Bullet point of impact

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irishlad

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I experienced an issue while shooting a group at 100 yards today. My point of impact on the target moved up with every shot. The first group of the day was fine, and about 40 min later I shot another group and the POI rose about 1/2inch with every shot. Several hours later it was back to normal. The rifle is a S&W1500 (Howa) chambered in 30-06. The load is 150g SST with 48g Varget and COL of 3.250. The rifle is usually very accurate. (shot 3 shot groups at .75MOA last week)

It is far from overheating as it barely gets luke warm witha 5 shot group and has more than ample time to cool between groups. (ambient temp is about
+4C)

The scope rings and bases are tight.

Has anyone encountered this particular problem? Is it a load problem? I have heard Varget is not prone to temperature deviations, but I am leaning that way.
 
If it was my rifle my first assumption would be barrel heating up.

If not that, then I would suspect an improper torque on an action screw, or some crap under the action buggering up the bedding. I assume the rifle is bedded. Is there oil under the action?
 
usually, I've found issues like that due to scope mounting problems, or bedding issues; perhaps a loose stock screw
 
moving

Are you using forend pressure on this barrel? I'm assuming a sporter weight barrel. Have you shot many groups previously at those temps? Are you keeping your ammo warm or just at ambient +4? I've seen similar issues with mirage off barrel, doesn't take much heat in cool weather to get heat waves and mirage. Not likely a gun/scope/mount issue if it went bact to normal a couple hours later.
 
With the cool ambient temperature,the cartridge is being inserted into an ever preheated chamber with each shot. Perhaps it is increasing pressure?

I have experienced this, but it was with a 300WM. Tried everything possible, and it always climbed as it warmed. By the 3rd shot it was really noticeable. I swapped on a heavier barrel, and it ended the tendency.
 
My 7mm Rem Mag used to do the same thing if I didn't let it cool completely between shots. Check to see if the barrel is free floating(sounds like there might be a pressure point at the foreend), let it cool 3-4 min between shots and try again.
If there is a pressure point, I'd recommend shimming the action and trying again before modifying the stock.
 
All mounting screws are tight. Group in question was shot from bench with sandbag under forestock and rear bag. Ammo was at outside ambient temp. (+4C) Will shoot several groups at next sight-in and will keep ammo room temp in my inside pocket and see if it makes any difference. If I experience the problem again, I will let the barrel cool for 10 min between shots and see if it improves the group. I wish I had a chrony to see if each shot increases in velocity.
 
Most common cause of this phenomena (i.e., vertical stringing) is an over heated barrel. Let the barrel cool down (to ambient temperature if you are patient enough) and shoot your group(s) again. If it still happens, then at least you've eliminated the most commonly cited cause for vertical stringing.
 
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