Advanced Marksmanship Shifting zero

[jaym_100]

Last week I sighted in my rifle. I went back a few days later and it was shooting 2 MOA high. It is shooting great groups and there was no horizontal shift. I don't know if it could be the temperature change along with the sun or if something is wrong with the scope. Scope is a mark 4.

 

[jaym_100]

Re: Shifting zero

By the way. My rings and base are tight.

 

[HOG0317]

Re: Shifting zero

I was taught that with a .308, a ~20 degree F change will move point of impact roughly 1 MOA either up or down. You said the "temperature change along with the sun" so I'm assuming it was hotter than when you zeroed, explaining the higher impact. How much hotter was it when you shot the second time than when you zeroed the gun?

 

[TimK]

Re: Shifting zero

What range is your zero. 2 MOA is huge. I doubt velocity increase alone could account for it, nor even getting out of your barrel's accuracy node.

Be suspicious of your scope. I had a Mark 4 that lost zero if you looked at it funny. Leupy replaced it.

Without intending any insult to your shooting, could it be parallax issues?

 

[jaym_100]

Re: Shifting zero

This isn't the first time this happened but it is the most it has moved. I don't think it is a parallax issue because once it is re zeroed I can put it in the same spot. I'm going to try and shoot friday and sunday see if I can figure it out. The rifle is free floating on and HS stock. The temperature was probably about 15 to 20 degrees warmer then the last time I shot.

 

[ubet]

Re: Shifting zero

next question, what distance is your zero? Did you zero on a bench, then move to prone? Did you use a rear bag, as opposed to your fist? Take your stock off? Anyone of those things could lead to a change in zero.

 

[Driftwood]

Re: Shifting zero

Did you clean your rifle? Do you need to clean your rifle? There really a host of different things that can throw off your cold bore zero. I like to track mine. I keep a page in the back of my logbook just for coldbore. I put all the relevent information in and then plot it on the same target, like I was shooting a string. I try to look for tendencies in my cold bore as its the one that counts most. If everything has been consistant,except for that one round, I wouldn't change anything. It could be something to do specifically with that one shot, either how you fired it or with the ammunition itself.

 

[Lefty06]

Re: Shifting zero

Well you got the rings and base covered. I lost a shoot on Sunday because of a loose base.

 

[Greg Langelius *]

Re: Shifting zero

Maybe it's a cheekweld issue.

 

[bm11]

Re: Shifting zero

I see my zero shift from session to session by 1-2/10'ths of a mil, but I live in Maine so it might be 60 degrees one day and 35 degrees the next.

 

[jaym_100]

Re: Shifting zero

Zero is at 100 yds. Rifle was cleaned before I shot. I have cleaned my rifle and shot several times without this happening. It wasn't just a single shot that was off. All of my 20 rounds were in the same spot. Shot prone with a rear bag off a bipod. Just like the last several times. Tomorrow I will be out again shooting. Maybe I can come up with some answers.

 

[TimK]

Re: Shifting zero

You can test your scope for tracking without shooting. It's how I discovered the problems with my Leupy Mark 4.

Lock the rifle down somehow so it's <span style="text-decoration: underline">solid</span>. Set up some sort of target at 100 yards or whatever you can manage and note where the crosshairs are. Twiddle the knobs around like you normally might and then return to zero. Your crosshairs should be back to the original spot. If you want to get fancy, you can put up a calibrated target and check the click values while you are at it.

Is the action tight in the stock? Stock bedded?

 

[Lindy]

Re: Shifting zero

An elaboration on this idea: Optically Checking Rifle Scopes

 

[jaym_100]

Re: Shifting zero

Thanks for the idea, I'll have to check into that. Everything is tight. I went thought everything and checked. Couldn't make it to the range today due to some BS.