Strange POI shift

[Roddy]

I am novice when it comes to long range shootong. I have developed a load (178 Amax over 43.5 grains Varget at 2.840) and last week I zeroed my rifle with it (at 100m) and trued my ballistic data.

Today I went to try some more shooting and confirm the data from last week as the wind was a bit calmer (it had been about 2mph but I never adjusted for it). The temperature was about the same.

Today I shot in the morning. My "range" faces SE or maybe S SE so the sun was in my eyes and there was a bit of mirage.

Here is a target I used a both days. The hits at the top and bottom are at 327m before and after I dialed in 1.7 mils. That was how I trued my data which seemed bang on.

Now when I dialed in 1.7 mils today I got this at the same 327 meters.

I was off quite a bit. About 4.2" high.

I tried at 176 meters with 0.4 mils dialed in to see what was up. Again I was really high, about 2.5".

Now I thought maybe my zero was way off but I checked at 100m and got this five shot group:

It was about 0.15 mils high and 0.15 mils right. Pretty close (and as a side note I think it's the best five shot group I ever fired. About 0.3".)

What could cause this? My loads haven't changed, my zero is very close to where it should be, my parallax was set. The sun was in my eyes and there was a bit of a mirage. I thought my turrets might have slipped bit they came back to what is probably where they were, the small error being a poor zero.

Any ideas? I'm new at this and don't know what happened. I won't be able to go shooting again until next week and wanted to make a range data card and set everything up.

 

[Mystic Precision]

What is the scope?

What is the rifle?

What type of stock?

What is the rests?

Jerry

 

[Roddy]

I guess I should have mentioned that stuff. The rifle is an AI AE MkIII in .308 with its chassis system.

Scope is a bit cheap it's a Vortex Viper PST 6-24x FFP. The zero stop is the shim type but I was going to redo it as all the shims are on one side. Mounted with an AI 18 MOA 30mm mount.

Rest is an Atlas bipod and TAB gear rear bag.

I don't think anything is too cheap or ready to fall apart. The weird thing is the zero is only off a click each way.

So why so different from last time? Same lot of powder hand trickled, same box of bullets, same outside temperature give or take a few degrees.

I thought I had done something stupid and just go"Ah ha! I forgot to bottom out my elevation!" but that's not the issue and I can't figure it out.

 

[Glock4ever]

Maybe it was a temp difference...

 

[Roddy]

Maybe it was a temp difference...

The temperature was a but colder so that would result in a lighter load, no? Also Varget is supposed to be very temperature insensitive.

Thanks for the lead though. I don't really know much about this. The error is almost definitely something I did or failed to do or take into account.

I thought maybe it was the sun or the shimmer of the mirage but you would think that would be like 0.1 mil error. I was off a lot more than that, about 0.4. The scope turrets, could they be an issue?

I spent about $5000 on this set up and went through 200 rounds getting my loads figured out just so this wouldn't happen!

 

[Roddy]

Okay kind of embarrassing. The reason the POI shifted is because I'm an idiot who can't follow simple instructions.

The zero stop shims are supposed to be inserted one after the other on opposite sides of the exposed turret until you can not easily fit any more, even if the turret goes a few clicks past zero.

Well I decided I was above instructions so I jammed as many as I could in from one side until I had a mushy pile and the turret stopped right at 0 or one down, or so I thought.

When I cranked it back down I was slipping the cap on the turret, hence the POI shift. The scope is not a bad design but it has to be used as intended. It is not a por man's mechanical zero stop.

The only thing I find weird now is why my 100 meter zero isn't an inch or so higher.

Anyways thank you very much for the replies and sorry to j up the Precision Rifle forum with such dumb questions.

 

[Mystic Precision]

Take the shims out and just look at the markings. Confirm that the scope WILL return to zero.

A scope of proper tracking will not vary. If you scope does, then you may have a stacking of errors. A bit of at close range, adds up to a big problem at LR.

Make sure the scope works before adding the variable of zero stop. Especially if the design is open to problems.

At least the load and rifle work well.

Jerry