How to adjust ghost ring sights on an 870?

[SPaikmos]

I just got my 870P with GRS. Problem is, the rear sight is adjusted almost all the way to the left. I'm wondering, are these things calibrated at the factory, or are they just set will-nilly and the end user has to fix it?

And, how do I sight it in? I was thinking to just move it to the center and forget about it... I presume the left/right movement is for windage, and center should be normal, right? Or is it necessary to sight it in since each gun is different?

I've also got GRS on my 590; at least that one looks like it came centered from the factory, but not sure what to do with all the adjustments on there....

Hmmm, now I think it would have been easier going with the bead....

[MossbergMan]

A couple of questions come to mind: Whos' GR is mounted on the 870?
Are they aftermarket?
They could be mounted off center if aftermarket. Of course the best thing, if you already haven't done it is benchrest the SG and fire a group at 25 to 50 yards (depending on range facilities) Save the target. Document the ammo used (BTW try different brands, they do shoot to different POI). Send target, with documentation to a good 'smith and I'm sure they can fix you up. All my SG stuff is done by Hans Vang in Chino Valley AZ.
Just some thoughts on the issue. Good luck.

[SPaikmos]

ah, ok. The GRS came standard, it's a model 24591.

I'm guessing this has to be done at a rifle range? Do they have bench rest setups for rent, or do I have to provide my own equipment to do this? I'm such a noob it isn't funny...