Range Report Lateral POI shift between loads

[turbo54]

Anyone care to toss around interior/exterior ballistic thoughts concerning drastic POI shifts from one load to the next?

Yesterday, I was shooting at 400 yards with my new A3G. After printing a 6 round group of 2" with some 175smks over 45gr Varget in virgin Lapua brass, I switched ammo to FGMM 168. I was shooting at a target about 14"x14", and WRONGLY assumed I would still be on paper. I fired a 10 round "group"...only to find 2 shots just BARELY scratched the far right edge of the paper. They hit about 2" low, but probably 8 or 9" right.

I've seen this phenomenon before, so im not super surprised (disappointed with waste of ammo though), but wonder what dynamics are involved with causing it.

I assume it has to do with the torsion in the barrel caused by a bullet being forced to turn by the rifling...if so, im amazed that torsion would cause an 8" lateral shift in POI.

 

[TiroFijo]

Re: Lateral POI shift between loads

Wow! that's a large POI shift. Normally I would expect about half of it.

The cause is the barrel "whips" during firing (harmonic vibrations) in the vertical AND the horizontal plane as well, for several reasons. If the bullet departs the barrel at a different time (and position) the orientation is different from your baseline zero.

 

[MALLARD]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">I've seen this phenomenon before, so im not super surprised (disappointed with waste of ammo though) </div></div>
hahaha , 10 rounds a waste of ammo ? i think not or i must waste allot of ammo

14x14 , if his group was dead center , he only drifted 7-10 inchs or 2moa as a result of the change in ammo , which really isnt to bad, at least not to me.

my immediate advice would be to try again , and see if the same thing happens the same way , at least then you would have something solid.

why , no idea , fouling affected your poi i guess. Or something to do with OCW/OBT ? hopefully someone smarter then me will chime in.

I see POI/POA shifts when i adjust charge weights using the same bullet/powder.

 

[BryanLitz]

Re: Lateral POI shift between loads

I would check the bedding to see of you can spot anything that's binding or crooked. Vertical POI shifts are totally normal between loads, but lateral is less common.

Bore curve might cause that effect even if the bedding is OK. Bore curve isn't necessarily bad for accuracy as long as you can zero your group.

The above is my non-gunsmith hunch. You might want to run it by a knowledgeable gunsmith to see if it sounds reasonable or if he has other ideas.

-Bryan

 

[turbo54]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">I would check the bedding to see of you can spot anything that's binding or crooked. Vertical POI shifts are totally normal between loads, but lateral is less common.

Bore curve might cause that effect even if the bedding is OK. Bore curve isn't necessarily bad for accuracy as long as you can zero your group.

The above is my non-gunsmith hunch. You might want to run it by a knowledgeable gunsmith to see if it sounds reasonable or if he has other ideas.

-Bryan </div></div>

I hope there is nothing wrong with the rifle/bedding! It's an expensive piece, and was bedded by GAP! Yesterday I ran some ladder tests using various loadings with 168, 175 and 180 SMKs. Fired from 600 yards, I was seeing only 1/2 minute shifts in POI.

As mentioned, it would be interesting to try this again and see if it repeats. I think it will.

 

[pcschwenke]

Re: Lateral POI shift between loads

My 5R milspec does exactly the same thing as you describe with the 168 & 175 Federal loads. The Hornady 168 TAP shoot to the same point of impact as the Federal 168 smk's.

Paul

 

[pcschwenke]

Re: Lateral POI shift between loads

I just checked my log book to get the specifics. The 168gr shot 1.25" to the right at 100 yards. At 400 yards is about 5 inches. Hope this helps. Both shot at .5 MOA or better.


Paul

 

[remr5]

Re: Lateral POI shift between loads

I'm not an expert but I know with my rifle my poi is different with my 168 and 175 Federal ammo. Just for fun try shooting two groups of the different ammo at 100yds. Use two of the same targets but shoot one group on each. Measure the change of impact and then multiply accordingly out to 400 yds.

