Reasons for lateral POI shift II

[dmike]

Turbo54's thread on this from 3 years ago didn't ever get a conclusive answer.

Yesterday I was working load dev at 400 and to warm up I tried some of the Remington Premier Match 168s. POI was about 1.5 MOA right of POA. I was thinking maybe my scope was off somehow but I left it. Switched to my loads, the 178 AMAX were right where I expected them to be. I don't buy that harmonics are whipping my barrel laterally like that, if the action is torqued in the z most whip would be in the z (vertical).

I'm wondering if this is somehow related to spin drift? Could some property of the 168 give it more traction, where the 178s are slipping more? Let's say the bullet is making about 14k revolutions during the flight. Circumference is nearly one inch. Doesn't that mean that a miniscule change in surface friction could make a big difference in lateral poi shift? Softer jacket with more rifling ridges for example. Isn't it a little like a belt sander with an abrasive speed of about 15000 fps? Some abrasives catch more than others.

Velocity was close on both rounds, without looking at the chrono I think they were both mid-2600s.

700 SPS varmint in 308
Manners t4 with dbm torqued to spec
bipod

Any thoughts?

 

[djskit]

I have challenges in this area as well. I have narrowed it done to how my right hand interacts with the stock at shot break and through recoil. But I can see it even at 100 yards as 1" of shift left or right from POA on any give shot. I'll eliminate it for a time, then it rears it's ugly head as it did in our steel match yesterday. Makes understanding the wind corrections a bitch.

 

[dmike]

I have challenges in this area as well. I have narrowed it done to how my right hand interacts with the stock at shot break and through recoil. But I can see it even at 100 yards as 1" of shift left or right from POA on any give shot. I'll eliminate it for a time, then it rears it's ugly head as it did in our steel match yesterday. Makes understanding the wind corrections a bitch.

I am undoubtedly (and unwittingly) applying counterproductive pressures to the weapon...prior to shot, at break, and through recoil. It will take me years to identify all my bad habits and address them. But this poi shift is ammo-specific and repeatable, whatever bad habits I have don't account for why one bullet shoots 1.5 MOA right of another.

In terms of spin drift, at the bullet's RPM rate it seems like a 0.0005% increase in the friction (actually traction) of the jacket could cause a 6" lateral shift at 400yds.

 

[Lowlight]

You're smoking crack... Spin drift at 400 yards, and now 6" of it.

Ya, has to be that, science doesn't lie right.

the idea of selling people on SD is the bane of the shooting community.

 

[Bender]

I am 100% sure it was Coriolis effect. No, 110%. You need the newest scope with the Coriolis button that negates earth's rotation. I have one to sell you, comes with a NV bridge.

 

[Sterling Shooter]

I am undoubtedly (and unwittingly) applying counterproductive pressures to the weapon...prior to shot, at break, and through recoil. It will take me years to identify all my bad habits and address them. But this poi shift is ammo-specific and repeatable, whatever bad habits I have don't account for why one bullet shoots 1.5 MOA right of another.

In terms of spin drift, at the bullet's RPM rate it seems like a 0.0005% increase in the friction (actually traction) of the jacket could cause a 6" lateral shift at 400yds.

Here's the short answer to your question: when any principle, element, or factor of shooting is different from shot to shot then a different result should be expected. Changing a bullet changes recoil resistance. This is exacerbated with any other ammunition component substitutions and bullet impact somewhere other than where aimed should be expected. Now take a look at your grouping when nothing is perceived to be different from shot to shot. Most likely it is still nothing more than a grouping of misplaced shots, that's to say, for most shots, the line of sight did not actually intersect the bullet path. How can this be? It shows even when you think your position has been rebuilt exactingly you do not have the sensitivity to understand when you have perfected the position. Thing is, you will likely never be able to rebuild the position exactingly enough to actually realize a literal zero, so understanding the total effect of component swaps is not likely to be fully understood either, your position inconsistency masks a full understanding. With this being the case, for the best results with whatever skill you bring to the firing line, annotate trends in your data book and adjust sights accordingly. You do not need to contemplate the specific cause of error in your process, only the need to be consistent and adjust sight when trends show the need. In your specific case, you know the bullet shoots to a different point than where called. O.K. so just adjust the sight and move on.

 

[dmike]

Thanks for the replies.

Frank: maybe we need a "stupidest marksmanship questions" thread for shit like this?

Bender: classic. Don't forget the tide chart.

Okay, SD is horseshit. I didn't know. Got it.

Sterling: Great points.

So if we had a master shooter on the gun and someone else randomly load the mag with 5 of each, take out wind, is a 1.5 MOA lateral shift between ammo types possible? I know I'm inconsistent (my point in post #3) but what's the most lateral shift you've seen that's not attributable to the shooter? I'm just trying to understand the primary factors (non shooter/position/wind related) in lateral shifts.

thanks guys

 

[Sterling Shooter]

Thanks for the replies.

Frank: maybe we need a "stupidest marksmanship questions" thread for shit like this?

Bender: classic. Don't forget the tide chart.

Okay, SD is horseshit. I didn't know. Got it.

Sterling: Great points.

So if we had a master shooter on the gun and someone else randomly load the mag with 5 of each, take out wind, is a 1.5 MOA lateral shift between ammo types possible? I know I'm inconsistent (my point in post #3) but what's the most lateral shift you've seen that's not attributable to the shooter? I'm just trying to understand the primary factors (non shooter/position/wind related) in lateral shifts.

thanks guys

You cannot disassociate yourself from execution of the firing tasks unless you put the rifle in a jig. The reality is you have contact with the gun and the ground and this contact must be controlled consistently. When your arc of movement is not controlled and/or recoil resistance is made different you will get a different result. The amount or magnitude of the result a different bullet will make is commensurate with the amount of difference between bullets. Regarding the shooter, who knows how much the effect will be from any changing relationship between himself, gun, and ground. Some changes like wobble have little effect, while angular error can cause a gross miss at extended distance. What matters is that you record calls and shots, annotating trends, and change sight setting to accommodate trends. At some point you will have a data book full of zeros for various wind and weather conditions for each rifle/loading you have taken the firing line.

 

[Bender]

Reasons for lateral POI shift II

Here's the short answer to your question: Aliens

Fixed it.

 

[Sterling Shooter]

Here's the short answer to your question: Aliens

Fixed it.

Not aliens, undocumented immigrants.

 

[dmike]

Stupid marksmanship answers.

Fixed it