Different zero

[TeaRex]

A few weeks back, I took my rifle out to a 0-500 yard class and got good, repeatable dope at 100yd increments. But I went to the range today and found that I was hitting ~0.4 mil high at up to 300 yards (Couldn't shoot further today)... my zero had shifted some.

When I took the class, it was ~65 deg and raining. Today, it was 80 deg, 80% humidity. I doubt anything atmospheric caused the shift (elevation was within 500 ft).

The ammo was from the same set of reloads, so I do not think that was the issue (140gr hybrid, 45.9gr IMR-4350). Group sizes were comparable.

The one idea I have is related to the action/chassis. Because the rifle was completely soaked at the class a few weeks back, I completely disassembled it and cleaned / oiled the gun (out of the chassis). I then reassembled it and used my Borka set to torque to the proper setting. Is it possible that taking apart and then re-assembling the rifle/chassis could have caused some harmonic differences and shifted the zero? Maybe the action screws were torques tighter/looser during the class? I didn't scrub that crap out of the bore or remove any copper. Just a light cleaning with bore guide.

Rifle is Rem700, 6.5x55, Bartlein 24" barrel with APA micro bastard brake, AICS chassis with viperskins, 5.5-22 NF NXS with Larue mount. Shooting was done prone with atlas bipod.

 

[Cranedaddy]

Probably the atmospheric conditions. Do you know that the pressure was? I wouldn't sweat it very much, just keep a good record of your zero and atmospheric conditions, including barometric pressure. Are you using a data book? There should be a place in your data book to log where your zero changes according to the conditions.

I had this same thing happen to me in a class. Had to adjust my zero -.3 mils down. Weather was pretty much opposite of what yours was. Went from hot as hell to cloudy with rain. Every rifle will respond different, you just have to record the changes so you know what to expect next time.

 

[Mad_Charlie]

Well, you have two changes, The weather, and the rifle was taken out of the stock, flip a quarter...

 

[witchhunter]

You took the action out of the stock? You will need to re zero every time.

 

[Gunfighter14e2]

You took the action out of the stock? You will need to re zero every time.

Interesting, I must be real lucky.

 

[TeaRex]

Thanks for the responses. My first intuition was atmospheric conditions, but I can't imagine they were that much different (Navasota TX vs Alvin TX). I played around with my ballistic calculator some, and it looked like it would take a lot of Density Altitude change to get that magnitude of change on POI.

 

[Dan Newberry]

Thanks for the responses. My first intuition was atmospheric conditions, but I can't imagine they were that much different (Navasota TX vs Alvin TX). I played around with my ballistic calculator some, and it looked like it would take a lot of Density Altitude change to get that magnitude of change on POI.

You're right... and I was going to suggest that. Probably the torque on the action wasn't the same when you got your zero, as it was after you removed the action and re-torqued things.

Different shooting positions (bench versus prone, etc) can also affect POI and zero. Too, when a scope is brand new (not sure this is the case but I'm just throwing it out there), the erector spring(s) and turret adjustments may need to settle in a bit before repeatability is there... running the turrets end-to-end several times before getting your initial zero can help in some cases.

Dan

 

[mb3]

Most likely the temp change. Having the action out of the stock can shift yout zero but, if you torque it back in to the same spec you should be gtg (or very slight change is possible).

 

[MitchAlsup]

I'm going to go with the disassembly/reassembly is the root of the problem.

 

[Barney88PDC]

Atmosphere within 500 yards doesn't do shit.

Did you check zero at 100? Taking it out of the stock may have caused a change that affected your zero. Second did you adjust the parallex out of the optic. NF is horrible for effecting accuracy with parallex.

 

[Pinecone]

Running my .308 load through JBM, a simple change in temp from 80 - 65 results in a 0.1 mil change in the 300 yard dope.

And a rainy day, could have meant a pressure change. YOu can check online weather data for the actual pressure those two days.

 

[TeaRex]

Pinecone - good idea. I hadn't though to go back and look at the the weather data for the previous trip.

Barney - I didn't check my 100yd zero. The 200 yd and 300 yd were consistently different than before, so I assume the 100yd would be the same, but I guess I'm going to have to check now.

Thanks for the responses.

 

[6brshooter]

I used imr4350 during last yrs h4350 shortage. i found it is clean burning and accurate. what it isn't is temp stable, in my 8 twist 243 the same load runs 3090@80* and 3020@65*. what does 50-70fps do to ur drops

 

[TeaRex]

I used imr4350 during last yrs h4350 shortage. i found it is clean burning and accurate. what it isn't is temp stable, in my 8 twist 243 the same load runs 3090@80* and 3020@65*. what does 50-70fps do to ur drops

JBM said ~0.1 mil with 100 fps lower MV at 300 yds... pretty minimal.

Did you find IMR 4350 and H4350 to be equally accurate clean (but IMR more temp sensitive)? I am down to my last lb of IMR and was thinking about re-working the load with H4350.

 

[6brshooter]

I got 6 lbs of imr4350 left, when it is gone i will switch back to my original h4350 load. The only reason i tried imr4350 was 5 rifles i shot used h4350 and last summer i got down to 3-4 lbs of h4350 so i decided to start subbing powder. If u lived closer i would sell u my imr4350 so i can start back on h4350. I think ur zero change stems from a couple things: change in temp effecting drop value and ur 100 zero, and also removing barreled action effecting zero. Ur load at 65* was zeroed, and when it was 85* that load was hotter and zero was off plus ur drops were slightly less. I have had some bedded rifles that shot great, but take them out of the stock and retorque them and they need a couple shots to settle in, usually they are close to original zero but u can't always count on it every time or with every rifle. As a sidenote, the best group i shot at 1 k was with that custom tikka Krieger243 using imr4350, light 8 mph fv 5 shots=4.25"& 4 of those were 2.1".