You don't have to do this but what I'm getting at is because the velocity and or properties of the rounds are different they will not be the same poi. As distance increases the difference magnifies, so difference of two inches at 100 yds will be eight inches at 400 yds( if my math is right).

If you know your dope for each round at your point blank range you can adjust when you change rounds.

In closing each bullets acts the way they do because of design (bc) velocity ect.

 

[remr5]

Re: Lateral POI shift between loads

The velocity and or properties of the rounds are different they will not be the same poi. As distance increases the difference magnifies, so difference of two inches at 100 yds will be eight inches at 400 yds( if my math is right).

If you know your dope for each round at your point blank range you can adjust when you change rounds.

In closing each bullets acts the way they do because of design (bc) velocity ect. Then add in barrel whip ans all the other good stuff.

 

[aqualung]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body">I would check the bedding to see of you can spot anything that's binding or crooked. Vertical POI shifts are totally normal between loads, but lateral is less common.

Bore curve might cause that effect even if the bedding is OK. Bore curve isn't necessarily bad for accuracy as long as you can zero your group.

The above is my non-gunsmith hunch. You might want to run it by a knowledgeable gunsmith to see if it sounds reasonable or if he has other ideas.

-Bryan </div></div>

^^ What does he know??




Sorry Bryan, I just couldn't resist.

Loved your book, just a couple of questions...

Neil

 

[aqualung]

Re: Lateral POI shift between loads

OK,

Here's a curveball for you. 7mm-08Rem. 150grn Sierra Matchkings, 41.1grn Varget. Zero'd at 100 yards, spot on.

Same rifle, same day, (within minutes), same cases, same primers, same tub of Varget, measured in the same way, but this time it's a Sierra 150grn Gameking. Impact shift of 2" laterally, but elevation is the same at 100.

Go figure.

Oh, switched back to the Matchkings, back on zero! Back to the Gamekings, yup, 2" right again, right on top of the previous Gamekings.

I have no idea, but <span style="font-style: italic">anybody</span> got any ideas?

Over to you Bryan!

Neil

 

[turbo54]

Re: Lateral POI shift between loads

If you define the length of the bore as the x-axis, and the horizontal plane as the x-y plane, only forces in the y direction can cause lateral shifts. The way I see it, as the bullet obturates into the rifling, it causes the barrel to stretch out and become longer. Also, the barrel is twisted in torsion as it reacts the forces of the bullet twisting in the rifling.

I assume these forces contribute to the lateral POI shifts.

My guess is that a smoothbore wouldn't exhibit the same behavior, but of course, a smoothbore isn't accurate enough to prove it!

By the way, I've also noticed a shift between 180smk and 180gameking, loaded identically.

 

[BryanLitz]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Paduan Learner</div><div class="ubbcode-body">OK,

Here's a curveball for you. 7mm-08Rem. 150grn Sierra Matchkings, 41.1grn Varget. Zero'd at 100 yards, spot on.

Same rifle, same day, (within minutes), same cases, same primers, same tub of Varget, measured in the same way, but this time it's a Sierra 150grn Gameking. Impact shift of 2" laterally, but elevation is the same at 100.

Go figure.

Oh, switched back to the Matchkings, back on zero! Back to the Gamekings, yup, 2" right again, right on top of the previous Gamekings.

I have no idea, but <span style="font-style: italic">anybody</span> got any ideas?

Over to you Bryan!

Neil </div></div>

That's an easy one

You see, the GameKings are <span style="font-style: italic">killing</span> bullets and MatchKings are <span style="font-style: italic">target</span> bullets. Everyone knows that when you shoot the <span style="font-style: italic">killing</span> bullets, you have your warrior face on, and that affects your stock-weld, which affects the lateral pressure on the comb of the rifle stock. That's where the lateral POI shift comes from

But for real, I got nuthin. All my speculatin' is based on different weight bullets with different pressures, barrel times, etc. Same charges with same weight bullets...? Ask a gunsmith? Let us know what they say.

-Bryan

 

[sinister]

Re: Lateral POI shift between loads

Seriously?

Record different zeroes for different ammo out of the same rifle.

