Scope Movement Over Time

[CVShooter]

I've always practiced the required sight-in procedures before hunting. With open sights, everything is always exactly where I left it. With scopes, however, I often have small differences after the rifle sits for several months. It's nothing too dramatic as I'm always on paper at 100 yards. But they often seem to need a couple of clicks to get it back where things should be. This year, things were more bizzare than usual in that my .308, sighted in dead-on at 100 was actually 3-4" high at 100. I don't buy high-end glass but it's not the cheapest stuff either. Ammo was exactly the same -- as in the same batch from handloads. After cleaning last season, that rifle pretty much sat in the safe. Again, this is more dramatic than usual.
Any idea what might cause a shift in the POA?

[HK Dave]

Here are some possible causes in no specific order:

-Temperature and/or altitude
-Parallax error
-Shooter error
-Scope Mount
-Scope

[Mute]

In addition to the list HK Dave put up, have you ever tested whether or not your rifle has a POI shift between a clean and a dirty bore?

[Divernhunter]

add stock movement on barrel

[CVShooter]

In addition to the list HK Dave put up, have you ever tested whether or not your rifle has a POI shift between a clean and a dirty bore?

That could have been it on this last go-round with the .308. I had cleaned with a copper solvent at the end of the season vs the ol' Hoppes 9.

[FeuerFrei]

Just not enough info to tell yet. 3-4 inches is pretty significant movement. Bolt gun? Semi auto? Lever? Need more details OP.

Could be the scope/mount or the stock warped due to moisture exposure during the hunt?
Stock to barrel contact changes can throw it off if it's a wood stock because of the moisture.
Action screws could have lost proper torque.

I'd use a Federal factory match load as a control and re-test before I started to delve further into it.

[CVShooter]

Just not enough info to tell yet. 3-4 inches is pretty significant movement. Bolt gun? Semi auto? Lever? Need more details OP.

Could be the scope/mount or the stock warped due to moisture exposure during the hunt?
Stock to barrel contact changes can throw it off if it's a wood stock because of the moisture.
Action screws could have lost proper torque.

I'd use a Federal factory match load as a control and re-test before I started to delve further into it.

In the case of the .308, we can rule out most of those issues. Synthetic stock on a mossberg mvp (bolt gun). Dry conditions on the hunt (western sierras). Humid conditions on the coast a few weeks ago but same results when taken to a range inland a ways where it was warm & dry. Action screws? The ones holding the action to the stock or something else? I'd fault my handloads but I confirmed with the copper hunting rounds from the same batch I've been using for 4-5 years.

Still doesn't explain a similar effect with the airguns (PCP, not piston) but those are cheaper scopes anyway.

[FeuerFrei]

Action screws holding the action to the stock. Bolt rifle's action screws have been known to loosen under recoil.

Current line up of MPV shows 6 different models. Some have standard sights and some have pic rails.

Rail screws are tight? Scope mounted to rail with some sort of steel/aluminum mount?
Was any blue or green Loctite or other thread locker used on the screws?

Any uncontrolled pressure on the barrel can cause shots to be thrown off. Stocks warping or otherwise changing shape can do it.

If you don't want to use a control factory ammo to test then you have to flip your lucky coin and reset the scope which is made by unknown factory and see if the thing looses/gains elevation.
It could be the scope. IDK.

[Mute]

One thing at a time.

- Re-torque all screws (action, scope mounts...etc.) to recommended specs.

- Shoot some groups. If consistent, clean bore and shoot again. If still a -problem, move on to ammo. Test ammo in another gun for consistency.

So on and so forth. Address one thing at a time. Don't try to correct multiple, possible issues at the same time.