Point of impact shift, I want to understand why...

Evil_Dark

Evil_Dark

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Hi there, this sunday I went shooting and adjust my rifle as I added a 20MOA rail and I modified my cheek rest to a better fit. After having a reasonable grouping at 100 yards (3/4"), I shifted at 200 yards. My ballistic calcultor gived me a drop of 3.8" for 200 yards, for my 30-06 140gr Nosler Custom competition. the temperature was around -4 celcius.

My target image have the inches increments drawed on it, so I decided to hold over at 4" instead of using my turret. The first shot was about 1/2" high, so I was happy with the result of my bal. calculator. The second was 1/2" higher again, so now 1" over the bullseye. So I wondered if I did flinched or it was any kinda my fault, so I continued. The third bullet was now 1 3/4" over the bullseye, and the fourth was 2" over... I shooted 7 rounds, and the group did stabilized at 2 1/2" over the bullseye, 2" higher than my original calculation.

My cartridges were at the ambient outside temperature, I did'nt wait so long before I shooted them, so the chamber temperature did'nt had time to warm the cartridge so much. My barrel IS NOT A BULL BARREL, so I Wonder if the rising temperature of my barrel may have lifted my point of impact at 200 yards (I did'nt wait soo long between the shots)? My barrel has 200-250 rounds through it. I have a SUper slam scope, I pretty confident that it is not the issue.

I want to understand some dynamics so I'll be able to go shooting at longer distances and be able to calculate more accurately my point of impacts!

Thanks,

Evil_Dark
 
Evil_Dark said:
Hi there, this sunday I went shooting and adjust my rifle as I added a 20MOA rail and I modified my cheek rest to a better fit. After having a reasonable grouping at 100 yards (3/4"), I shifted at 200 yards. My ballistic calcultor gived me a drop of 3.8" for 200 yards, for my 30-06 140gr Nosler Custom competition. the temperature was around -4 celcius.

My target image have the inches increments drawed on it, so I decided to hold over at 4" instead of using my turret. The first shot was about 1/2" high, so I was happy with the result of my bal. calculator. The second was 1/2" higher again, so now 1" over the bullseye. So I wondered if I did flinched or it was any kinda my fault, so I continued. The third bullet was now 1 3/4" over the bullseye, and the fourth was 2" over... I shooted 7 rounds, and the group did stabilized at 2 1/2" over the bullseye, 2" higher than my original calculation.

My cartridges were at the ambient outside temperature, I did'nt wait so long before I shooted them, so the chamber temperature did'nt had time to warm the cartridge so much. My barrel IS NOT A BULL BARREL, so I Wonder if the rising temperature of my barrel may have lifted my point of impact at 200 yards (I did'nt wait soo long between the shots)? My barrel has 200-250 rounds through it. I have a SUper slam scope, I pretty confident that it is not the issue.

I want to understand some dynamics so I'll be able to go shooting at longer distances and be able to calculate more accurately my point of impacts!

Thanks,

Evil_Dark
Click to expand...

Barrel heat is likely causing the shift imho. That's why people use heavy barrels in long range competitions. Try shooting one then switch to a different rifle for ten minutes. Then shoot the second shot.. Repeat for your group size. If that doesn't cure it then you may have vertical stringing and your load doesn't suit the gun.
 
Couple things...possible mirage if bad can effect this and not sure on your shooting routine, inconsistent cheek weld could be part as well as slapping or poor trigger control even eye fatigue, generally this is a human error problem.
 
Was your parallax set to 200yds? How confident are you in the scope? I wouldn't think 7 shots in 3 minutes would move your POI that much, but I could be wrong.
 
Well, I just remarked that the barrel is barely touching the stock at the very end, or just half a milimeter gap, so I suspect that the temperature of the barrel made a shift in the bullet speed, and the barrel touching the tip of the stock may not have helped. I will dismantle everything to grind some of the barrel groove in the stock to have a bigger gap to ensure the barrel to never be able to touch the stock!

Kinda weird that I've never noticed this before...

Evil_Dark
 
50 shots, roughly 2 hours of shooting. I began to shoot at 200 yards when I had 25 shots fired. Fired 15 shots at 200Yards, then 10 at 300 yards on a steel gong.

Evil_Dark
 
In cold wether barrel temperature can do funny things.
When your amo is cold and your chamber is hot, the longer the round is in the warmer the powder will be when you pull the trigger so little higher velocity.

By how much, I never checked on chronograph, but on clay birds at 400 yards the bullet hits just high if I take too much time. The effect will be proportionnal to powder temperature stability
 
From a cold barrel, plot your poi with number of rds... put the targets horizontally and see where each shot goes.

It is very common for factory barrels to warp as they heat up and change their POI.

Jerry
 
I will buy myself a chronograph to note the differences speed depending on changing factors, like barrel temps, chamber temps and cartridge temp. I may be avle to correlate some causes of poi shifts.

Evil_Dark
 
Evil_Dark said:
A Browning A-Bolt 30-06 synthetic stock, with stainless steel barrel, with muzzlebrake/BOSS system.
My parralax was ajusted and verified to 200 yards, the BOSS adjustement was tight.

Evil_Dark
Click to expand...

Synthetic stocks aren't great for precision purposes. Just a side note. You need a stock where you can torque the action down.
 
My guess is barrel temp. Years ago I shot a 22-250 in a 788 Rem. If I shot a group as fast as I could reload and get a good sight picture, the bullet impact would wander toward the top right. If I shot from a cold barrel and waited two minutes between rounds for the rest of the group, it would put them into a half inch.
 
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