Just in 308 the same rifle will put ball, tracer, 155s (two different Sierras, Hornady, Berger, and Lapua), 165s (Game King hollow and soft points and Nosler Ballistic Tips), 168s (Sierra, Hornady, Berger, and Lapua), 175s, and 180s in different places OUT OF THE SAME GUN WITHOUT TOUCHING THE SCOPE.

Besides different velocities and weights the bullets will all have different ogive and boat-tail profiles, contact and bearing surface area, and jump to rifling. Shoot ten-shot groups of each cartridge above all out of the same gun on to the same target without touching the turrets and look for a composite group -- duh, the rounds will not print into the same hole.

The angular variation will look worse the farther out you go.

 

[turbo54]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sinister</div><div class="ubbcode-body">Seriously?

Record different zeroes for different ammo out of the same rifle.

Just in 308 the same rifle will put ball, tracer, 155s (two different Sierras, Hornady, Berger, and Lapua), 165s (Game King hollow and soft points and Nosler Ballistic Tips), 168s (Sierra, Hornady, Berger, and Lapua), 175s, and 180s in different places OUT OF THE SAME GUN WITHOUT TOUCHING THE SCOPE.

Besides different velocities the bullets will all have different ogive and boat-tail profiles, contact and bearing surface area, and jump to rifling. Shoot ten-shot groups of each cartridge above all out of the same gun on to the same target without touching the turrets and look for a composite group -- duh, the rounds will not print into the same hole. </div></div>

I think some of you may have missed the objective of this thread. I didn't start this with the purpose of asking if it was normal, or if it was OK etc etc. I started it because I wanted to begin a discussion of what dynamics and forces work together to cause a 9" lateral shift at 400 yards.

 

[TiroFijo]

Re: Lateral POI shift between loads

Turbo 54, there are other issues at work, like bolt lug contact, action flex (it is not symmetrical), recoil lug fit, bedding, head space, how tight and uniformely seated is the case in the chamber, etc. that are probably more important than torsion due to the rifling.

If you are interested in the theory, get a copy of "Rifle accuracy facts" by H. R. Vaughn.

 

[gstaylorg]

Re: Lateral POI shift between loads

You see, the GameKings are <span style="font-style: italic">killing</span> bullets and MatchKings are <span style="font-style: italic">target</span> bullets. Everyone knows that when you shoot the <span style="font-style: italic">killing</span> bullets, you have your warrior face on, and that affects your stock-weld, which affects the lateral pressure on the comb of the rifle stock. That's where the lateral POI shift comes from

Very nice, Bryan. Bait the man at your own risk, Paduan! What chapter is that in by the way...I must read more about the effect of "warrior face" on trajectory when I get home (LOL).

Turbo54, did you record MVs for the 175/Varget and FGMM loads by any chance? Although not nearly as large in magnitude at the shift in POI you described, I have seen a similar result when switching from FGMM 168s to 175s (all other conditions essentially equal). It has been suggested above that barrel harmonics could be the reason for this observation. However, (and this is just speculation on my part) that doesn't seem like the most likely explanantion to me. I've watched plenty of slo-mo videos of rifle barrels during firing. Certainly there is harmonic movement during this process, however it appears to my eye as though the magnitude of barrel motion in the horizontal is always much less than that in the vertical. Maybe I'm getting the wrong impression due to the views shown in the videos (mostly from the side), however there are some you can find from above and behind and they look the same. If you buy that argument, the a horizontal shift in POI of the magnitude you described should be due to some other factor(s)???

So what other differences between the two rounds might account for the lateral shift in POI? Now this is where I'm on shaky ground, so don't flame me, I just think it is an interesting question and am trying to continue the discussion of said phenomenon. What I keep coming back to in my mind is an effect after the projectile has left the barrel. Possibly different spin rate, possibly a greater effect of wind on one round than the other, a combination of both, or ??? That is why I asked about the MVs of the two loads. My other question would be did you compare the two loads at closer ranges? If you were 8-9" right at 400 yd, that translates to about 2.0 MOA. If the difference were purely external, I might expect to see very little lateral shift at closer range between the two loads, such as 100 yd. By analogy, it should become even greater at 500 or 600 yd.

