Help me understand my zero shift

[mzvarner]

I developed a load for my 308 ( 20" Rem 700) over the winter. I ended up going with 43.5 gr of 4064 over a 178 A-MAX in Hornady match brass (the brass has been reloaded about 6 or 7 times now). This gave me an average velocity of 2610, SD of 16. I had not gotten this rifle out in a while as I was busy finishing up school and taking my boards. I got my rifle out yesterday and my POI was about 14 MOA high at 100 yards. My buddy brought his chrony along and i ran a few rounds through it and was between 2516 and 2550. Once I finally got it all re-zeroed it was holding POA/POI just fine.

So my question is how does seasonal temp changes affect a load? Or was this user error (maybe I bumped the scope getting into the safe). I also noticed that my groups have opened up a little. I used to shoot half minute all day now I am averaging 3/4 to 1 moa. I did remove the copper at one point and my groups opened up to minute of barn after that (so maybe I still need a little more copper fouling?)

Should I redevelop a load? I am not seeing any pressure signs and bolt opens smooth each time.

thanks

 

[EXTREMEPREJUDICE]

Well, I don't run 4064 but I can't believe any modern propellant would shift over 14 inches at 100 yds due to temp. In fact, you indicate the muzzle velocity was "less" than during your original development so the POI should drop, not raise. My guess is you bumped something........................

 

[Darkside-Six]

The temp differences will cause your velocity to fluctuate but not your POI by that much. I experienced something simular. Did my load developement back in March when it was around 30 degrees out. my load is also 43.5gr of IMR 4064 but i'm pushing 175 smk's out of a 24" barrel. when i did my load developement they had an average velocity of 2745. Well i chrono'd them again in july and the same load measures with an average of 2660. that didn't effect my POI at 100 at all. if you were off by 14" at 100 then yo bumped something.

 

[mzvarner]

I agree that I must have bumped something, but it is just odd. I always make it a point to check my turrets to make sure they are back at my 100 yard zero before I start for the day. I learned this lesson the hard way. I have read somewhere that guys were having problems with Vortex ( I am running a Viper PST 4-16) and recoil affecting the internals of the scope. I am not sure what calibers these scopes were on ( I can understand a 50 knocking stuff around, but a 308?)

 

[29aholic]

You left elevation in your scope from a previous range trip maybe shooting at 500yds?

 

[seanhagerty]

Being off one revolution seems most likely. Unless you bumped it, or took is apart.

 

[Sterling Shooter]

Temperature may have an effect on trajectory. Keep a data book to understand the effect. In general, a 20 degree temperature change is worth about 50 feet per second change in velocity; and, a 50 fps change in velocity has about a 1 minute value.

 

[Sterling Shooter]

The temp differences will cause your velocity to fluctuate but not your POI by that much. I experienced something simular. Did my load developement back in March when it was around 30 degrees out. my load is also 43.5gr of IMR 4064 but i'm pushing 175 smk's out of a 24" barrel. when i did my load developement they had an average velocity of 2745. Well i chrono'd them again in july and the same load measures with an average of 2660. that didn't effect my POI at 100 at all. if you were off by 14" at 100 then yo bumped something.

Could it have been the velocity in March was 2660 and then in July 2745?

 

[Darkside-Six]

Could it have been the velocity in March was 2660 and then in July 2745?

That's what you would have thought, but it wasn't the case. Could have been that the chrono was off a little but i'm not sure.

 

[vh20]

I agree that I must have bumped something, but it is just odd. I always make it a point to check my turrets to make sure they are back at my 100 yard zero before I start for the day. I learned this lesson the hard way. I have read somewhere that guys were having problems with Vortex ( I am running a Viper PST 4-16) and recoil affecting the internals of the scope. I am not sure what calibers these scopes were on ( I can understand a 50 knocking stuff around, but a 308?)

Is the barrel free-floated on this R700? Is it a wooden stock that may have changed with the season, putting pressure on the barrel somewhere? 14 MOA is almost unheard of in POI shift unless something major happened.

I had one of the Viper PST 6-24s that came apart inside. It was on a .338LM with a brake. I think all the ones that I've seen that had legitimate, documented failures reported here were on rifles of at least 300 WinMag in caliber, and with brakes. There is a good argument being made that it isn't the large calibers, but rather the brakes that they can't stand up to (reverse recoil impulse, similar to the way some air rifles can eat up scopes that aren't designed for them).

 

[mzvarner]

Is the barrel free-floated on this R700? Is it a wooden stock that may have changed with the season, putting pressure on the barrel somewhere? 14 MOA is almost unheard of in POI shift unless something major happened.

I had one of the Viper PST 6-24s that came apart inside. It was on a .338LM with a brake. I think all the ones that I've seen that had legitimate, documented failures reported here were on rifles of at least 300 WinMag in caliber, and with brakes. There is a good argument being made that it isn't the large calibers, but rather the brakes that they can't stand up to (reverse recoil impulse, similar to the way some air rifles can eat up scopes that aren't designed for them).

My R700 is free floated in a B&C medalist. I am starting to believe that I maybe my turret caps were loose and the last time I shot (out to around 500 yards) I thought I returned to zero, but my turrets just slipped.

On another note do you think I am going to have issues with my scope now? I just ordered a Ross Schuler break for my 308?

 

[mb3]

I would suspect a big change of 14 moa would be in your rifle. Check the torque on all your bolts. I havd found the biggest change was from the action bolts getting loose.

 

[RobertB]

Your a rev off I bet. What kind of scope do you have on it? None of my rifles have ever done that and some have gone through very drastic climate changes for a long time.

 

[vh20]

My R700 is free floated in a B&C medalist. I am starting to believe that I maybe my turret caps were loose and the last time I shot (out to around 500 yards) I thought I returned to zero, but my turrets just slipped.

On another note do you think I am going to have issues with my scope now? I just ordered a Ross Schuler break for my 308?

I have no idea. All the ones I know of with the problem like mine had were 6-24s and yours isn't. I don't know if that has anything to do with it or not, but I don't remember any 4-16s being mentioned. Mine failed during the early load-development phase, so I wasn't changing my zero because I was always at 100 yds. What I noticed was my group sizes kept getting bigger and bigger, even when I back-tracked to an earlier load that had been working fairly well before. Also, the very first thing I noticed was a black "flake" of material that suddenly appeared in one of the upper quadrants of the reticle one day. Shortly after that is when my groups began going south.