Zero adjustments for 0-10000 ft

[jdmecomber]

This year I have had a couple issues with my rifle setup. My zero has shifted. One from the abuse of a quad I would assume
which resulted in 2 moa low, one from the rifle bipod leg falling in a hole that caused the gun to tip over and hit on the bell of the scope which caused a 2 moa right and 2 moa low error. This also caused the scope to be out of level.

The rifle has a NF Steel base with the 4 screws locitite, the rings are Barret no gap 250$ and the scope is S&B PM2.

The recent coues deer trip to Arizona, I also experienced about a .5 moa shift high when I took two shots in the field to make sure the gun arrived after a plane flight. The ballisitics programs told me I should only have a .03 inch shift in zero from 28.4 Baro to 25 ish.

I will include some pics of my zero check before I left for Arizona in 32 degrees and my infield check in Arizona at around 60 degrees 25 baro.


I was talking with my gunsmith and he had an elk hunt in Colorado this year. He zeroed his gun at sea level and at 9000 ft at 200 yards his rifle was off 2.2 inches about 2 moa for his infield check.

Lets hear some thoughts on this

 

[ShtrRdy]

I don't have any first hand experience on what your saying, and there has been lots of talk on whether or not that much elevation change would affect zero. To my way of thinking a 10,000 foot change in elevation will change the zero on your rifle. This is because the air density will be much less at 10,000 feet and therefore the bullet will get to the target faster. If the bullet gets to the target faster there will be less time for gravity to pull down on the bullet therefore the bullet will impact a little higher.

 

[Barrelnut]

I kinda ran the numbers for a 6.5 CM I have with a 140 AMax at 2700 FPS. Rifle is zeroed at 200. Went from the 29.74 station pressure is was zeroed at to 25.00 station pressure. It was not enough to make any MOA POI change at 200. Zero was the same.
But looking at the trajectory curve at 175 yards it would be .5 MOA high.
Is it possible that you were just shooting at say 175 yards in AZ when checking zero?

 

[jdmecomber]

I don't have any first hand experience on what your saying, and there has been lots of talk on whether or not that much elevation change would affect zero. To my way of thinking a 10,000 foot change in elevation will change the zero on your rifle. This is because the air density will be much less at 10,000 feet and therefore the bullet will get to the target faster. If the bullet gets to the target faster there will be less time for gravity to pull down on the bullet therefore the bullet will impact a little higher.

From a real quick adjustment in my calculator 20.6 Baro which would be around 10000 feet only has a .25 inch difference in zero. So it would seem that the gun/Scope combo is shifting in the rings or some offer place. I am not an expert in this stuff.

 

[jdmecomber]

I kinda ran the numbers for a 6.5 CM I have with a 140 AMax at 2700 FPS. Rifle is zeroed at 200. Went from the 29.74 station pressure is was zeroed at to 25.00 station pressure. It was not enough to make any MOA POI change at 200. Zero was the same.
But looking at the trajectory curve at 175 yards it would be .5 MOA high.
Is it possible that your were just shooting at say 175 yards in AZ when checking zero?

No 100 yards. Even at the extreme 20.6 Baro it barely makes a difference.

 

[jdmecomber]

I have heard of some lighting issues shifting zero but I have never been able to verify it at a distance of 100 yards.

 

[Barrelnut]

Ya know I have banged cheap scopes and rings around (all I own it seems) a heck of a lot harder than what you described above and never had the zero change. IMO there is something going on with the rings or scope.

 

[dok7mm]

The easy way is putting your high country weather/altitude info from your Kestrel, into a ballistic app.
This year I went to the extreme and took my LabRadar to Colorado @ 9500 ft. and checked my velocity and POI, while scouting a new to me area. When I checked my zero, it was 1" high at 200. When I put the Kestrel info in Shooter app, it showed 2 clicks adjustment to my original 1,545' 200 yd zero, so it was spot on.

 

[Bravo 4]

Ya know I have banged cheap scopes and rings around (all I own it seems) a heck of a lot harder than what you described above and never had the zero change. IMO there is something going on with the rings or scope.

This...
I would expect your top tier scope to hold zero through some harsh treatment. Every mechanical device is prone to failure and I would have the scope checked out. Good luck, keep us posted on how things turn out.

 

[WildRose]

I have heard of some lighting issues shifting zero but I have never been able to verify it at a distance of 100 yards.

IF it were me I'd shoot a box test with it, then take it for a bumpy ride on the four wheeler again and repeat the box test.

A 10,000' change in elevation should cause some shift in your zero but not that much.

Downrange the thinner air is going to make a difference in your LR trajectory. If everything is right it should be riding high.

I know you have first class equipment but it appears something isn't right.