Zero shift with elevation of targets at the same distance

Bolivar

Bolivar

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Outside Barrie ON, Native to South Shore of NS
I've noted this phenominon before and I am a bit mystified.

Yesterday I was playing with my Savage VLP at 100 yards shooting at a playing card near the bottom of my target board. Prone with a Harris Bi-pod. Rear stock supported on a sand bag.

I found I had to lower my previously established 100 zero by 0.3 mil rads to hit point of aim

Then I switched to a bulleye target higher up in the middle the same target board (where I normally place them). This target was about 2 ft higher than the playing card.

Not touching the scope, I shot a nice tight group - about 0.3 mil rads below my point of aim.

Why does a 2ft shift in the elevation a target cause such a shift in POI? Is there a way to eliminate this shift with better technique?
 
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Did you dry fire before your first shots and how was the group. For me I found if I didn't dry fire or was rusty I would "push" through my first couple shots dipping my barrel just slightly, dropping point of impact. Having the target high or low on the target board shouldn't do anything at 100. What setup are you running?
 
What scope are you running graduated in mils; Nightforce?

3 milli-radian is 10.8 inches at 100 yards. Are you saying that you adjusted for 10.8 inches and got 21.6"? Whatever it is that you did, or think you did, it doesn't have anything to do with a 2 foot change in target elevation at 100 yards.
 
My first guess would be that your parallax is not set correctly. Do you have a scope with side focus or adjustable objective? To check your parallax set your rifle up on the bipod and rear bag pointing at your target. Without touching the rifle move your eye around behind the scope. If the crosshairs do not remain stationary on the target, but rather swim over the target face your parallax is set incorrectly. If it is, small changes of your eye position behind the scope will cause a change in point of impact.

If you have a scope with fixed parallax and it is not set correctly for the distance you are shooting you will have to be very careful to have the exact same eye position every time you shoot.
 
wapitiwhacker said:
My first guess would be that your parallax is not set correctly. Do you have a scope with side focus or adjustable objective? To check your parallax set your rifle up on the bipod and rear bag pointing at your target. Without touching the rifle move your eye around behind the scope. If the crosshairs do not remain stationary on the target, but rather swim over the target face your parallax is set incorrectly. If it is, small changes of your eye position behind the scope will cause a change in point of impact.

If you have a scope with fixed parallax and it is not set correctly for the distance you are shooting you will have to be very careful to have the exact same eye position every time you shoot.
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After further thought, I am leaning this to being the likely answer. I did adjust the parallax but I might not have gotten it just right. I will have to check that next time I am shooting groups. At least this is something I can fix.
 
Dogleg said:
What scope are you running graduated in mils; Nightforce?

3 milli-radian is 10.8 inches at 100 yards. Are you saying that you adjusted for 10.8 inches and got 21.6"? Whatever it is that you did, or think you did, it doesn't have anything to do with a 2 foot change in target elevation at 100 yards.
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He said .3 Millirads so its closer to 1.08 in.
 
NormB said:
He said .3 Millirads so its closer to 1.08 in.
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OK, it says that NOW, but even so the difference in shot angle for 2 feet at 100 yards is a ginormous .38 of a degree. There is no way .38 of a degree is going to make 1 inch of difference, but it's easy to prove. Alternate shots at 2 targets placed 24 inches apart without touching a turret and see what you get.
 
Dogleg said:
OK, it says that NOW, but even so the difference in shot angle for 2 feet at 100 yards is a ginormous .38 of a degree. There is no way .38 of a degree is going to make 1 inch of difference, but it's easy to prove. Alternate shots at 2 targets placed 24 inches apart without touching a turret and see what you get.
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I've already proved per my original post. I am trying to fix it ;)
 
Zero Shift

The zero shift you describe could be the result of cartridge cases not full of propellant. When the muzzle is tipped down primer flame has a larger surface area of powder to ignite and this results in a bit more velocity. The same load fired horizontally has less velocity.


If the cases were full or slightly compressed zero shift could be bipod tension.
 
Dogleg said:
All you've proven is that your second group hit an inch lower than the first. It wouldn't be the first barrel to shoot high or low when it warmed up.
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That could be I suppose but I am crossing my fingers that this is not the case. This is a heavy, free floated barrel with a bedded action. If I get this much zero shift from barrel heating I am going to have a hell of a time competing with this rifle.:eek:

Chinbullco said:
The zero shift you describe could be the result of cartridge cases not full of propellant. When the muzzle is tipped down primer flame has a larger surface area of powder to ignite and this results in a bit more velocity. The same load fired horizontally has less velocity.

If the cases were full or slightly compressed zero shift could be bipod tension.
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Cases are pretty full. 223 - 75 Amax - 25 Gn Varget. Both targets were "up" relative to my prone shooting position.
 
Bolivar said:
That could be I suppose but I am crossing my fingers that this is not the case. This is a heavy, free floated barrel with a bedded action. If I get this much zero shift from barrel heating I am going to have a hell of a time competing with this rifle.:eek:



Cases are pretty full. 223 - 75 Amax - 25 Gn Varget. Both targets were "up" relative to my prone shooting position.
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They make new barrels every day. Most of the rifles on the line aren't wearing the pipe they were born with.
 
A change in your position to a target that is slightly lower or higher than a target previously fired on is affected by minute changes in the position as you raise (or lower) the point of aim. Small bore shooters who shoot on a verticle 3 bullseye target array. Actually there are 6 targets (2 each, side by side). The top target is for sighters and the two below are for score. Unless you are prepared to deal with a slight shift of point of impact going from the sighting bull to the lower scoring bull, an impact of 3/8" of an inch (at 50 yards) can be expected. Not a bit deal but the effect can be confusing. Basically the difference can be attributed to your hold and physique.
 
Grinch said:
A change in your position to a target that is slightly lower or higher than a target previously fired on is affected by minute changes in the position as you raise (or lower) the point of aim. Small bore shooters who shoot on a verticle 3 bullseye target array. Actually there are 6 targets (2 each, side by side). The top target is for sighters and the two below are for score. Unless you are prepared to deal with a slight shift of point of impact going from the sighting bull to the lower scoring bull, an impact of 3/8" of an inch (at 50 yards) can be expected. Not a bit deal but the effect can be confusing. Basically the difference can be attributed to your hold and physique.
Click to expand...

You nailed it.......
 
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