd need to see the same experiment repeated with a faster camera(the bullet is blurred so it's moving faster than the frame rate where as the pistol is moving slower), a sealed rifle chamber and a rest to convince me. This could be an optical illusion and certainly not enough to draw hard conclusions. Over 40 years and roughly 500k rounds of shooting including in the military, and competitive pistol, long range and high power, I've never concerned myself with recoil in this context. Even when firing large caliber, high recoiling rifles and pistols. Cool video though...

I don't believe that, out of a rifle, the OP's modest changes in bullet weight and inconsequential change in dwell time have any effect on recoil displacement, but...

CryptKeeper5 said:

If this were true, then stretch that barrel out to 20" as in a .44rifle and you'd have 5mm of upward movement roughly assuming the same load beginning velocity. EVERYTHING I've ever read or observed has told me it's had zero affect on accuracy. But everyone's entitled to their own opinions...No matter how steady you think you are, I can't accept a free hand experiment as gospel.

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Do the math. Angular inertia is a function of the mass X the rotational radius squared. Make it easy and assume center of mass (COM) at the midpoint of the gun: 2.5#, 8" handgun vs. 7.5#, 40" rifle. Three times the mass and five times the COM radius. 3 x (5 squared) is 75 times the angular inertia. Even 5 times (assume 5 times the angular energy imparted) the dwell time doesn't begin to overcome the inertia to make dwell time and vertical displacement a straight ratio.. Over simplified yes, but physics is what it is.

CryptKeeper5 said:

Bullets are NOT impacted by recoil. The recoil process doesn't happen until the bullet has departed the muzzle(Newtons third law of motion).....

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Maybe so, but Newtons third law would seem to make it likely... being the opposite of impossible. Equal and opposite reactions happen along the same timeline. So with the time that a bullet is in the barrel, something must be happening on the other end.

Seems to me, harmonics in a short, very, very stiff (did I say short) handgun barrel, don't well explain why different loads shoot to such different POI. Recoil impulse seems the more likely culprit. But obviously that can't be since bullets aren't impacted by recoil

CryptKeeper5 said:

If this were true, then stretch that barrel out to 20" as in a .44rifle and you'd have 5mm of upward movement roughly assuming the same load beginning velocity. EVERYTHING I've ever read or observed has told me it's had zero affect on accuracy. But everyone's entitled to their own opinions...No matter how steady you think you are, I can't accept a free hand experiment as gospel.

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its not an opinion, it’s physics. And no it would not scale that way because a 20” long rifle is heavier and longer and because of that it has a much greater moment of intertia, and the center of gravity is closer to the bore axis. A handgun will rotate around its center of gravity more than a rifle will.

Lets just say I'd need to see a LOT more PROOF than a singular homemade video and general high school physics principles to convince me to change my mind. We'll just leave it at that.

Go do your own experiment then. I gave you proof that you can see with your own eyes. Do the math on how much the muzzle has to rise relative to the breech on a 20” barrel to move the bore axis 1” at 100 yards.

I have no need, want or desire to conduct one. Because in my real world practical shooting experience recoil has no affect on bullet placement. "Proof" is far more than a single video by a single person. Of the books I've read regarding rifle and pistol accuracy I've yet to read one stating recoil in THIS context has any measurable affect. I'm not here to get into a pissing match about it. You have your opinion on what you see I have mine. Until this experiment is repeated and done in a more controlled environment...I'll remain the skeptic.

I don’t have an opinion, I have an understanding of the laws of physics. If your not willing to do any experimenting or research or reading then maybe you shouldn’t be refuting statements on the subject.

Look, you have a grainy video of a bullet exiting a barrel at higher speed than your shutter/frame rate. Until I see a crystal clear picture of both in a CONTROLLED environment I say you have little more than your opinion. You wanna have a physics debate on an amateur firearms forum? Wrong place, wrong time....You shouldn't be dealing in absolutes after not adhering to the basics of the scientific method. Repeatability is paramount. I've wasted enough time on this...

The video is merely supporting evidence to demonstrate what physics says must be happening. You cannot accelerate an object without generating a force in the opposite direction. I can explain it to you but I can’t understand it for you.

AAAHHHH, They said no math!!!!

Amazing:
Take the 357 caliber and load up full house loads using H110/ww296/AA#9/2400 and load up 110gr/125gr/180gr/200gr bullets and shoot them in a 2 1/2" bbl/4"bbl/6"bbl/10"bbl firearm For some odd reason the lighter bullets will always hit lower (powder choice doesn't matter) when using the same sight picture/scope with the same settings.

Odd that the the heavier bullets will always hit on the up swing of the barrels harmonics in every firearm!!! Don't think so??? Go to the revolver forum and ask real world owners and what they have tested/learned & the sights they've bought or changed and why.

Same with a 16"/20"/24" bbl'd 223rem Shoot 52gr bullets VS 80gr bullets and full house loads. Doesn't matter the firearm or powder, the lighter bullet will impact lower.

Must be nice to be able to always have the heavy bullets leave the muzzle when the harmonics are at the high end/upward swing.

(Newtons third law of motion).

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Requires the equation to be balanced instantaneously.
Otherwise I would fire howitzers offhand and just put them down quickly before they recoil.

I find it amazing that someone, with even a lot of “experience”, that is not remotely concerned about the actual physics of barrel dwell time and bore axis attitude, refutes known science so ferociously.

And when found wrong, decided a discussion forum wasn’t a place for the discussion, or just didn’t have the time to waste. Except in the beginning, when they thought they were going to browbeat their incorrect assumptions to humans that understand with their unprovable merits.
Even the amateurs get it…

someguy2800 said:

I can explain it to you but I can’t understand it for you.

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Proof a human can do a thing, even compete successfully (though I wouldn’t accept proof of it), and not understand it.