Cold bore, follow up shots, and temperature questions

[DontBurnMyFlag]

Hey all,

Got an odd occurrence the other day with my new rifle. Its an Accuracy International Arctic Warfare in 308. Currently shooting 168gr Federal BTHP.

Back in August I zeroed it at 100 yards with a warm bore in 90 degree temperatures.

Yesterday I go to take a cold bore shot in 60 degree temperature. My cold bore shot is dead on at 100 yards. No issues.

But any shot following that was off 2 MOA high left consistently. Adjusted and came down 2 minutes and right two minutes and im back to zero for the rest of the day. No issues following that.

Now I have heard of temperature affecting the cold bore shot. But have not heard of it affecting my follow-ups unless it was zeroed for a cold-bore shot.

The rifle is used. Previous owner has probably 2000+ rounds through it. Its department issued and everything has been inspected by armorers. The scope is tight and the rings have all been tightened to the appropriate specs.

It is stored in a safe in the trunk of my car. Its subjected to some slight bumping around. But nothing jarring enough to knock the zero off the scope or anything.

My questions are

1. How do I correct this?
2. What caused this?

I am relatively new to the precision rifle field. So be gentle haha.

 

[BSA1]

Did you fire your first shot from a clean, dry (free of any oil or Breakfree) bore?

 

[DontBurnMyFlag]

I punched the barrel with a boresnake, but it wasn't a spotless perfectly cleaned rifle. I tend to shoot with a slightly fouled barrel for consistency.

But yea, this shot was fired with a clean barrel in comparison to my previous zero.

 

[wally]

That seems like and awfully large shift at 100 yards, you sure you didn't end up using ammo from a different lot after that first shot?

 

[DontBurnMyFlag]

Its possible that lots get mixed up. Especially since we get ammo issued to us. But its all the same manufacturer and grain at all times. Velocity holds steady and never fluctuates more than 20 FPS give or take.

 

[Sunray]

60 degree temperature isn't cold enough compared to 90 to affect anything. Difference between warm and cold barrels is though. Mind you, with that rifle it's not likely given the stock it has. Most likely to be ammo related or the stock screws got loosened. Bedding issues will move groups too.

 

[wally]

Consistent velocity but different POI for the same POA with different lots would seem to be pretty common, otherwise matches wouldn't need to allow "sighting" shots at the start.

Precision and accuracy are two different but related things. Precision is shooting small groups, ignoring the bias error with respect to the POA. Accuracy is deviation of POI from POA, its basically impossible for imprecise ammo to be consistently accurate.

In theory with precise ammo you can remove the bias error when zeroing, but seems in the real world different ammo lots have different bias errors.

See how many shots or outings you can go before it shifts again. If the shift only happen at the start of a new outing, I think something was being knocked about between outings.

 

[gspn]

You have a sample size of one occurrence. Tough to draw any conclusions based on a one time event.

Repeat your cold bore experiment a few more times and come back with teh results. It won't take more than two or three shots each time.

For now I'm with those who suggest you may have fired different types of ammo.

I usually zero my rifle when it's a legit 90 to 105 degrees, and I hunt with it when it's 28 to 50 degrees and have never had a POI shift from temperature...and I shoot a 20 year old A-Bolt.

 

[taliv]

my guess is it's mechanical, possibly inside the scope, or rings. or something in the muzzle. (like suppressor or brake is loose or something)

there are a lot of things that can change your POI. how your holding the rifle, for instance, which would be my second guess. much less likely are environmental, like a change in lighting between the 1st and 2nd shots.

let us know how further testing goes

 

[243winxb]

different cleaning process

Clean bore, first shot, goes high, but not 2" at 100 yds. , more like 1/4 to 1/2" . I suggest a different cleaning process. I store with BF CLP. Clean it out with Hoppe's number 9 , patch it dry and shoot. If not going to shoot right away, barrel will be good to go when needed. No worse then a dirty barrel riding around in the trunk. Note where the first shot strikes, after cleaning, for a few cycles.

 

[DontBurnMyFlag]

Thanks guys

I suppose you're right. Too little information to make an accurate hypothesis at this time. I will take it out this week and see what happens. It will be an indoor 100 yard range, temp controlled, and same lot of ammo. I will update the results.