On the other hand, if the lateral shift was due to barrel harmonics or some other (internal) issue that was differentially affecting trajectory between the two projectiles within the barrel, and/or at, or very near the muzzle, it seems as if you should expect to see a shift at 100 yd of similar magnitude (possibly a little smaller, but close to 2.0 MOA). I know that's not an answer, but it might at least help you to narrow down the possible causes. Anyhow, if you come up with the answer, be sure to let us know, this is a very interesting question.

 

[turbo54]

Re: Lateral POI shift between loads

I didn't have my chrono set up that day, so no MV's... I don't think there was enough wind that day to account for the shift, and all 16 shots (6 of the 175smk/45Varget, and 10 of the FGMM 168) were fired inside about 20 minutes. The wind coundn't have been two bad, being I was getting 1/2 MOA from the 175/Varget loadings.

Thinking a little more - doesn't seem there should be much reason for a barrel to tend to whip up and down more than side to side? The way I see it, there are two forces at play that would affect the barrel up/down and not side/side:

1. Gravity
2. The moment created because recoil isn't reacted by your shoulder through a straight line of action directly back from the barrel, but instead through a line of action passing through the recoil lug and your shoulder.

I'm curious enough that I intend to print some groups again to show you guys. I'm confident this phenomenon will repeat.

 

[gstaylorg]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body"> 2. The moment created because recoil isn't reacted by your shoulder through a straight line of action directly back from the barrel, but instead through a line of action passing through the recoil lug and your shoulder. </div></div>

This is along the lines of what I was thinking, and I see what you mean about wind not being a likely factor.

If you purposely allow the weapon to hop on the bipod, the tendency for upward (rather than sideways) movement is always markedly more noticeable, for me at least. There can be a slight horizontal component to a purposely allowed hop, but it seems to be present mostly because I wasn't properly positioned behind the weapon, whereas the vertical component will always appear if you don't load the bipod correctly.

I had always assumed this was because of higher momentum in the vertical plane around the horizontal axis through the center of gravity (not sure I stated that clearly, but analogous to idea of having recoil push the barrel to the right rather than up if you fire the weapon turned 90 degrees on its right side). Horizontal momentum (causing lateral shift in POI) around the vertical axis should be comparatively less unless you're not lined up behind the weapon properly. It will be interesting to hear whether you see a similar difference at closer range, or can pinpoint the actual factor underlying the difference in the two loads. Anyhow, thanks for the interesting post.

TiroFijo - thanks for the tip on the Vaughn book, just ordered it and it looks like a good read.

 

[sinister]

Re: Lateral POI shift between loads

A .308 Winchester caliber rifle locked into a mechanical rest with linear bearings or free-travel V-block is going to recoil consistently.

You are still going to get horizontal and vertical displacement due to different bullet profiles, weights, and velocities.

I've had up to two minute (right) variations in POI between 168 SMKs and 180 Ballistic Tips in the same rifle at 300 Meters.

 

[remr5]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sinister</div><div class="ubbcode-body">A .308 Winchester caliber rifle locked into a mechanical rest with linear bearings or free-travel V-block is going to recoil consistently.

You are still going to get horizontal and vertical displacement due to different bullet profiles, weights, and velocities.

I've had up to two minute (right) variations in POI between 168 SMKs and 180 Ballistic Tips in the same rifle at 300 Meters. </div></div>

+1

 

[damoncali]

Re: Lateral POI shift between loads

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">
Thinking a little more - doesn't seem there should be much reason for a barrel to tend to whip up and down more than side to side? </div></div>

There is a good reason that the barrel moves up-down more than left-right. It has to do with the load path (bullet to bolt to lug to stock to shoulder) not being in line, and the fact that rifles are far more symmetric from side to side than up to down. (The distribution of mass matters).

If you're interested in this, read Harold Vaughn's book, "Rifle Accuracy Facts". It's good stuff.