Change of Point of Impact

D_Bell

Active member
I have been working on a load for my 223 over the last week, I compared targets from 2 separate outings with the exact same load specs:

OAL
Cases
Powder and Powder weight
Primers
Bullets

Bullet impact on the first target was at the 4 o'clock position from the bullseye, and the second target the impact was at 7 o'clock position from the bullseye. Both groups were under 1/2".

I confirmed my scope rings, bases, and action were all tight.


***This is not the first time I have noticed this, and this has happened with different rifles.

Has anyone else had similar issues like this?
 
It can be anything from light/mirage, different placement of a support bag on the forearm or worst scene scenario if the barrel has pressure from the forearm as in non-free-floated. I've seen light angles come into play much more than I ever realized until I was taking a class in becoming an M/O instructor.

Greg
 
I've seen heat warp do that but the typical scenario is the first group is shot fairly quickly then there is a pause in the shooting as the heat spreads then the second group is shot into the new POI. With your two groups being so tight heat warp is unlikely unless you see a little horizontal stringing within the two groups.
 
Yes, I was at the range at different times of the day, and both times there was little to no wind.

Both groups were shot as the second string of rounds down range.
 
Yep, w/no wind, could be,light direction/intensity, different placement on bags/shooter's hold on stock, cheek pressure and probably a couple more..........

Regards,
hm
 
How about ambient presser or temperature.
Both could make a difference. You did not say how much the groups moved a 1/2 inch 5" ????
 
3 questions:
What rifle is it? I have a Rossi that did this, I finally quit shooting it.
Was there possibly a change in the sizing die setting?
Silly question; is it possible the target was rotated 90 degrees from the first outing?
 
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Originally Posted By: jetman How about ambient presser or temperature.
Both could make a difference. You did not say how much the groups moved a 1/2 inch 5" ????

The difference was almost an inch.
 
Originally Posted By: reloader3263 questions:
What rifle is it? I have a Rossi that did this, I finally quit shooting it.
Was there possibly a change in the sizing die setting?
Silly question; is it possible the target was rotated 90 degrees from the first outing?

The rifle is a REM 700 in a 223.

All the brass was sized with the same FL sizing die at the same time.
 
Originally Posted By: D_BellOriginally Posted By: jetman How about ambient presser or temperature.
Both could make a difference. You did not say how much the groups moved a 1/2 inch 5" ????

The difference was almost an inch.

Personally, I would blame a 100 yard 1" shift in POI (off good bench/rest) and steady/no wind condition as you described on (most likely):

1) Light. I found that bright sunlight, especially from 4 o'clock affected windage noticeably (w/aperture front & rear sight). Adding a "blinder" to shooting glasses corrected that issue. Side light seemed to open my groups, not necessarily shift POI.

Here's a pretty good discussion on the old saw "Light's up, sights up", which definitely affects elevation w/post front sight. (Personally I was not able to document w/aperture sights).

Quote:Where I first heard the 'lights up/sights up. lights down/sights down' was with shooting NRA High Power with a Service Rifle i.e. post front sight. The common explanation at the time was that as the light gets brighter (sunny day) the black bullseye (all we can really see with the naked eye) gets a bit 'washed' and the lighter colored 'white' (buff) area gets brighter - making the bull appear slightly smaller. Correspondingly, when the sun goes behind a cloud or on an overcast day, the black seems darker and 'bigger'. Where this affects the aiming is with a six-o'clock hold (pumpkin on a post) as the apparent 'edge' of the bull changes location slightly. The amount is fairly small - 1/2-3/4 moa typically, but enough that the mantra took hold. Some shooters use a center-mass hold in an effort to counter or at least minimize the effect. In theory (assuming this theory is correct) this shouldn't really affect people shooting with aperature sights (as Keith mentioned) or with a scope, as any one 'edge' of the bull or aiming point doesn't really matter any more.

A few years ago I noticed it still seemed to bite me even when shooting with match rifle (aperature, or circle-inside-a-circle-inside-a-circle) sights, or even with a scope (Any Rifle/Any Sight). Not as much, but still *some*, and definitely repeatable. I asked around a bit and never really got a solid answer other than some people noticed it, some didn't. A few theories about differences in mirage off the ground displacing the target image optically, etc. but nothing really accepted as firmly the 'lights up/sights up' mantra. Usually someone cites examples of leaving a scope zeroed on a target from early in the morning til later in the day, checking periodically and seeing the crosshairs moving all over the place (I'd really, really, REALLY like to see a condensed video clip of that!) so the mirage theory may have some merit. Others debunk it by claiming the observed movement is from heating of the tube (or the gun if its mounted on one). Add into that some ranges get weird thermal updrafts and down drafts and it gets hard to separate the theory from the real world effects.

In the end... shooting and observing the effects in the real world are probably the best solution. Taking notes definitely helps, but keep in mind what applies at one range might not at another. Some days every shot is a sighter...

2) Rifle recoiling differently from one group to the next due to:
a] Placement of rifle on the bags different (watch sling studs hanging on bags, etc.)
b] Cheek pressure on stock
c] Shooter's grasp of rifle (lots if little differences such as butt pressure on shoulder, holding forend down on bags or not, etc. affect recoil direction and amount which can affect POI before bullet exits barrel)


Regards,
hm
 
Thanks for the info, but both groups I am referring to are both nice groups...Under 1/2" in size.

I've noticed this happen on different rifles, so this isn't just on one rifle.
 
Good topic. I vote lighting. I shoot right from the front porch so I get quite a bit of experience shooting after work. Too many times I've hung on a little too long in the evening at last light when testing groups. Everything is going fine then at a certain point the group will be high for no other reason than the sun has gone down as far as I can figure. I shoot towards the west at my place.
 
Originally Posted By: DiRTY DOGBedding your action might help wandering POIs, especially in a tupperware stock.

The stock is a H-S Precision with a Alum. bedding block. Also, the action is torqued to specs.
 
Originally Posted By: TrapShooter12It's head placement. You were not looking through the scope exactly like you did the first time.
This would be my first thought also. But there are many, many other factors too.

Many years ago there was a article in "The Precision Shooting" magazine about a similar problem. In short, the author found that in different times of day, his highly accurate rifle would have a different POI (100 yrds)!.


Why? The guess was - temp, humidity, etc.


Let some one else shoot your rifle and see if it happens to them the same way.....



 
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