Suddenly, at my ususual 100m indoor range, my AR15 rear sight needs 1 MOA (4 clicks) down adjustment from its original 100m zero at the same range.
I have ruled out a number of causes and now I am at a loss.

Specifics:
* Bushmaster DCM rifle
* Free floated Shilen 20" stainless match barrell, which had about 2000 shots
* Grouping of shots has been and remains excellent
* The said down adjustment appears to be here to stay and was necessary at different lanes on the same 100m indoor range
* Same ammo (Lapua cases with 52 gr Sierra bullets) with identical reloading specs
* No changes in barrell cleaning procedure

So what can be the cause of this sudden rise of my POI?
One of the reasons why this causes me some concern is that now I have only two clicks down adjustment left to play with when going to other ranges (and I have a tapered front post thus not quite suitable for fine-adjustment).

Any thoughts are well appreciated!

Did you change primer brand?

Did you change primer lot number?

Did you change powder lot number?

At 2000 rounds your barrel's throat may have lengthened enough to reduce velocity a little.

As long as it's still shooting small groups, I wouldn't worry about it. Temperature has an effect on ammo velocity, but since you are shooting indoors, I doubt that is the cause. Cold temperatures reduce velocity, high temperatures increase velocity, especially when using ball powders.

Many thanks for the ideas!! My further thoughts:

--> Unchanged primer brand (CCI small rifle)
--> Yes, new batch of CCI primers (but personally in many thousands of shots I've never noticed any significant differences in POI between CCI batches)

--> I use vihtavuori N133. Naturally I progress through different batches of this powder, but never noticed a significant change in POI. Of course this is no guarantee that VV has changed something with their powder.

--> Lengthened barrel throat is interesting. Would that be able to cause such a sudden (instead of gradual) change?

--> Temperature at the range is a constant

If all the equipment remained constant then its the nut behind the trigger and how they perceives the target.

Lighting should also be considered, lights down sights down, lights up sights up.

Thanks again.
Lighting is identical to previous situation. It's an indoor 100m range and lighting hasen't been altered.
I'm not quite sure what you mean by 'nut behind the trigger'. Probably referring to the shooter and not some kind of bolt that I'm not aware of.
Anyway, it's the same nut (me) and I cannot think of any reason why within one week, there would be such a sudden and stable change in POI (i.e. how I perceive the target).
I guess this remains a mistery.

Someone turned your scope elevation knob if everything is tight and nothing else has changed.

Did you change to a new jar of powder?

what type of iron sights are being used?

I'm using fixed A2 Iron sights (1/4x1/4 adjustment capabilities).

And Yes, I did switch to another jar of powder. Hower, I'm shooting the same reloaded ammo in my other competition AR15 rifle, and with that rifle POI did not change. Because of that, I believe I can rule out ammo as being the cause.

Probably my next move will be checking the (torque on the) barrel nut.

Your description is a little confusing. A2 front and rear on a free floated setup is really atypical. An A2 sight is typically bottomed out at zero, but you have clicks left. Now you're talking about checking the torque on the barrel nut, suggesting you have retorqued it recently, otherwise it would not be a factor in the change in POI.

If you have removed the barrel from the upper and reinstalled it, you can expect a change in POI as it's near impossible to get it back exactly the same as it was.

It seems far more likely that something has been whacked and moved. I would probably accept the POI change and move on. Again, an A2 rear isn't designed to have clicks left at zero and, with M855, should be at correct elevation at 25m and 300m. The procedure for adjusting zero elevation is not moving the rear, it's raising or lowering the front sight post. You could certainly adjust that if you're married to a 100m zero. While you're probably not shooting M855, you get the idea.

Thanks. Sorry if my description is confusing. It is a Bushmaster DCM Competition Rifrle, with a free float barrel. The original barrel has been replaced with a Shilen barrel about 18 months ago. I do not really expect that the barrel nut has come loose. Moreover, that would most likely (also) result in sideward deviations.

I have a square and tapered front sight post installed, so fine adjustment using the front sight post is not an option. I prefer a zero art 100m because that is the 90% of my range time. Other frequent shooting ranges are 300m and 50 m; both require up-adjustment at the rear sight. I prefer to have some (more than 2x1/4) downward slack at the rear to be able to adjust for changing light conditions when shooting outside or at other indoor ranges.

You are probably right that I should simply accept this and move on. After all, the lower POI is consistent and groups are fine. For that, and to obtain some more downward adjustment capability at the rear, I will need to do some work on my front sight post.

A small correction: when pre-zeroing at 25m with A2 irons and 20" barrel, the rear is set to 8/3+1 click (fixed rear) or 6/3+2 clicks (removable rear). After pre-zeroing at 25m you go back down on the rear to either 8/3 or 6/3 for the 300m zero, which of course has to be confirmed

I would guess that since front sight is mounted to free floated rail of some sort, that at some point rail got bumped and threw off your zero.  I would also check to make sure all mounting hardware for rail is secure.

A2 front sight is one unit with gas block.

Well, maybe the FSB got bumped hard enough to lose zero?

Carbon ring builds up in my competition barrel and my pressures/velocity builds enough after 200 rounds without cleaning that I can see it on the target.

Other things to look at,...

flash hider if equipped is still tight, centered, not cracked, no copper build up some place odd
check crown carefully for damage
borrow a bore scope and check out your bore for stainless steel erosion, it kind of crumbles and alligator skins versus just wearing smooth.
maybe you need a good cooper removing solvent session if your barrel is copper in badly.

just a few ideas to consider.

Thanks for the suggestions.
No flash hider on this rifle.
My bore scope shows the bore to be in fine condition and minimal copper fouling. Crown is also pristine.

I think the only thing left is simply to accept it without understanding what happenend (and that I hate).